Taxonomic history

In 1876, Louis Normand, a physician at the Naval Hospital in France, identified a small worm (∼0.25 mm in length) in the faeces of soldiers returning from Cochin-China (present-day Vietnam) with severe diarrhoea (Bavay, Reference Bavay1876). Bavay (Reference Bavay1876), his colleague, named it Anguillula stercoralis. During a subsequent autopsy of a soldier with similar symptoms, Normand discovered a larger worm (∼2 mm) in the small intestine, which Bavay (Reference Bavay1877) identified as a separate species, Anguillula intestinalis. Shortly thereafter, Bavay (Reference Bavay1877) observed larvae in cultured stools, mistakenly attributing them to A. intestinalis. These forms were later recognized as the rhabditiform larvae (from Greek rhabdos, ‘rod’, referring to the rod-like, 3-part oesophagus characteristic of the feeding larval stages), parasitic adults and filariform larvae (from Latin filum, ‘thread’, and forma, ‘shape’, referring to the elongated/cylindrical oesophagus) of a single species, now known as Strongyloides stercoralis (Grassi, Reference Grassi1879). The relationship between these stages was clarified by Grassi and Parona in 1878, who established the genus Strongyloides and named the parasite Strongyloides intestinalis (Grassi, Reference Grassi1879). In 1881, Perroncito cultivated free-living adults from larvae and referred to them as Pseudorhabditis stercoralis, a designation later corrected by Leuckart, who confirmed they belonged to the same species (Speare, Reference Speare1986). The nomenclature was ultimately resolved by Stiles and Hassall in 1902, who assigned the definitive name Strongyloides stercoralis (Stiles and Hassall, Reference Stiles and Hassall1902).

Host range

Since its original description in humans (Bavay, Reference Bavay1876), S. stercoralis has also been identified in dogs (Fulleborn, Reference Fulleborn1914), cats (Brown, Reference Brown1969) and NHPs (Penner, Reference Penner1981). The taxonomic status of dog-infecting S. stercoralis remains unresolved. Although human and canine strains of S. stercoralis are morphologically indistinguishable, early cross-infection experiments showed that human-derived strains did not consistently establish long-term infections in dogs and vice versa (Fulleborn, Reference Fulleborn1914; Fülleborn, Reference Fülleborn1927; Sandground, Reference Sandground1928; Augustine and Davey, Reference Augustine and Davey1939; Galliard, Reference Galliard1939, Reference Galliard1951; Sandosham, Reference Sandosham1952; Grove and Northern, Reference Grove and Northern1982). More recently, population genetic/genomic studies have supported the existence of a dog-specific lineage (cox1 lineage B), along with a separate lineage shared among humans, dogs, cats and NHPs (cox1 lineage A) (Jaleta et al., Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017; Nagayasu et al., Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017; Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Bradbury et al., Reference Bradbury, Pafčo, Nosková and Hasegawa2021). It has been proposed that human-infecting S. stercoralis likely originated in wild canids and adapted to humans following the domestication of dogs (Nagayasu et al., Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017; Liu et al., Reference Liu, Sarker, Sripa, Tangkawattana, Khieu, Nevin, Paterson and Viney2025). Evidence of occasional introgression between the human and canine lineages suggests they are not fully reproductively isolated (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024; Liu et al., Reference Liu, Sarker, Sripa, Tangkawattana, Khieu, Nevin, Paterson and Viney2025).

The occurrence of S. stercoralis in cats remains somewhat enigmatic. Historically, S. stercoralis was not considered a natural parasite of felines, as early experimental infections of cats using human- or dog-derived isolates either failed to establish or resulted in only transient patency (Tuira, Reference Tuira1919; Sandground, Reference Sandground1925b; Reference Sandground1926a,Reference Sandgroundb, Reference Sandground1928; Augustine and Davey, Reference Augustine and Davey1939; Kadhim, Reference Kadhim1968). Although numerous reports have described S. stercoralis infection in cats, few included species-level confirmation (Zhao and Bradbury, Reference Zhao and Bradbury2024). The first unequivocal identification was reported by Wulcan et al. (Reference Wulcan, Dennis, Ketzis, Bevelock and Verocai2019), in which phylogenetic analysis of a 522 bp cox1 fragment placed the isolate within the human-dog-shared lineage of S. stercoralis.

In 1935, Mirza and Narayan isolated an S. stercoralis-like parasite from the intestine of an Arctic fox (Vulpex alopex) (Mirza and Narayan, Reference Mirza and Narayan1935). Morphometric analysis showed that the parasite had a shorter tail (∼40 µm) but was otherwise indistinguishable from S. stercoralis in humans. Based on this, they proposed it as a variety of S. stercoralis, naming it S. stercoralis var. vulpi (Mirza and Narayan, Reference Mirza and Narayan1935). This parasite has not been reported in foxes since, and it remains unclear whether foxes are true natural hosts. A separate species, Strongyloides vulpis, was described from red foxes (Vulpes vulpes) by Petrov in 1940 (Speare, Reference Speare1986), but original descriptions are now inaccessible. As a result, the taxonomic validity of both S. stercoralis var. vulpi and S. vulpis remains unresolved.

Taxonomic history

In 1905, von Linstow O (Reference von Linstow1905) proposed the name Strongyloides fuelleborni for a species identified in chimpanzees (Pan troglodytes) and yellow baboons (Papio cynocephalus) in Africa. In his original description, he reported that this species characteristically shed larvae rather than eggs in faeces; however, it was later clarified that he had intended the opposite. Chandler AC (Reference Chandler1925) regarded S. fuelleborni as a variant of Strongyloides papillosus (Wedl, Reference Wedl1856) Ransom, 1911, whereas Sandground (Reference Sandground1925b) and Goodey (Reference Goodey1926) considered it a distinct species based on the morphology of the free-living female, particularly the prominent vulvar lips and the narrowing of the body immediately posterior to the vulva.

In 1923, Hung and Höppli (Reference Hung and Höppli1923) described Strongyloides simiae from Asian macaques (Macaca sp.), distinguishing it from S. fuelleborni and S. cebus by the presence of cuticular striations in parasitic females. However, subsequent studies revealed that transverse cuticular striations are common across all Strongyloides species, rendering these morphological differences insufficient to support S. simiae as a separate species. Consequently, S. simiae is regarded as a junior synonym of S. fuelleborni (Speare, Reference Speare1986).

The discovery and subsequent investigations of a S. fuelleborni-like species in PNG led to a taxonomic revision of S. fuelleborni. The New Guinea parasite was designated S. fuelleborni kellyi in 1991, while the original African species described by von Linstow has since became S. fuelleborni fuelleborni (Viney et al., Reference Viney, Ashford and Barnish1991). However, this subspecific distinction has recently been challenged by molecular data showing that S. f. kellyi shares genotypes with the Asian clade of S. f. fuelleborni, suggesting they are taxonomically identical (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). While these findings point to a synonymy between S. f. kellyi and Asian S. f. fuelleborni, further morphological and genomic comparisons of isolates from Asia, Africa and the Pacific are needed before any formal taxonomic conclusions can be drawn (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c).

Host range

Humans and NHPs are the only confirmed natural hosts of S. f. fuelleborni (Speare, Reference Speare1986). Experimental infections with S. f. fuelleborni have been attempted in dogs (Canis lupus familiaris), cats (Felis catus), brown rats (Rattus norvegicus), house mice (Mus musculus) and guinea pigs (Cavia porcellus), but only dogs developed patent infections, which were short-lived and self-limiting (Sandground, Reference Sandground1925b; Rego, Reference Rego1972).

Taxonomic history

During a 1971 parasitological survey in Kiunga, PNG, Allen Kelly detected Strongyloides eggs in human stool samples (Kelly and Voge, Reference Kelly and Voge1973). In some children, this infection was associated with a severe protein-losing enteropathy known as swollen bell syndrome (SBS). This condition was characterized by the presence of large numbers of Strongyloides eggs in stool, often in long mucous strings. In other children, eggs were passed individually, and there were no apparent signs of disease, despite high worm burdens. As it was assumed that only a single egg-producing Strongyloides sp. infected humans in PNG, an unknown co-factor was proposed to explain the development of SBS in some infants with heavy infections but not in others (Ashford et al., Reference Ashford, Barnish and Viney1992).

Subsequent morphological analysis of adult Strongyloides from PNG children revealed a close resemblance to S. fuelleborni von Linstow, 1905. However, the absence of NHPs on the island of New Guinea, combined with the paucity of reported S. fuelleborni infections in humans from regions between Africa and New Guinea at the time, left the identity of this S. fuelleborni-like nematode (S. cf. fuelleborni) unresolved (Viney et al., Reference Viney, Ashford and Barnish1991; Ashford et al., Reference Ashford, Barnish and Viney1992).

Using scanning electron microscopy, Viney et al. (Reference Viney, Ashford and Barnish1991) observed subtle morphological differences between S. fuelleborni isolated from African NHPs and S. cf. fuelleborni from PNG humans, specifically in the peri-vulval cuticle of parasitic females and the position of the phasmidial pore in free-living males. A parallel isoenzyme electrophoretic analysis showed that most S. cf. fuelleborni isolates (22/26) grouped with African S. fuelleborni; however, 4 isolates unexpectedly clustered with S. ransomi from local pigs (Viney and Ashford, Reference Viney and Ashford1990). The authors speculated that this may have resulted from participants submitting pig faeces in place of their own (Viney and Ashford, Reference Viney and Ashford1990). Notably, the Asian clade of S. fuelleborni was not included in these analyses (Viney and Ashford, Reference Viney and Ashford1990; Viney et al., Reference Viney, Ashford and Barnish1991). Nevertheless, based on the available data, Viney et al. (Reference Viney, Ashford and Barnish1991) designated S. cf. fuelleborni as a subspecies of S. fuelleborni, naming it S. f. kellyi in honour of its discoverer, Allen Kelly.

This taxonomic designation was later challenged by Dorris et al. (Reference Dorris, Viney and Blaxter2002), who conducted phylogenetic analysis of a 330 bp region of 18S rRNA and found that S. f. kellyi clustered more closely with S. venezuelensis and S. ransomi than with S. f. fuelleborni, prompting calls to elevate it to species status. However, these findings have been criticized due to the use of formalin-fixed specimens, which are prone to DNA degradation and sequencing artefacts, particularly given the limitations of early molecular techniques (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). A subsequent genotyping study based on the 18S rRNA hypervariable regions HVR-I (432 bp) and HVR-IV (252-259 bp) similarly identified 1 PNG human isolate (1/8) clustering with S. venezuelensis and S. ransomi, distinct from S. f. fuelleborni (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). However, the majority of PNG isolates (7/8) grouped within the Asian clade of S. f. fuelleborni (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). The authors postulated the existence of 2 genetically distinct Strongyloides nematodes in PNG: S. f. kellyi which is likely synonymous with the Asian-Pacific clade of S. f. fuelleborni, and a second, undescribed species corresponding to the genospecies identified by Dorris et al. (Reference Dorris, Viney and Blaxter2002), closely related to S. ransomi of pigs. Notably, S. ransomi causes a protein-losing enteropathy in piglets similar to SBS (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c), and this second egg-producing Strongyloides sp. infecting infants in PNG may explain the previously confusing epidemiology of SBS in that nation.

Host range

Humans are the only known hosts of S. f. kellyi in New Guinea. Prior to the molecular era, investigations into potential zoonotic reservoirs, including pigs, chickens and dogs, found no evidence of infection in these animals (Viney et al., Reference Viney, Ashford and Barnish1991). Given recent taxonomic insights suggesting that S. f. kellyi is likely synonymous with the Asian-Pacific clade of S. f. fuelleborni (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c), such findings are not unexpected. Ashford et al. (Reference Ashford, Barnish and Viney1992) proposed that S. fuelleborni has adapted to exclusive human-to-human transmission in New Guinea without an NHP reservoir, a hypothesis supported by experimental and epidemiological findings from Africa (Pampiglione and Ricciardi, Reference Pampiglione and Ricciardi1972a; Hira and Patel, Reference Hira and Patel1980). However, whether humans are the only natural reservoir in New Guinea remains to be confirmed. This question is particularly relevant given reports of invasive NHP species in parts of the Pacific, including New Guinea (Kemp and Carter, Reference Kemp and Carter2022), which could act as mobile zoonotic reservoirs if they come into contact with humans.

The novel, undescribed genospecies identified by Zhao et al., (Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c) and Dorris et al. (Reference Dorris, Viney and Blaxter2002) may have a zoonotic reservoir in local animals, particularly pigs, given its close genetic relationship to the porcine parasite S. ransomi and the relative ubiquity of domestic pigs in PNG. Future investigations into this nematode in both humans and potential animal reservoirs in New Guinea should employ molecular or advanced morphological tools capable of differentiating Strongyloides spp.

Taxonomic history

Strongyloides felis was first described by Chandler AC (Reference Chandler1925) in cats from Kolkata, India. Chandler AC (Reference Chandler1925) suspected that S. felis might be a variety or subspecies of S. stercoralis, tentatively naming it S. stercoralis var. felis. This subspecies was later elevated to specific status by Goodey (Reference Goodey1926).

Host range

Domestic cats (Felis catus) are the only confirmed natural host of S. felis (Speare, Reference Speare1986). The species has been reported in cats from India (Chandler AC, Reference Chandler1925), Australia (Speare and Tinsley, Reference Speare and Tinsley1986, Reference Speare and Tinsley1987) and Thailand (Jitsamai, Reference Jitsamai2019). However, in the Thai case, the morphological identification was questionable, as it described a hexagonal stoma in the free-living female, a diagnostic feature characteristic of the parasitic female (Speare, Reference Speare1986). Experimental infections of humans (Homo sapiens) and pigs (Sus scrofa) with S. felis did not result in patent infections (Speare, Reference Speare1986).

Taxonomic history

In 1927, during the necropsy of a cat from Louisiana, United States (US), Price and Dikmans (Reference Price and Dikmans1941) observed several tumour-like lesions in the large intestine. A similar case was reported in Florida 3 years later, with nodular masses also found in the colon of another cat. Microscopic examination in both cases revealed small nematodes embedded within the nodules, with no parasites detected elsewhere in the intestine. Recovery of intact specimens was hindered by the brittleness of formalin-fixed tissues; however, measurements from 2 incomplete worms indicated an estimated body length of 5000 µm, substantially larger than that of any known Strongyloides sp. in cats at the time (2370–3330 µm). Based on the relatively large size of the parasitic female and the presence of characteristic nodular colonic lesions, Price and Dikmans (Reference Price and Dikmans1941) designated a new species, Strongyloides tumefaciens. No additional morphological descriptions of S. tumefaciens have since been published.

In 2019, Wulcan et al. (Reference Wulcan, Dennis, Ketzis, Bevelock and Verocai2019) reported similar nodular lesions in the colonic wall of Strongyloides-infected domestic cats from the Caribbean Island of St. Kitts. Worms recovered from these lesions were molecularly identified as S. stercoralis based on a 522 bp fragment of cox1. This finding prompted the authors to question the taxonomic validity of S. tumefaciens (Wulcan et al., Reference Wulcan, Dennis, Ketzis, Bevelock and Verocai2019). However, in the absence of additional molecular or morphological analyses of either S. tumefaciens or S. stercoralis in feline hosts, the taxonomic status of S. tumefaciens remains unresolved.

Host range

Since its initial discovery, S. tumefaciens has been reported in domestic cats from the US (Malone et al., Reference Malone, Butterfield, Williams, Stuart and Travasos1977; Lindsay et al., Reference Lindsay, Blagburn, Stuart and Gosser1987) and Brazil (Moura et al., Reference Moura, Jorge, Nascimento, Riet-Correa, Abel, Cavalcante, Oliveira and Bezerra2016), as well as in wild cats (Felis chaus) from India (Dubey and Pande, Reference Dubey and Pande1964). All of these reports relied on colonic pathology for species identification, a criterion later questioned by Wulcan et al. (Reference Wulcan, Dennis, Ketzis, Bevelock and Verocai2019).

Taxonomic history

Strongyloides planiceps was first identified by R.T. Leiper in rusty-spotted cats (Prionailurus planiceps) from Malaysia in 1927 (Rogers, Reference Rogers1939). Rogers (Reference Rogers1939) described this species and initially named it Strongyloides cati. However, he later recognized that the name S. cati had already been used by Brumpt (Reference Brumpt1936) to describe a Strongyloides sp., now known as S. felis, from domestic cats in India. As Brumpt (Reference Brumpt1936)’s designation lacked a formal description, it was treated as a nomen nudum and held no official taxonomic standing. Nonetheless, to avoid confusion, Rogers (Reference Rogers1943) renamed the species S. planiceps. Although S. cati (Rogers, Reference Rogers1939) remains the technically valid name under the rules of nomenclature, S. planiceps has become the widely accepted name in the scientific literature (Speare, Reference Speare1986).

Host range

Natural infections with S. planiceps have been reported in raccoon dogs (Nyctereutes procyonoides) (Horie et al., Reference Horie, Noda, Noda and Higashino1981; Fukase et al., Reference Fukase, Chinone and Itagaki1985; Sato et al., Reference Sato, Suzuki and Aoki2006), Japanese weasels (Mustela itatsi) (Fukase et al., Reference Fukase, Chinone and Itagaki1985), red foxes (Vulpes vulpes schrencki) (Miyamoto and Inaoka, Reference Miyamoto and Inaoka1982), Japanese red foxes (Vulpes vulpes japonica) (Horie et al., Reference Horie, Ohnishi and Kobayashi1989) and domestic cats (Horie et al., Reference Horie, Noda, Noda and Higashino1981; Fukase et al., Reference Fukase, Chinone and Itagaki1983), most exclusively from Japan.

Natural infection in domestic dogs (Canis lupus familiaris) was suggested in 3 studies (Arizono, Reference Arizono1976; Horie et al., Reference Horie, Noda, Noda and Onishi1980; Fukase et al., Reference Fukase, Chinone, Itagaki, Aihara, Ohkuma, Shimamura and Shibuya1984). Arizono (Reference Arizono1976) described a strain of S. planiceps isolated from a dog in Kyoto, Japan, and maintained through serial passage in puppies; however, details of the original infection were not provided. Horie et al. (Reference Horie, Noda, Noda and Onishi1980) experimentally infected cats with a Strongyloides sp. isolated from dogs and later detected larvated eggs in feline faeces, leading the authors to suspect the parasite was S. planiceps. The pre-inoculation infection status of the cats, however, was not assessed, leaving open the possibility of prior infection. Similarly, Fukase et al. (Reference Fukase, Chinone, Itagaki, Aihara, Ohkuma, Shimamura and Shibuya1984) recovered Strongyloides larvae from canine faeces and experimentally infected both dogs and cats. Larvated eggs were detected in the faeces of both hosts, and adult parasitic females recovered from the small intestine were morphologically identified as S. planiceps. Although this study strongly suggests that dogs can serve as suitable hosts, the identification was based on experimentally infected animals rather than naturally infected dogs. Therefore, the status of dogs as natural hosts of S. planiceps remains circumstantial and requires further confirmation.

Patent experimental infections of cats and dogs with S. planiceps derived from wild carnivores have been documented (Horie et al., Reference Horie, Noda, Noda and Higashino1981; Fukase et al., Reference Fukase, Chinone and Itagaki1985). In the study by Horie et al. (Reference Horie, Noda, Noda and Higashino1981), patency lasted from 0.9 to 3.6 years in cats and from 26 days to over 1 year in dogs. Fukase et al. (Reference Fukase, Chinone and Itagaki1985) did not report the duration of patency in experimentally infected cats.

Parasitic female

Parasitic females are slender and serpentine, measuring 1.5–10 mm in length and 27–95 µm in maximum width. The body is cylindrical, tapering slightly at the anterior end and abruptly at the tail. The thin body wall is covered by a finely striated cuticle. Tail is short and cone shaped. The head has a circumoral elevation but lacks lips, with a shallow, bilaterally symmetrical stoma. Cephalic papillae are indistinct, and amphids are positioned laterally. A single dome-shaped cervical papilla is present at the excretory pore level. The nerve ring crosses the oesophagus in the anterior quarter. The oesophagus is cylindrical, with a muscular anterior portion and a posterior part composed of 3 glandular nuclei (1 dorsal, 2 subventral). The intestine consists of 40 cells, each with a single nucleus, in 2 rows leading to a short rectum. The excretory system opens just posterior to the nerve ring. The reproductive system is didelphic, with equal, opposed uteri and reflexed ovaries, but lacks seminal receptacles. The vulva, situated approximately two-thirds of the body length from the anterior end, is a transverse slit with distinct margins. Eggs are arranged in a single row within the uterus. Paired nerve endings are present near the vulva, where the cuticle dorsal to the vulva is modified at its junction with the hypodermis (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996; Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024).

Strongyloides spp. can be readily distinguished morphologically from other nematodes that may occur in the intestines of humans and animals by having only the parasitic female stage. Unlike most parasitic nematodes, Strongyloides spp. lack parasitic males, despite early reports suggesting otherwise (Faust EC, Reference Faust1933). Parasitic females typically reside within the mucosal layer of the small intestine but may also be found freely in the intestinal lumen, particularly in cases of severe pathological reaction (Page et al., Reference Page, Judd and Bradbury2018). In humans and dogs, hyperinfection with S. stercoralis can lead to disseminated strongyloidiasis, where parasitic females have been identified in the mucosa of small bronchi and bronchioles (Higenbottam and Heard, Reference Higenbottam and Heard1976). In such severe cases, both parasitic females and eggs may be recovered from sputum and faeces (Bisoffi et al., Reference Bisoffi, Buonfrate, Montresor, Requena-Mendez, Munoz, Krolewiecki, Gotuzzo, Mena, Chiodini and Anselmi2013; Mati et al., Reference Mati, Raso and de Melo2014).

Free-living female

Free-living females are comparatively smaller (up to 1.5 mm in length) but broader (up to 85 µm at their widest point). They have a spindle-shaped body with a marked central enlargement to accommodate the egg-filled uterus, which occupies most of the body cavity. The head bears 2 lateral cephalic lobes, each containing small, inconspicuous papillae in subdorsal, lateral and subventral positions. Amphids are located posterior to the lateral papillae. The mouth is dorso-ventrally elongated, with a laterally compressed subglobular stoma bordered anteriorly by a collar-like cuticular structure. The rhabditoid oesophagus consists of a muscular corpus subdivided into anterior and posterior portions, a narrow isthmus and a terminal bulb with a well-developed valvular apparatus. The nerve ring encircles the oesophagus at the posterior end of the isthmus. The intestine comprises 22 cells arranged in dorsal and ventral rows. A short rectum leads to a subterminal anus, which has a slight lip-like swelling along its posterior edge. Phasmids are located laterally near the midpoint of the gradually tapering, finely pointed tail. The reproductive system is didelphic, with equal and opposed uteri and reflexed ovaries; the anterior branch runs along the right side of the intestine, and the posterior branch along the left. The vulva is positioned near mid-body and is associated with a short vagina; the terminal portion of the uterus serves as a seminal receptacle (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996; Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024).

Free-living Strongyloides females must be carefully distinguished from free-living rhabditoid nematodes commonly found in soil, which can contaminate faecal cultures, particularly when samples are collected from bare ground. Strongyloides free-living females possess 2 broad lateral lips bearing 6 small papillae, whereas free-living rhabditoid nematodes typically have 4–6 lips with more prominent papillae. The buccal capsule in rhabditoids is longer and cylindrical with parallel sides, in contrast to the shorter and more compact buccal structure of Strongyloides. The oesophagus in Strongyloides is clearly divided into 4 regions, while in Rhabditis, the anterior muscular region is often reduced or absent, and the posterior corpus commonly expands into a muscular bulb, producing a characteristic mid-bulb morphology. Tail morphology also provides a useful differentiator; most free-living rhabditoid nematodes have a long, slender and whip-like tail, whereas the tail of Strongyloides is finely tapered and relatively short, typically not exceeding 15% of the total body length (Sandground, Reference Sandground1925a; Speare, Reference Speare1986; Grove, Reference Grove1996).

Free-living male

Free-living males are slightly smaller than females, measuring up to 1.2 mm in length and 55 µm at maximum width. The body wall, cuticle, head, oesophagus, intestine and excretory system are similar to those of the female. The reproductive system consists of a straight, tubular structure with an unreflexed testis extending from just posterior to the oesophagus to the mid-body. The seminal vesicle and vas deferens are poorly differentiated. The short cloaca houses a pair of equal, blade-like spicules with laterally bent, knob-like anterior ends and 2 supporting ribs that extend nearly to the tip. A thin membrane along the ventral edge gives the spicules a bow-like appearance. The gubernaculum is laterally compressed, with wing-like extensions forming a T-shaped posterior end in cross-section. The caudal papillae include a single unpaired papilla on the anterior cloacal lip and 6 bilaterally arranged papillae: 1 subventral preanal, 2 subventral adanal (anterior and posterior), 1 lateral postanal, 1 subventral postanal and 1 subdorsal postanal. The tail is shorter and broader than that of the female, ventrally curved when fixed and tapers abruptly to a point (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996; Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024).

Several morphological features distinguish free-living Strongyloides males from rhabditoid males. In Strongyloides, the testis is broad, blunt-ended and unreflexed, whereas in rhabditoids, it is typically narrow and reflexed posteriorly. The spicules of rhabditoids are strongly curved, often appearing fused at the tip and lacking a ventral membrane. In contrast, Strongyloides spicules are less curved, possess a ventral membrane and are often extruded and diverge laterally in fixed specimens. The gubernaculum of Strongyloides includes a distinct medial plate separating the spicules, while in rhabditoids, the medial plate, if present, is typically small and does not extend between the spicules. Additionally, caudal alae are absent in Strongyloides but present in rhabditoids, where they typically bear 9 pairs of stalked papillae (Speare, Reference Speare1986; Grove, Reference Grove1996).

Egg

Eggs shed by parasitic and free-living females of Strongyloides are thin shelled and ellipsoid, with slightly flattened poles. They typically measure 40–55 µm in length, with a width ranging from one-half to three-quarters of the length. Each egg contains an underdeveloped rhabditiform larva at the time of passage (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

Careful differentiation is required in faecal samples to distinguish Strongyloides eggs from those of hookworms, Ternidens deminutus, Oesophagostomum spp. and trichostrongylids. Strongyloides eggs are generally smaller, have thinner shells and usually contain a developing larva rather than the 8- or 16-cell morula typical of these other species when freshly voided (Figure 1).

Figure 1.

Exemplar eggs of selected intestinal nematodes, with size indicated in micrometres (µm).

Figure 1 Long description

The image shows five different intestinal nematode eggs, each labeled with its name and size indicated in micrometres on the y-axis. From left to right, the eggs are Strongyloides fuelleborni, Hookworm, Oesophagostomum spp., Ternidens deminutus and Trichostrongylus spp. The y-axis ranges from 0 to 100 micrometres, providing a scale for comparison of egg sizes. Each egg has distinct visual characteristics, with variations in texture and pattern visible within the shells.

Rhabditiform larva

Four rhabditiform stages of Strongyloides spp. (L1r-L4r) have been identified. They are differentiated primarily by the increasing size following each moult (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996; Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024).

The first-stage rhabditiform larva (L1r) of Strongyloides spp. measures 180–240 µm in length and 14–15 µm in width, with a mean length around 210 µm. The oesophagus comprises nearly one-third of the body length and is structurally similar to that of the free-living adult. The head has 2 cephalic lobes separated by a transversely elongated, oval mouth. The cylindrical stoma is 5–8 µm long, with a slightly thickened posterior wall. The nerve ring is initially located near the anterior end but migrates to the posterior isthmus prior to the first moult. The intestine consists of 22 uninucleate cells arranged in dorsal and ventral rows. The rectum is short, with the anus located 40–60 µm from the tail tip. A prominent, rhomboid-shaped genital primordium, containing 5–9 nuclei, lies ventrally near the mid-intestine. Although larval length nearly doubles before the first moult (depending on culture conditions), the oesophagus grows only slightly. There are no detectable morphological differences between L1r larvae originating from eggs of parasitic versus free-living females (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

The L1r can be distinguished from other soil-transmitted nematodes by its shallow buccal cavity (4–8 µm), straight intestine and prominent lateral genital primordium. Hookworm larvae, in contrast, possess a smaller, more refractile genital primordium (<4 µm), while Ternidens spp. and Rhabditis spp. have longer tails and deeper buccal cavities (Schulte and Poinar, Reference Schulte and Poinar1991; Bradbury, Reference Bradbury2019; Buonfrate et al., Reference Buonfrate, Tamarozzi, Paradies, Watts, Bradbury and Bisoffi2022).

The second-stage rhabditiform larva (L2r) varies in morphology depending on whether development proceeds through the homogonic (direct) or heterogonic (indirect) life cycle. Larvae destined to become L3r (heterogonic development) undergo modest elongation and increased body width, measuring approximately 400 μm in length and 18–25 μm in width. In homogonic development, L2r transform into iL3 and exhibit more pronounced internal changes: the oesophagus elongates from 30% to 45% of body length, with the posterior portion becoming less muscular and more glandular, and oesophageal gland nuclei become more prominent. Intestinal cells (except the first and last pairs) divide, increasing the total from 22 to 40 nuclei. A notched tail of the emerging filariform larva forms within the cuticle in preparation for ecdysis. In both developmental pathways, the head is reorganized such that the cephalic lobes of the third-stage larva become lateral, in contrast to the dorsal and ventral positions observed in L1r and early L2r (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

The third-stage rhabditiform larva (L3r) typically measures 550–700 μm in length and 22–30 μm in width. The oesophagus remains rhabditiform, increasing slightly in length and complexity, while the intestinal lumen widens. The genital primordium continues to expand, showing preliminary gonadal development. Morphological sexual differentiation begins to appear in L3r and becomes increasingly apparent as larvae approach L4r. The cuticle thickens, and head and tail structures mature; however, this stage remains non-infective and restricted to environmental development (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

The fourth-stage rhabditiform larva (L4r) represents the final larval stage in the heterogonic cycle. These larvae typically measure 700–900 μm in length and 30–40 μm in width, with females slightly larger than males. The oesophagus adopts a structure resembling that of the free-living adult, including distinct corpus, isthmus and terminal bulb regions. Gonadal development progresses significantly: ovaries and testes are readily distinguishable and lie adjacent to the intestine. Though morphologically similar to free-living adults, L4r larvae are distinguishable by their smaller size and incomplete gonadal maturation. In females, the vulva is visible as a shallow, slit-like invagination but remains closed to the exterior (Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024). This stage precedes the final moult into sexually mature free-living adults (Little, Reference Little1966a; Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

Infective third-stage larva

In the homogonic (direct) life cycle, some L2r bypass the free-living phase and develop directly into infective third-stage filariform larvae (iL3) (Grove, Reference Grove1996; Streit, Reference Streit2008; Viney and Lok, Reference Viney and Lok2015). The choice between homogonic and heterogonic development can be influenced by environmental factors such as temperature, humidity and host availability (Streit, Reference Streit2008, Reference Streit2017). In most Strongyloides spp., including S. stercoralis and S. felis, free-living development is limited to a single generation (Viney and Lok, Reference Viney and Lok2015). However, under optimal culture conditions, some species may undergo multiple successive free-living generations. Hansen et al. (Reference Hansen, Buecher and Cryan1969) reported 2–3 such generations in S. fuelleborni, comprising exclusively females that reproduced only in the presence of males from the first generation, while Premvati (Reference Premvati1958a) observed only 1 generation in the same species. S. planiceps has been observed to complete up to 9 consecutive free-living generations (Yamada et al., Reference Yamada, Matsuda, Nakazawa and Arizono1991). iL3 represent the infective stage responsible for initiating new parasitic infections (Grove, Reference Grove1996; Streit, Reference Streit2008; Viney and Lok, Reference Viney and Lok2015).

Strongyloides iL3 are slender and serpentine, measuring 400–700 µm in length and 12–20 µm in width. The cylindrical, filariform oesophagus constitutes approximately 40–45% of the total body length. The cuticle is finely striated, and the lateral alae are double, spaced about 4 µm apart, extending to the tail, where they form a distinctive notched tip. The head bears 2 inconspicuous lateral cephalic lobes with small subdorsal and subventral papillae, and a lateral amphid. The mouth is small and pore-like, with a shallow, laterally compressed stoma. The intestinal cells are arranged in dorsal and ventral rows, consisting of 40 nuclei, where the anterior and posterior pairs are uninucleate and the remainder binucleate. The excretory system resembles that of the free-living adult stages (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996; Castelletto et al., Reference Castelletto, Akimori, Patel, Schroeder and Hallem2024).

iL3 can be readily distinguished from other nematode larvae by their slender shape, elongated filariform oesophagus and, most notably, the notched tail tip, which is considered pathognomonic for the genus Strongyloides. No other nematodes exhibit this feature. Unlike hookworm iL3, Strongyloides iL3 are unsheathed, having shed the cuticle of the previous stage during ecdysis (Little, Reference Little1966a, Reference Fukase, Chinone and Itagakib; Speare, Reference Speare1986; Grove, Reference Grove1996).

Autoinfective larva

Strongyloides stercoralis and S. felis are the only Strongyloides species for which an autoinfective cycle has been documented (Speare, Reference Speare1986; Buonfrate et al., Reference Buonfrate, Bradbury, Watts and Bisoffi2023). The third-stage autoinfective filariform larvae (L3a) share overall morphology and structural proportions with iL3 but are generally shorter and stouter. Their body length rarely exceeds 500 µm, with a width-to-length ratio of approximately 1:4 (Schad et al., Reference Schad, Smith, Megyeri, Bhopale, Niamatali and Maze1993; Kim et al., Reference Kim, Joo, Ko, Na, Hwang and Im2005; Buonfrate et al., Reference Buonfrate, Tamarozzi, Paradies, Watts, Bradbury and Bisoffi2022, Reference Buonfrate, Bradbury, Watts and Bisoffi2023).

Recently, S. stercoralis larvae exhibiting morphological features intermediate between L3a and parasitic females were observed in respiratory tract specimens from 2 human cases in Australia (Zhao et al., Reference Zhao, Koehler, Truarn, Bradford, New, Speare, Gasser, Sheorey and Bradbury2025b). These were identified as fourth-stage autoinfective filariform larvae (L4a) and are distinguished from L3a by their cone-shaped tail and a more developed genital primordium. The oesophagus remains filariform and occupies 37–46% of body length, a proportion intermediate between that of L3a and parasitic females. The genital rudiment is notably enlarged and contains a developing vulva enclosed within cuticular layers (Zhao et al., Reference Zhao, Koehler, Truarn, Bradford, New, Speare, Gasser, Sheorey and Bradbury2025b). Similar ‘juvenile’ parasitic females of S. stercoralis had been previously described in experimentally infected dogs (Faust EC, Reference Faust1933) and marmosets (Mati et al., Reference Mati, Raso and de Melo2014), although Faust EC (Reference Faust1933)’s description may represent a mixture of parasitic and free-living forms (Zhao et al., Reference Zhao, Koehler, Truarn, Bradford, New, Gasser, Sheorey and Bradbury2025a). Detailed morphometric data for this stage remain unavailable.

Parasitic female

Morphological features useful for species identification in the parasitic female include stomal shape (in en face view), the number and arrangement of lobes on the circumoral elevation, ovary configuration and tail morphology (Figures 2 and 3). Stomal shape can be categorized into 4 types: simple, angular, complex and those with oesophageal teeth (Speare, Reference Speare1986). A simple stoma lacks angularity and may appear round, oval or dumbbell-shaped. Angular stomas include square, rectangular, hexagonal or badge-like configurations. Complex stomas are partitioned into multiple chambers with radiating subdivisions extending from a central cavity. A stoma with oesophageal teeth has anterior projections arising from the oesophagus and extending to the stomal margin (Figure 2A). While identifying the exact stomal shape may be technically challenging, classification into one of these categories is typically achievable and alone permits differentiation of several Strongyloides species (Speare, Reference Speare1986; Sato et al., Reference Sato, Tanaka, Une, Torii, Yokoyama, Suzuki, Amimoto and Hasegawa2008).

Figure 2.

Distinguishing morphological features of parasitic females of Strongyloides spp. (a) Stomal shape in en face view; (b) ovary type, based on its orientation relative to the intestine; (c) tail morphology in lateral view, highlighting the degree of tapering. Modified from Speare (Reference Speare1986). See Table 2 for species-specific morphological features.

Figure 2 Long description

The image shows three sections. The first section (a) illustrates various stomal shapes: round, dumbbell, dumbbell with oesophageal teeth, X-shaped, hexagonal and rectangular. The second section (b) depicts ovary types: straight and spiral. The third section (c) shows tail morphologies: bluntly rounded, narrowly tapered and narrowly tapered to a point.

Figure 3.

Parasitic females of Strongyloides spp.: (a) Strongyloides stercoralis, (b) Strongyloides fuelleborni, (c) Strongyloides felis, (d) Strongyloides planiceps. Each panel shows the full body of the nematode. Insets highlight the stoma and tail regions, with the stoma shown in en face (EF), lateral (L) and dorsoventral (DV) views, and the tail in lateral view. Modified from Little (Reference Little1966a) and Speare (Reference Speare1986).

Figure 3 Long description

The image shows four panels labeled a, b, c and d, each depicting the full body of a parasitic female nematode. Insets highlight the stoma and tail regions. In panel a, the stoma is shown in en face, lateral and dorsoventral views. Panel b displays similar views with a different stoma shape. Panel c presents another stoma configuration in the same views. Panel d shows yet another stoma type, again in en face, lateral and dorsoventral views. Each nematode illustration includes a scale bar indicating 100 micrometers for the full body and 10 micrometers for the stoma views.

The circumoral elevation is often divided into paired lobes, with species-specific variation in the number. In lateral or dorsoventral views, lobes may be absent or present in groups of 2, 4, 6 or 8. Species with 2 lobes typically have broad lateral lobes; those with 4 include lateral, dorsal and ventral lobes. In 6-lobed species, lobes are positioned laterally, subventrally and subdorsally, while 8-lobed species also have distinct dorsal and ventral lobes (Little, Reference Little1966a, Reference Littleb; Arizono et al., Reference Arizono, Matsuo and Yoshida1976). The prominence of these lobes varies among species, making enumeration less reliable in some cases. Although theoretically useful for differentiation, lobulation is generally considered a minor criterion, used only when other distinguishing features fail to separate species (Speare, Reference Speare1986).

The didelphic ovary in parasitic females may be either straight (recurrent) or spiral. In spiral types, the ovaries follow the course of the intestine without encircling it. The anterior arm typically exhibits more pronounced spiralling than the posterior, and both arms maintain a consistent anatomical relationship with the intestine, spiralling uniformly in an anticlockwise direction from the anterior end (Figure 2B). Ovary type can aid taxonomic classification, but it is not a definitive diagnostic feature by itself. In species with spiral ovaries, females may reach sexual maturity before spiralling becomes morphologically apparent (Little, Reference Little1966a, Reference Littleb; Speare, Reference Speare1986; Grove, Reference Grove1996).

Tail morphology is another distinguishing feature among Strongyloides spp., with variation in the degree of taper and tip shape. Tails may range from narrowly tapered to bluntly rounded (Figure 2C). S. felis has a finely tapered, often pointed tail, whereas S. stercoralis typically has a less acutely tapered, blunt-ended tail (Speare, Reference Speare1986). Although not exclusively diagnostic, tail morphology can support preliminary species differentiation, particularly between S. felis and S. stercoralis in feline hosts, pending confirmation through more definitive features, such as stomal shape (Speare, Reference Speare1986).

Free-living female

The morphology of the free-living female Strongyloides spp. is largely conserved across species. Differentiation relies primarily on 2 peri-vulval features: post-vulval narrowing and vulval rotation. In some species, the body diameter remains relatively constant posterior to the vulva (Figure 4A), while in others, a reduction in diameter is observed (Figure 4B). S. fuelleborni and S. felis have pronounced post-vulval narrowing, whereas S. stercoralis shows a moderate reduction of approximately 15%. In species with less pronounced narrowing, the reduction is generally below 10% (Speare, Reference Speare1986; Grove, Reference Grove1996). A limitation of this trait is its environmental sensitivity; for instance, in S. fuelleborni, the degree of narrowing may vary with temperature (Premvati, Reference Premvati1958b).

Figure 4.

Vulval morphology in free-living females of Strongyloides spp. (a) Absence of post-vulval body narrowing (S. stercoralis, S. planiceps); (b) presence of post-vulval body narrowing (S. fuelleborni, S. felis); (c) absence of posterior vulval rotation (the vulval slit forms an angle of 90°–100° with the longitudinal body axis) (S. stercoralis, S. planiceps); (d) presence of posterior vulval rotation (the vulval slit forms an angle greater than 100° with the longitudinal body axis) (S. fuelleborni, S. felis). Modified from Speare (Reference Speare1986).

Figure 4 Long description

The image shows four illustrations labeled a, b, c and d. Illustration a depicts the absence of post-vulval body narrowing. Illustration b shows the presence of post-vulval body narrowing. Illustration c illustrates the absence of posterior vulval rotation, with the vulval slit forming an angle of 90 to 100 degrees with the longitudinal body axis. Illustration d shows the presence of posterior vulval rotation, with the vulval slit forming an angle greater than 100 degrees with the longitudinal body axis.

By contrast, vulval rotation represents a more stable and taxonomically reliable feature. In most species, including S. stercoralis and S. planiceps, the vulval slit or short vagina forms an angle of 90°–100° with the longitudinal body axis (Figure 4C). However, in S. fuelleborni and S. felis, this angle exceeds 100°, producing a distinctly posteriorly rotated vulval appearance (Figure 4D). Unlike post-vulval narrowing, vulval rotation appears unaffected by environmental conditions and is considered a reliable criterion for species differentiation (Speare, Reference Speare1986).

Free-living male

Taxonomically important features in the free-living male include spicule morphology, gubernaculum structure and the arrangement of caudal papillae. Most Strongyloides spp. possess sharply pointed spicule tips, though some species have blunted, hooked or laterally projected tips. Spicule curvature varies from straight to markedly curved; however, this trait has limited diagnostic value due to measurement variability and observer subjectivity (Speare, Reference Speare1986). A more consistent taxonomic character is the shape of the spicule’s ventral membrane, which may appear convex, straight or concave (Figure 5A). Most species have a straight membrane, but its prominence varies, being more pronounced in S. fuelleborni compared to S. stercoralis (Speare, Reference Speare1986; Grove, Reference Grove1996).

Figure 5.

Distinguishing morphological features of free-living males of Strongyloides spp. (a) Spicule morphology, showing the shape of the ventral membrane; (b) tail showing the arrangement of caudal papillae, including (1) subventral preanal papilla, (2) anterior adanal papilla, (3) posterior adanal papilla, (4) lateral papilla, (5) subventral postanal papilla and (6) subdorsal postanal papilla. The subventral preanal papilla is aligned with the 2 adanal papillae in the left diagram but not in the right, as indicated by the red dashed line. The arrow indicates the preanal organ. Modified from Speare (Reference Speare1986). See Table 2 for species-specific morphological features.

Figure 5 Long description

The image A shows three illustrations of spicule morphology labeled as straight, convex and concave. The image B shows two diagrams of tail structures with numbered caudal papillae: (1) subventral preanal papilla, (2) anterior adanal papilla, (3) posterior adanal papilla, (4) lateral papilla, (5) subventral postanal papilla and (6) subdorsal postanal papilla. The left diagram shows alignment of the subventral preanal papilla with the adanal papillae, while the right diagram shows a different arrangement, indicated by a dashed line and an arrow pointing to the preanal organ.

The gubernaculum is relatively conserved across Strongyloides spp., except in S. serpentis, which displays a uniquely straight dorsal border. Minor differences in the dorsal pole and width-to-length ratio are observed among species but are not sufficient for definitive identification (Speare, Reference Speare1986).

The arrangement of caudal papillae is among the most reliable taxonomic markers (Speare, Reference Speare1986). Key distinguishing features include the position of the subventral preanal papilla relative to the preanal organ and the longitudinal alignment of the subventral preanal and adanal papillae (Speare, Reference Speare1986). For instance, in S. stercoralis, the subventral preanal papilla is aligned with both adanal papillae, whereas in S. fuelleborni, the anterior adanal papilla is dorsally displaced (Figure 5B). Additionally, S. stercoralis has a more anteriorly located preanal organ relative to the cloacal opening, in contrast to its more posterior placement in S. fuelleborni (Speare, Reference Speare1986).

The position of the phasmidial pore in the free-living male is particularly informative in distinguishing subspecies of S. fuelleborni. In S. f. kellyi, the phasmidial pore lies midway between the 2 post-cloacal papillae, while in S. f. fuelleborni, it is situated adjacent to the anterior-most cloacal papilla (Viney et al., Reference Viney, Ashford and Barnish1991). Owing to their minute size, phasmidial pores require visualization using scanning electron microscopy.

Other distinguishing features

The developmental stage present in freshly voided faeces can assist in preliminary species identification. While all Strongyloides spp. are oviparous, hatching may occur internally, resulting in the passage of L1r in faeces, as observed in S. stercoralis and S. felis, or externally, with eggs passed in faeces and hatching occurring in the environment, as seen in S. fuelleborni and S. planiceps. Some other animal-infecting Strongyloides spp., such as S. ratti, may shed both eggs and L1r in faeces (Speare, Reference Speare1986; Viney and Lok, Reference Viney and Lok2015).

However, faecal stages alone are unreliable for definitive species identification, particularly when examination is delayed. For example, eggs of S. f. fuelleborni may hatch into L1r within 6–10 h at 23–25 °C and within 2–7 h at 37 °C (Cordi and Otto, Reference Cordi and Otto1934). Infection intensity may also influence which stage is excreted. Although rare, eggs of S. stercoralis have been documented in freshly voided faeces during hyperinfection (Bisoffi et al., Reference Bisoffi, Buonfrate, Montresor, Requena-Mendez, Munoz, Krolewiecki, Gotuzzo, Mena, Chiodini and Anselmi2013; Mati et al., Reference Mati, Raso and de Melo2014).

The detection of Strongyloides L1r, parasitic females or eggs in respiratory or urinary tract specimens is indicative of disseminated S. stercoralis infection (Buonfrate et al., Reference Buonfrate, Tamarozzi, Paradies, Watts, Bradbury and Bisoffi2022). S. stercoralis is currently the only species known to cause dissemination in natural infections (Buonfrate et al., Reference Buonfrate, Bradbury, Watts and Bisoffi2023).

Strongyloides stercoralis

Genotyping of Strongyloides was first attempted by Ramachandran et al. (Reference Ramachandran, Gam and Neva1997) using PCR–restriction fragment length polymorphism (RFLP). This study analysed partial 28S rRNA and ITS across multiple Strongyloides spp., including S. stercoralis. While all human-derived S. stercoralis isolates were indistinguishable, several distinct RFLP profiles differentiated them from a canine strain originally isolated from a naturally infected beagle and subsequently maintained in laboratory dogs at the University of Pennsylvania (the UPD strain) (Schad et al., Reference Schad, Hellman and Muncey1984). To identify more informative markers for species discrimination, Hasegawa and colleagues examined HVR-I to HVR-IV of 18S rRNA and a 722 bp fragment of cox1. Although species-specific clustering was observed, these loci did not consistently distinguish S. stercoralis isolates from humans, dogs and chimpanzees (Hasegawa et al., Reference Hasegawa, Hayashida, Ikeda and Sato2009, Reference Hasegawa, Sato, Fujita, Nguema, Nobusue, Miyagi, Kooriyama, Takenoshita, Noda and Sato2010).

Two subsequent genotyping surveys in Asia demonstrated the phylogenetic divergence of S. stercoralis into 2 lineages based on cox1 sequence data: one infecting both humans and dogs (cox1 lineage A) and the other found exclusively in dogs (cox1 lineage B) (Jaleta et al., Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017; Nagayasu et al., Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017). Jaleta et al. (Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017) also analysed the 18S rRNA HVR-I and HVR-IV regions, finding that 18S rRNA HVR-IV haplotype A corresponded with cox1 lineage A, while 18S rRNA HVR-IV haplotype B corresponded with cox1 lineage B. Furthermore, they observed that although cox1 and 18S rRNA HVR-IV sequences reliably distinguished the 2 lineages, 18S rRNA HVR-I haplotypes did not correlate with host specificity.

The genotyping scheme established by Jaleta et al. (Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017) and Nagayasu et al. (Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017) has since been expanded in multiple population genetic studies. cox1 lineage A has been identified globally in humans, dogs, cats and NHPs (Hasegawa et al., Reference Hasegawa, Sato, Fujita, Nguema, Nobusue, Miyagi, Kooriyama, Takenoshita, Noda and Sato2010; Laymanivong et al., Reference Laymanivong, Hangvanthong, Insisiengmay, Vanisaveth, Laxachack, Jongthawin, Sanpool, Thanchomnang, Sadaow and Phosuk2016; Jaleta et al., Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017; Thanchomnang et al., Reference Thanchomnang, Intapan, Sanpool, Rodpai, Tourtip, Yahom, Kullawat, Radomyos, Thammasiri and Maleewong2017; Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Basso et al., Reference Basso, Grandt, Magnenat, Gottstein and Campos2019; Beknazarova et al., Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019; Wulcan et al., Reference Wulcan, Dennis, Ketzis, Bevelock and Verocai2019; Aupalee et al., Reference Aupalee, Wijit, Singphai, Rödelsperger, Zhou, Saeung and Streit2020; Sanpool et al., Reference Sanpool, Intapan, Rodpai, Laoraksawong, Sadaow, Tourtip, Piratae, Maleewong and Thanchomnang2020; Salant et al., Reference Salant, Harel, Moreshet, Baneth, Mazuz and Yasur-Landau2021; Repetto et al., Reference Repetto, Braghini, Risso, Argüello, Batalla, Stecher, Sierra, Burgos, Radisic and Cappa2022; Borrás et al., Reference Borrás, Pérez, Repetto, Barrera, Risso, Montoya, Miró, Fernandez, Telesca and Britton2023; Nosková et al., Reference Nosková, Modrý, Baláž, Červená, Jirků‐Pomajbíková, Zechmeisterová, Leowski, Petrželková, Pšenková and Vodička2023, Reference Nosková, Svobodová, Hypská, Cerezo-Echevarria, Kurucová, Ilík, Modrý and Pafčo2024; Beiromvand et al., Reference Beiromvand, Ashiri, de Ree, Harbecke, Rödelsperger, Streit and Rafiei2024; de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024). In contrast, cox1 lineage B has been reported exclusively in dogs from Southeast Asia (Jaleta et al., Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017; Nagayasu et al., Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017), South Asia (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024) and Australia (Beknazarova et al., Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019). An exception to this is the recent identification of a human isolate from Bangladesh carrying the dog-specific mitochondrial (cox1 lineage B) and nuclear (HVR-IV haplotype V) genotypes, suggesting possible incidental zoonotic transmission (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024).

To date, genotyping surveys have identified 6 18S rRNA HVR-IV haplotypes (A, B, C, E, J, V) and 11 HVR-I haplotypes (I–XII, XV) within S. stercoralis (Bradbury et al., Reference Bradbury, Pafčo, Nosková and Hasegawa2021). 18S rRNA HVR-IV haplotype A is the predominant genotype detected in humans, dogs and NHPs worldwide (Bradbury et al., Reference Bradbury, Pafčo, Nosková and Hasegawa2021). 18S rRNA HVR-IV haplotype B has been reported in dogs from Cambodia (Jaleta et al., Reference Jaleta, Zhou, Bemm, Schär, Khieu, Muth, Odermatt, Lok and Streit2017) and Australia (Beknazarova et al., Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019), while haplotype V was identified in canine isolates from Bangladesh and is likewise considered dog-specific (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024). Other haplotypes include C, isolated from a human infection in Southeast Asia but maintained via serial passage through laboratory dogs in Australia (Putland et al., Reference Putland, Thomas, Grove and Johnson1993); E, detected in canine and human strains from Australia and China, respectively (Beknazarova et al., Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019; Zhou et al., Reference Zhou, Fu, Pei, Kucka, Liu, Tang, Zhan, He, Chan and Rödelsperger2019); and J, reported in a human strain from the USA (Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019). Due to the high variability of 18S rRNA HVR-I haplotypes within host-specific S. stercoralis isolates, this region has not been considered a reliable marker for inferring host specificity (Bradbury et al., Reference Bradbury, Pafčo, Nosková and Hasegawa2021).

Two population genomics studies have provided new insights into the host specificity of S. stercoralis (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024; Liu et al., Reference Liu, Sarker, Sripa, Tangkawattana, Khieu, Nevin, Paterson and Viney2025). de Ree et al. (Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024) analysed whole-genome sequences from worms isolated from 7 human hosts and 1 dog in Bangladesh. Their results largely support the previously described 2-lineage population structure of S. stercoralis. However, 2 dog-derived isolates possessed a nuclear genome containing 18S rRNA HVR-IV haplotype V and a mitochondrial genome corresponding to cox1 lineage A, suggesting historical inter-lineage hybridization (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024). In a second, larger-scale study, Liu et al. (Reference Liu, Sarker, Sripa, Tangkawattana, Khieu, Nevin, Paterson and Viney2025) examined S. stercoralis from sympatric human (n = 26) and dog (n = 12) populations in Bangladesh, Cambodia and Thailand. Whole-genome analysis revealed that human- and dog-derived S. stercoralis form 2 largely distinct populations that diverged genetically approximately 8000–12 000 years ago. Nevertheless, evidence of introgression was detected, consistent with the findings of de Ree et al. (Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024), suggesting that reproductive isolation is incomplete. Also, 1 dog harboured a worm possessing human-type mitochondrial and nuclear genomes, supporting the potential for occasional cross-species transmission (Liu et al., Reference Liu, Sarker, Sripa, Tangkawattana, Khieu, Nevin, Paterson and Viney2025). Together, these studies support the hypothesis first proposed by Nagayasu et al. (Reference Nagayasu, Aung, Hortiwakul, Hino, Tanaka, Higashiarakawa, Olia, Taniguchi, Win and Ohashi2017), that S. stercoralis originally parasitized canids and subsequently adapted to humans and other hosts, likely following the domestication of dogs. Although the human- and dog-infecting S. stercoralis have diverged genetically, they have not undergone complete speciation.

In a separate study investigating the clinical relevance of S. stercoralis genotypes, Repetto et al. (Reference Repetto, Braghini, Risso, Argüello, Batalla, Stecher, Sierra, Burgos, Radisic and Cappa2022) analysed cox1 haplotypes in patients from South America and the Caribbean. No significant differences in clinical presentation were observed between haplotypes; however, reactivation of strongyloidiasis following ivermectin treatment was significantly less frequent in infections with cox1 haplotypes carrying the I152V mutation. These findings require validation in larger, geographically diverse cohorts.

Strongyloides fuelleborni fuelleborni

Genetic characterization of S. f. fuelleborni has been conducted on isolates from humans and NHPs across multiple African and Asian countries, as well as from imported African NHPs in St. Kitts and a UK safari park (Table 3). Most studies targeted partial cox1 and 18S rRNA, while 3 examined the complete mitochondrial genome (Ko et al., Reference Ko, Haraguchi, Hara, Hieu, Ito, Tanaka, Tanaka, Suzumura, Ueda and Yoshida2023; de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024; Richins et al., Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025). Across studies, S. f. fuelleborni isolates consistently showed allopatric clustering. At the cox1 locus, sequences grouped broadly into African and Asian clades, with each geographic cluster (A–I) containing isolates from sympatric human and NHP populations (Hasegawa et al., Reference Hasegawa, Kalousova, McLennan, Modry, Profousova-Psenkova, Shutt-Phillips, Todd, Huffman and Petrzelkova2016; Thanchomnang et al., Reference Thanchomnang, Intapan, Sanpool, Rodpai, Tourtip, Yahom, Kullawat, Radomyos, Thammasiri and Maleewong2017, Reference Thanchomnang, Intapan, Sanpool, Rodpai, Sadaow, Phosuk, Somboonpatarakun, Laymanivong, Tourtip and Maleewong2019; Frias et al., Reference Frias, Stark, Lynn, Nathan, Goossens, Okamoto and MacIntosh2018; Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Janwan et al., Reference Janwan, Rodpai, Intapan, Sanpool, Tourtip, Maleewong and Thanchomnang2020; Ko et al., Reference Ko, Haraguchi, Hara, Hieu, Ito, Tanaka, Tanaka, Suzumura, Ueda and Yoshida2023; de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024; Richins et al., Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025). Human isolates from PNG grouped within the Asian clade, suggesting possible historical introduction via human migration (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). At the 18S rRNA locus, 5 HVR-I haplotypes (XII–XVII) and 11 HVR-IV haplotypes (K–U) have been identified (Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Richins et al., Reference Richins, Sapp, Ketzis, Willingham, Mukaratirwa, Qvarnstrom and Barratt2023, Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025). Asian-Pacific strains, regardless of host species, consistently belonged to HVR-IV haplotype S and HVR-I haplotype XIV (Sato et al., Reference Sato, Torii, Une and Ooi2007; Hasegawa et al., Reference Hasegawa, Hayashida, Ikeda and Sato2009; Thanchomnang et al., Reference Thanchomnang, Intapan, Sanpool, Rodpai, Tourtip, Yahom, Kullawat, Radomyos, Thammasiri and Maleewong2017; Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Janwan et al., Reference Janwan, Rodpai, Intapan, Sanpool, Tourtip, Maleewong and Thanchomnang2020; de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024; Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c). In contrast, African strains showed greater haplotypic diversity at both loci (Hasegawa et al., Reference Hasegawa, Hayashida, Ikeda and Sato2009, Reference Hasegawa, Sato, Fujita, Nguema, Nobusue, Miyagi, Kooriyama, Takenoshita, Noda and Sato2010, Reference Hasegawa, Kalousova, McLennan, Modry, Profousova-Psenkova, Shutt-Phillips, Todd, Huffman and Petrzelkova2016; Barratt et al., Reference Barratt, Lane, Talundzic, Richins, Robertson, Formenti, Pritt, Verocai, Nascimento de Souza and Mato Soares2019; Richins et al., Reference Richins, Sapp, Ketzis, Willingham, Mukaratirwa, Qvarnstrom and Barratt2023, Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025).

Hasegawa et al. (Reference Hasegawa, Kalousova, McLennan, Modry, Profousova-Psenkova, Shutt-Phillips, Todd, Huffman and Petrzelkova2016) hypothesized that S. f. fuelleborni diversified genetically through geographic dispersal and isolation associated with the migration of Old World primates from Africa to Asia by the end of the Miocene. If this hypothesis is correct, it becomes important to assess whether African and Asian S. f. fuelleborni differ sufficiently, both genomically and morphologically, to warrant recognition as distinct taxa. Viney et al. (Reference Viney, Ashford and Barnish1991) described subtle morphological differences between S. f. kellyi from PNG and S. f. fuelleborni from Africa. No morphological studies of S. f. fuelleborni from Asia have been reported. If S. f. kellyi is truly synonymous with the Asian clade of S. f. fuelleborni, these morphological distinctions may justify subspecific differentiation between the African and Asian lineages (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c).

To investigate the evolutionary history of S. f. fuelleborni, Ko et al. (Reference Ko, Haraguchi, Hara, Hieu, Ito, Tanaka, Tanaka, Suzumura, Ueda and Yoshida2023) examined mitochondrial gene arrangement patterns in Asian isolates and identified 2 distinct types. Type A, observed in wild isolates from Myanmar and Japan, contained a single tRNA-Met gene. Type B, found in captive NHPs from Japanese zoos, included 2 copies of this gene. Ko et al. (Reference Ko, Haraguchi, Hara, Hieu, Ito, Tanaka, Tanaka, Suzumura, Ueda and Yoshida2023) suggested that Type A may represent the ancestral state of the S. f. fuelleborni mitochondrial genome. Subsequently, Type A was also identified in human isolates from Bangladesh (de Ree et al., Reference de Ree, Nath, Barua, Harbecke, Lee, Rödelsperger and Streit2024). Richins et al. (Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025) analysed mitochondrial genomes from captive NHPs of putative African origin housed in a UK safari park and identified a novel arrangement, designated Type C. This arrangement also included 2 copies of the tRNA-Met gene but differed from Types A and B in overall gene order. Additionally, the mitochondrial genomes of these UK isolates were substantially larger (∼24 kilobases) than those of Asian S. fuelleborni (∼16 kilobases), due to expanded intergenic regions of unknown function. While the findings of Richins et al. (Reference Richins, Sapp, Juhasz, Cunningham, LaCourse, Stothard and Barratt2025) support a distinction between Asian and African clades of S. f. fuelleborni, they should be interpreted with caution, as the worms were derived from zoo-kept African NHPs of unknown duration in captivity. Further investigation using wild-caught African isolates is necessary to determine whether the mitochondrial arrangement observed in the UK samples reflects natural variation or adaptations acquired during captivity.

Strongyloides fuelleborni kellyi

Prior to the molecular identification of S. f. fuelleborni in New Guinea (Zhao et al., Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c), S. f. kellyi was considered the only non-S. stercoralis Strongyloides nematode infecting humans in the region (Zhao et al., Reference Zhao, Constantinoiu and Bradbury2025a). The only sequence data attributed to S. f. kellyi at that time was a 330 bp fragment of 18S rRNA (Dorris et al., Reference Dorris, Viney and Blaxter2002), which includes the HVR-I (Hasegawa et al., Reference Hasegawa, Hayashida, Ikeda and Sato2009). This sequence (AJ417029) was found to be identical to those of S. cebus (AJ417025) and S. papillosus (AJ417027) and clustered separately from S. f. fuelleborni and S. stercoralis in the same analysis (Dorris et al., Reference Dorris, Viney and Blaxter2002). Zhao et al., (Reference Zhao, Haidamak, Noskova, Ilik, Pafčo, Ford, Masiria, Maure, Kotale, Pomat, Gordon, Navarro, Horwood, Constantinoiu, Greenhill and Bradbury2025c) later demonstrated that the genospecies identified by Dorris et al. (Reference Dorris, Viney and Blaxter2002) was inconsistent with the formal designation of S. f. kellyi proposed by Viney et al. (Reference Viney, Ashford and Barnish1991) and likely represents an undescribed Strongyloides sp. infecting humans in PNG.

Strongyloides planiceps

Fourteen sequences of S. planiceps are available from three studies (Hasegawa et al., Reference Hasegawa, Hayashida, Ikeda and Sato2009, Reference Hasegawa, Sato, Fujita, Nguema, Nobusue, Miyagi, Kooriyama, Takenoshita, Noda and Sato2010; Ko et al., Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020), targeting cox1 (n = 9), 18S rRNA (n = 4) or 28S rRNA (n = 1) loci. All sequenced isolates were obtained from raccoon dogs (Nyctereutes procyonoides) in Japan. Phylogenetic analyses of cox1 sequences (Hasegawa et al., Reference Hasegawa, Sato, Fujita, Nguema, Nobusue, Miyagi, Kooriyama, Takenoshita, Noda and Sato2010; Ko et al., Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020) and concatenated 18SrRNA and 28S rRNA sequences (Ko et al., Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020) confirmed that S. planiceps is genetically distinct from S. stercoralis and S. f. fuelleborni. Intraspecific genetic diversity within S. planiceps has not yet been studied.

Strongyloides species of unknown identity

Published sequences of unidentified Strongyloides spp. from dogs and cats include 5 cox1 sequences and 2 18S rRNA sequences from Australian dogs (Beknazarova et al., Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019) and 32 sequences of cox1 (n = 28), 18S rRNA (n = 2), 28S rRNA (n = 1) and mitochondrial genome (n = 1) from cats in Myanmar (Ko et al., Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020).

Beknazarova et al. (Reference Beknazarova, Barratt, Bradbury, Lane, Whiley and Ross2019) conducted metabarcoding of faecal DNA from 20 dogs and 4 humans in remote northern Australia. In 2 dogs, they identified a Strongyloides sp. that was phylogenetically basal to all known S. stercoralis isolates at the cox1 locus. One of these dogs also harboured unique 18S rRNA HVR-I and HVR-IV haplotypes (genotype VIII/F). These findings should be interpreted with caution, as they may represent transient passage of Strongyloides DNA from other hosts consumed in the dog’s diet or from environmental sources rather than true infections. Nonetheless, the possibility of a novel, undescribed species, or a genetically distinct S. stercoralis lineage endemic to Australian dogs, cannot be ruled out.

Ko et al. (Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020) sequenced partial cox1, 18S rRNA, 28S rRNA and the mitochondrial genome from 70 Strongyloides isolates obtained from 19 cats in Myanmar. All 18S rRNA sequences were identical. Phylogenetic analysis of mitochondrial protein-coding genes and cox1 placed these isolates in a sister clade to S. stercoralis. Ko et al. (Reference Ko, Suzuki, Canales-Ramos, MPPTH, Htike, Yoshida, Montes, Morishita, Gotuzzo and Maruyama2020) suggested that this cat-derived Strongyloides sp. may represent S. felis, but the absence of morphological characterization makes this conclusion speculative. Similarly, Jitsamai (Reference Jitsamai2019) analysed a 708 bp fragment of 18S rRNA from Strongyloides isolates obtained from cats in Thailand and found that they formed a sister group to S. stercoralis and S. procyonis. Although morphological examination of the free-living stages supported identification as S. felis, this diagnosis is dubious due to deviations from established morphological criteria. Moreover, the 18S rRNA sequences from this study are not publicly available in GenBank, which precludes further verification.