The parasite faunas of wood mice (Apodemus sylvaticus) and bank voles (Myodes glareolus) have often been studied by infectious disease ecologists and parasitologists. This has provided important fundamental information on how communities of parasites and other symbionts are structured and on how transmission is determined. Having studied bank voles and wood mice over many years ourselves, we have frequently encountered capillariid nematodes at gastrointestinal sites. Nematodes belonging to this family form a large group (several hundred species) infecting vertebrates and include multiple species with transboundary and disease-causing potential.

We noticed that two species seemed to occur in wood mice and bank voles – both in our own surveys and in the literature – but it had puzzled us for some time that there were significant inconsistencies between authors in respect of which species was reported at which gastrointestinal locality.  Placing these species either in the genus Capillaria or Aonchotheca, some authors have reported recovering them from the small intestine, others from the stomach. Although these sections of the gut are adjoining, physiologically they are quite different environments. To survive in their hosts, parasites are adapted to cope with the hazards likely to be encountered in their preferred sites. Parasitologists surveying the parasites of wood mice and bank voles from locations in the British Isles have largely reported just one species, i.e. Capillaria muris-sylvatici – either in the stomach or small intestine. This contrasts with workers from the European mainland who have reported two different but closely related species. For one of these, the location has invariably been the small intestine (Aonchotheca annulosa). For the other, reports have varied, as in the British literature, either stomach or small intestine.

We decided to reexamine the wider literature on these worms, including the original descriptions from the 1800s. Much of the original descriptive literature on these parasites has been published in what might be regarded as non-mainstream journals, such as proceedings of scientific societies, museums, and universities, and in languages other than English. This has made access to the original reports frustrating and in some cases almost impossible, as indeed we also experienced in gathering the relevant documents.

In our paper, we confirm that A. annulosa is a valid species parasitizing the small intestine of both bank voles and wood mice, but not the stomach. In contrast, the other species, which has been referred to variously as Capillaria murissylvatici, C. muris-sylvatici, C. muris sylvatici and Aonchotheca murissylvatici was originally poorly described and inappropriately named. We conclude that the first satisfactory description was provided by Kalantarian in 1924 who named the parasite Capillaria halli. Since the reorganization of the genus Capillaria, this species is now best referred to as Aonchotheca halli. Moreover, we conclude that this species is a gastric specialist and does not normally parasitize the small intestine of its host. It predominantly infects bank voles, although it can also infect other cricetids (the mammalian family containing voles and hamsters) and sometimes murids (the mammalian family containing rats and mice). All of this inevitably leads to the conclusion that reports of A. halli (under its various previous names) from the small intestines of wood mice and other rodents were misidentifications.

To summarise, in our paper we conclude that A. annulosa is a valid species that is typified by male worms possessing a long spicule (about 1mm) and lives in the small intestine of its hosts. It has a wide host range amongst mammals, centred in murids, but which includes cricetids and even encompasses primates. In contrast, A. halli is a gastric specialist, parasitizing predominantly bank voles and other cricetids and is typified by male worms with a much shorter spicule (less than 0.35 mm).

Finally, our genetic analyses (in preparation) already confirm that worms recovered from stomachs form one genetic clade whereas those from the small intestine form another, confirming that they are two distinct species. Importantly, in harmonising the nomenclature for these species, we hope that we have firmly linked the biological information from more than a century of past studies to corresponding biological species that can be recognised by molecular methods. This is especially important as these species are increasingly likely to be identified by molecular methods in the future.

The latest paper of the month; ‘Aonchotheca annulosa and Aonchotheca murissylvatici, which is which? A reappraisal of the gastrointestinal Aonchotheca (Nematoda: Capillariidae) species common in wood mice and bank voles‘ by, Jerzy M. Behnke and Joseph A. Jackson is available open access.