INTRODUCTION
Toxocariasis, a highly underrated and neglected disease, is caused by Toxocara canis and Toxocara cati, which are important intestinal nematodes of dogs and cats, respectively [Reference Chen1–Reference Nijsse4]. Of these, T. canis is the most common causative agent of human toxocariasis [Reference Manini5–Reference Martínez8]. Humans acquire toxocariasis by accidental ingestion of embryonated T. canis eggs (from water or soil contaminated by infected cat/dog faeces) or encapsulated larvae (from raw/undercooked meat); toxocariasis usually does not show any symptoms [Reference Cong3, Reference Campos-da-Silva9, Reference Santos10]. However, there are four general manifestations of toxocariasis: covert, visceral larva migrans, ocular larva migrans and neurotoxocariosis. Once the infective T. canis larvae are ingested, the helminths migrate into internal organs, which may lead to severe illnesses including neurotoxocarosis, visceral larva migrans, and eosinophilic meningoencephalitis [Reference Chen1, Reference Chen2, Reference Manini5, Reference Cong6, Reference Helsen, Vandecasteele and Vanopdenbosch11–Reference Fan, Liao and Cheng14].
Toxocariasis is of increasing concern because of the increasing numbers of pet and stray dogs. Indeed, with the improvement in the standard of living, increasing numbers of people are acquiring pet dogs, which are potentially important sources of T. canis infections [Reference Santos10, Reference Khademvatan15, Reference Kroten16]. In addition to the high resistance of T. canis eggs to harsh environmental conditions, pet dogs have increased the risk of this infection in humans. Consequently, investigators have focused more on the global seroprevalence of Toxocara spp. in humans [Reference Santos10, Reference Alvarado-Esquivel13, Reference Fan17–Reference Antolová22]. There are 56 ethnicities in China [Reference Zhang23]. However, information concerning T. canis seroprevalence in different ethnic groups is limited. To our knowledge, Toxocara infection has been reported in several groups: children in Sichuan (11·49%) [Reference Luo24], Shandong, and Jilin provinces (19·3%) [Reference Cong3]; clinically healthy individuals, pregnant women and psychiatric patients in Shandong Province (12·25%) [Reference Cong6]; and asthma patients in Nanjing City [Reference Li25], but these studies were only related to the Han and Korean ethnic groups.
Because of the possible impact of different cultural backgrounds, customs, and habits, investigation of the risk factors of seroprevalence of T. canis infection in different ethnic groups is essential. Most Koreans in China live in Yanbian Korean Autonomous Prefecture, Jilin Province. Dog meat is a popular food for Koreans and is usually eaten either well cooked or raw. Inner Mongolia and northeast China are the most common residences of Mongols. Although Mongols have adapted to modern society, the characteristics that drive people to migrate to where there is water and grass are more or less maintained in this ethnic group. The Han ethnic group is the largest population group in China. Some Hans live together with other ethnic groups and have adopted similar living and dietary habits. For example, in rural areas of northeast China, some Hans also like to eat raw/uncooked meat and drink untreated well or river water. Manchus are now living across the country with similar diets and culture to the Hans. The present study aims to estimate the T. canis seroprevalence in four main ethnic groups (Han, Manchu, Mongol, Korean) in Jilin and Shandong provinces (northeastern and eastern China) and assess the risk factors associated with infection in these ethnic groups. This study will provide useful baseline information for planning effective prevention and control of T. canis infection in different ethnic groups in China.
MATERIALS AND METHODS
Sample collection and laboratory tests
This study was approved by the Ethics Committee of Jilin Agricultural University (Approval No. JAUAEC2013-003). A total of 1842 blood samples were collected from Han (n = 802), Korean (n = 520), Manchu (n = 303), and Mongol (n = 217) subjects in Jilin and Shandong urban and rural areas by medically trained staff between June 2013 and August 2015. The purpose and procedures of the study were explained to all participants, and written informed consent was obtained from each of them. Volunteers/guardians provided informed consent on behalf of all child participants. The sera were collected with agreement from the volunteers. Blood samples (5 ml) were taken from the elbow vein of each participant using a plain tube. Serum samples were separated by centrifuging at 4000 rpm for 8 min. The sera were collected in Eppendorf tubes and stored at 4 °C for 24–72 h until transportation in an ice box to the Laboratory of Veterinary Parasitology, College of Animal Science and Technology, Jilin Agricultural University, Changchun, Jilin Province, where they were kept at −20 °C until analysed.
Serological tests
All the serum samples were tested for Toxocara IgG antibodies using a commercially available enzyme immunoassay Toxocara kit (Diagnostic Automation Inc., USA). The positive and negative controls were supplied in the kit and used in each test. When the absorbance reading was ⩾0·3 OD units, the sample was considered to be positive. Doubtful samples were retested. All the operations were performed according to the manufacturer's instructions [Reference Cong6, Reference Alvarado-Esquivel13].
Statistical analysis
The variation in seroprevalence of Toxocara-infected participants (y) of different variables including age (x1), gender (x2), residential place (x3), residential area (x4), ethnicity (x5), cat at home (x6), dog at home (x7), water sources (x8), consumption of raw/undercooked meat (x9), and exposure to soil (x10) were analysed by χ 2 test using SAS version 9.1 (SAS Institute Inc., USA). Each variable was included in a binary logic model as an independent variable by multivariate regression analysis. P < 0·05 was considered statistically significant. Odds ratios (ORs) and their 95% confidence intervals (CIs) were estimated to examine the strength of the association between Toxocara positivity and the selected condition.
RESULTS
Seroprevalence in different ethnic groups
The overall seroprevalence of Toxocara in the present study was 16·07%. The seroprevalence ranged from a minimum of 11·22% in Manchus to a maximum of 20·58% in Koreans (Table 1). In the Han ethnic group, the seroprevalences of Toxocara in Weihai, Qingdao, and Changchun cities were 14·32%, 13·81%, and 14·50%, respectively (Table 1). The highest (18·75%) seroprevalence of Toxocara infection was found in the 30–39 years age group (Table 1). Females have a higher seroprevalence (16·67%) than males (11·35%; Table 1). In the Korean ethnic group, Toxocara seroprevalences were 24·42%, 15·26%, and 22·12% in Weihai, Qingdao, and Changchun cities, respectively (Table 1). Koreans aged 50–59 years had the highest (24·05%) seroprevalence (Table 1). In the Manchu ethnic group, Toxocara seroprevalences were 16·67%, 14·61%, and 11·49% in Weihai, Qingdao, and Changchun cities, respectively (Table 1). T. canis seroprevalences in Manchus living in urban and suburban/rural areas were 8·24% and 20·30%, respectively (Table 1). The seroprevalences in different age groups varied from 5·41% to 23·81% (Table 1). In the Mongol ethnic group, Toxocara seroprevalences in Weihai, Qingdao, and Changchun were 18·06%, 20·69% and 12·93%, respectively (Table 1). Toxocara seroprevalence in different age groups varied from 7·41% to 25·00% (Table 1).
Socio-demographic and risk factors associated with Toxocara seropositivity in different ethnic groups
Risk factor analysis
For the study populations, the effects of age, gender, city, residential area, ethnicity, cat at home, dog at home, water sources, consumption of raw/undercooked meat, and exposure to soil contaminated with Toxocara eggs based on seropositivity were considered in the evaluation by the forward stepwise logistic regression analysis using Fisher's scoring technique; this was described by the equation
$$\eqalign{y & = 0{\cdot}5049 \times 4 + 0{\cdot}4729 \times 7 + 0{\cdot}3555 \times 9 \cr & \quad + 1 {\cdot} 2599 \times 10 - 2{\cdot} 2364}.$$
Four variables, including living in suburban or rural areas, dog at home, exposure to soil, and consumption of raw/undercooked meat were found to be potential risk factors for Toxocara infection, for which the ORs were 1·657 (95% CI 1·209–2·272), 1·605 (95% CI 1·192–2·161), 1·427 (95% CI 1·106–1·840), and 3·525 (95% CI 2·523–4·925), respectively.
For the Han group, logistic regression analysis showed that gender (OR 1·56, 95% CI 1·04–2·35, P = 0·0306), dog at home (OR 1·73, 95% CI 1·11–2·71, P = 0·0151), and exposure to soil (OR 2·96, 95% CI 1·92–4·58, P < 0·0001) were significantly associated with Toxocara infection (Table 2). Meanwhile, exposure to soil (OR 1·68, 95% CI 1·09–2·60, P = 0·0187) and consumption of raw/undercooked meat (OR 1·78, 95% CI 1·16–2·74, P = 0·0080) were identified as being associated with Toxocara infection in Koreans (Table 2). Furthermore, residential area (OR 2·84, 95% CI 1·42–5·57, P = 0·0023), water sources (OR 2·94, 95% CI 1·49–5·78, P = 0·0013), and exposure to soil (OR = 3·67, 95% CI 1·68–7·98, P = 0·0006) were identified as the major risk factors for Toxocara seropositivity in Manchus (Table 2). In the present study, exposure to soil (OR 2·87, 95% CI 1·27–6·49, P = 0·0090) was the only factor associated with Toxocara seropositivity in Mongols (Table 2).
Odds ratio of the risk factors associated with seropositivity to Toxocara in different ethnic groups in northern China
OR, Odds ratio; CI, confidence interval.
DISCUSSION
The overall T. canis seroprevalence in the study populations was 16·07% (95% CI 14·39–17·75). This rate is higher than the 11·49% reported in children in Chengdu, China [Reference Luo24], 12·25% in clinically healthy individuals, pregnant women, psychiatric patients in Shandong Province, Eastern China [Reference Cong6], 6·4% in pregnant women in southern Brazil [Reference Santos10], 12·03% in the Roma and non-Roma populations of Eastern Slovakia [Reference Antolová22], 15·5% in Brazilian schoolchildren [Reference Cassenote21], and 4·7% in psychiatric patients in Mexico [Reference Alvarado-Esquivel13]. However, the rate is lower than the 19·3% reported in children in Shandong and Jilin provinces [Reference Cong3], 48·4% in a large urban setting in northeast Brazil [Reference Mendonça19], 50·6% in southern Brazil [Reference Schoenardie20], 40·6% in mountain aboriginal adults in Taiwan [Reference Fan17], and 86·75% among primary schoolchildren in the capital area of the republic of the Marshall Islands [Reference Fu18]. Many factors, including diagnostic methods, geographical conditions, the timing of sample collection, sample sizes, sanitation, and life style of the evaluated population, might have contributed to the differences observed in seroprevalence rates.
In the present study, Koreans showed the highest T. canis seroprevalence of the four ethnic groups studied, but the differences in the rates were not statistically significant in different ethnic groups (P > 0·05). However, this result is in contrast with the findings of a previous study in which Koreans were reported to have lower seroprevalence than Hans [Reference Cong6]; this difference could be attributed to the different sample sizes, different sampling times, and individual ethnic differences.
It is well known that dogs are the most important definitive host of T. canis. The parasite's eggs can survive not only in the faeces and fur of domestic dogs but also in soil and water after their faeces have been discharged into these areas [Reference Cong6, Reference Cassenote21, Reference Paoletti26]. Hence, dogs at home and exposure to contaminated soil were undoubtedly risk factors for T. canis infection in humans [Reference Cong3, Reference Cong6]. The present study has shown that exposure to soil is associated with T. canis infection in all four ethnic groups, which suggests that the problem of polluted soil is widespread in these areas.
However, only in the Han ethnic group were having dogs at home (OR 1·73, 95% CI 1·11–2·71, P = 0·0151) associated with T. canis infection (Table 2). Actually, dogs at home and contact with dogs are not necessarily risk factors unless there is lack of essential measures (e.g. not washing hands) after having contact with dogs’ faeces. The high T. canis burden in the Manchu ethnic group would be partially explained by this phenomenon. Manchus passionately raise dogs, especially in suburban and rural areas, where the majority of the people use untreated water from wells or rivers for drinking. The seroprevalence of T. canis was rather high in Manchus for those raising dogs and those living in rural areas. Furthermore, Han women handle raw meat and vegetables more frequently and spend more time with their pets than men, which may account for their significantly higher T. canis seroprevalence than the rate observed in males (P = 0·0316). In China, the increasing number of pet dogs and inadequate inspection and quarantine measures enhance the potential toxocariasis risk for humans. In addition, the old Korean tradition of consuming dog meat and the habit of consuming raw/undercooked meat, which may contain encapsulated larvae, across the groups can also be associated with the Toxocara infection in humans [Reference Cong6, Reference Cassenote21, Reference Taira27–Reference Noh29]. Improved and integrated strategies and measures are required for the effective prevention and control of toxocariasis in these ethnic groups in northern China.
CONCLUSION
The present study has shown that the overall seroprevalence of Toxocara infection in the various ethnic groups examined from Jilin and Shandong provinces, northern China, was 16·07%. The lowest seropositivity (11·22%) was recorded in the Manchu group, while the highest seropositivity (20·58%) was recorded in Koreans. The study revealed the seroprevalence of Toxocara infection in Manchus and Mongols for the first time. Living in suburban or rural areas, dogs at home, exposure to soil, and consumption of raw/undercooked meat are risk factors of Toxocara infection. Moreover, more attention should be given to the Han women and Manchus who come in contact with unboiled water. Hence, establishment of good health habits including washing hands before meals and after contact with soil, regular deworming of dogs, and eating well-cooked meat should be promoted. Investigation of soil contamination between cities should be further studied.
ACKNOWLEDGEMENTS
This work was supported by the National high-tech R&D Programme of China (863 programme) (2013AA102806), National Natural Science Foundation of China (31272552, 31272541), Science and Technology Development Programme of Jilin Province (20160519011JH), Special Funds for Industrial Innovation of Jilin Province (2016C063) and the Key Scientific and Technological Project of Jilin Province (20140204068NY).
DECLARATION OF INTEREST
None.
