INTRODUCTION
Toxoplasma gondii, an opportunistic protozoan parasite, can infect nearly all warm-blooded animals worldwide, including humans [Reference Dubey1–Reference Montoya and Liesenfeld5]. It is estimated that nearly a third of the human population in the world has been infected by T. gondii [Reference Dubey1]. The seroprevalence of T. gondii is evolving worldwide, subject to complex environmental, socioeconomic and health-related practices [Reference Pappas6]. Humans acquire T. gondii infection primarily through the consumption of undercooked or raw meat containing tissue cysts from an infected intermediate host, unwashed vegetables and fruits, or drinking water contaminated by oocysts from the excrement of infected cats [Reference Flegr4–Reference Torgerson7].
China is a multi-ethnic country with 56 ethnic groups. The Han nationality represents the majority of the Chinese nation, encompassing 92% of the national population; the remaining 8% are ethnic minorities. In the 55 ethnic minorities, Korean, Manchu, and Mongolian represent the largest populations. Each ethnic minority has its own lifestyle and eating habits. Human T. gondii seroprevalences have been reported in many regions and different research groups, including pregnant women [Reference Cong8–Reference Liu10], female sterilized patients [Reference Li11], psychiatric patients [Reference Cong12, Reference Zhu13], children [Reference Meng14, Reference Xin15], and cancer patients [Reference Wang16, Reference Cong17]. Moreover, a previous study reported T. gondii infection in the Bai and Han ethnic groups in southwestern China [Reference Li18]. However, limited information on T. gondii infection in the Korean, Manchu, and Mongol ethnic groups is available in China. Thus, the present study was conducted to determine the seroprevalence and possible risk factors for human T. gondii infection in the Korean, Manchu, Mongol and Han ethnic groups in eastern and northeastern China.
MATERIALS AND METHODS
Serum samples
This study was approved by the Ethics Committee of Jilin Agricultural University. A cross-sectional study was conducted and a total of 1842 blood samples were randomly collected from the Han (n = 802), Korean (n = 520), Manchu (n = 303) and Mongol (n = 217) groups in eastern and northeastern China from June 2013 to August 2015 (Fig. 1). The individuals' occupations and names were not recorded to ensure confidentiality. The purpose and procedures of the study were explained to all participants, and written informed consent was obtained from them all. Volunteers/guardians provided informed consent on behalf of all child participants. The sera were collected with agreement from the volunteers. Approximately 5 ml of venous blood samples were drawn from participants. Blood samples were left overnight at room temperature to allow for clotting and centrifuged at 3000 rpm for 10 min. The sera were collected in Eppendorf tubes and stored at 4 °C for 24–72 h until being transported in an ice box to the College of Animal Science and Technology, Jilin Agricultural University, Changchun, Jilin Province, People's Republic of China, where they were kept at −20 °C until tested.
Fig. 1. Geographical distribution of the sampled regions in eastern and northeastern China.
Socio-demographic and behavioural data collection
A questionnaire was used to collect data from study participants during blood sample collection which assessed socio-demographic and behavioural data. Socio-demographic data, including age, gender, birthplace and residence, were obtained from all participants, and behavioural data, including the presence of cats and dogs at home, the consumption of raw/undercooked meat, the consumption of unwashed vegetables and fruits, the source of drinking water, and exposure to soil were also obtained. These variables were selected based on the literature.
Serological tests
Sera were analysed for the presence of IgG and IgM antibodies against T. gondii using commercially available enzyme immunoassay kits (Demeditec Diagnostics GmbH, Germany) according to the manufacturer's instructions. Positive and negative serum controls were included in every plate. To avoid bias, the serology test was performed in a double-blind manner. Samples from different ethnic groups were randomly mixed, and the person performing the test was unaware of the source of the samples. Optical densities were measured by a photometer at a wavelength of 450 nm. Values higher than the cut-off (10 IU/ml) were considered positive. Values ±20% of the cut-off were considered to be equivocal and were re-tested.
Statistical analysis
Results were analysed with the SPSS v. 19.0 software package (SPSS Inc., USA). To compare the frequencies among groups, the Mantel–Haenszel test and, when indicated, Fisher's exact test were used. Bivariate and multiple analyses were used to assess the association between the characteristics of the subjects and T. gondii infection. Variables were included in the multiple analysis with P⩽0·25 in the bivariate analysis [Reference Pappas6, Reference Cong8]. The adjusted odds ratios (aORs) and 95% confidence interval (CIs) were calculated by multiple analysis using multiple unconditional logistic regression. P < 0·05 was considered statistically significant.
RESULTS
The overall prevalence of anti-T. gondii IgG antibodies in the examined participants was 13·79% (254/1842, 95% CI 12·22–15·36). Of these, Mongol ethnicity had the highest T. gondii seroprevalence (19·36%, 42/217, 95% CI 14·10–24·61), followed by Korean ethnicity (15·77%, 82/520, 95% CI: 12·64–18·90), Manchu ethnicity (12·87%, 39/303, 95% CI 9·10–16·42) and Han ethnicity (11·35%, 91/802, 95% CI 9·15–13·54). Han ethnicity had a significantly lower seroprevalence than other ethnicities (P < 0·001). Moreover, anti-T. gondii IgM antibodies were found in 23 (1·25%) participants, and six participants were positive for both IgG and IgM antibodies. Detailed information is summarized in Table 1.
Table 1. IgG and IgM anti-Toxoplasma gondii antibodies in different ethnic groups in eastern and northeastern China
Pos., Positive.
Risk factor analysis
For the Han group, bivariate analysis showed a number of socio-demographic characteristics and behavioural characteristics with a P value ⩽0·25, including area of residence, the consumption of raw vegetables and fruits, the consumption of raw/undercooked meat, exposure to soil, and source of drinking water (Table 2). Accordingly, multiple analysis of these socio-demographic and behavioural characteristics showed that the consumption of raw vegetables and fruits (aOR 1·654, 95% CI 1·083–2·528), the consumption of raw/undercooked meat (aOR 2·092, 95% CI 1·364–3·210) and the source of drinking water (aOR 1·607, 95% CI 1·045–2·472) were significantly associated with T. gondii infection in the Han group (Table 3). Likewise, having a cat at home was identified to be associated with T. gondii infection in the Korean (aOR 2·913, 95% CI 1·786–4·752), Manchu (aOR 2·400, 95% CI 1·171–4·918) and Mongol (aOR 2·188, 95% CI 1·055–4·535) groups (Table 3). Moreover, the consumption of raw/undercooked meat was identified as another predictor of T. gondii seropositivity in the Mongol group (aOR 2·490, 95% CI 1·275–4·861) (Table 3).
Table 2. Socio-demographic factors associated with Toxoplasma gondii seropositivity in different ethnic groups by univariate analysis
Pos., Positive.
Table 3. Multiple analysis of selected characteristics of the participants and their association with Toxoplasma gondii infection
aOR, Adjusted odds ratio; CI, confidence interval; Ref., reference.
a The included variables were those with a P⩽0·25 obtained in the bivariate analysis.
b Adjusted by region and the rest of the characteristics included in this table.
DISCUSSION
Our cross-sectional study estimated the seroprevalence of T. gondii infection in the Korean, Manchu, Mongol and Han ethnic groups in eastern and northeastern China. To our knowledge, this is the first report of T. gondii infection in the Korean and Manchu ethnic groups in eastern and northeastern China. T. gondii seroprevalence in the Mongol, Korean, and Manchu groups was significantly higher than that in the Han group. Apart from the impact of the limited number of samples in this study, lifestyle, eating habits and living environment may have contributed to high T. gondii seroprevalence in the Mongol, Korean and Manchu groups. However, the T. gondii seroprevalence in the present study was much higher (7·9%) than the national average estimated for the general population [Reference Zhou19], but lower than that in the Bai ethnic group, which presents a 32·3% seroprevalence [Reference Li18]. These differences may have been caused by several factors, including geographical conditions, the type and size of the population evaluated, lifestyle, the number of cats, and the specificity and sensitivity of the detection methods used.
T. gondii seropositivity has been reported to be related to age [Reference Li18, Reference Markovich20, Reference Nowakowska21]. It is well known that T. gondii is an opportunistic pathogen; thus, the acquisition of T. gondii infection is a result of immunosuppression due to a higher probability of contact with the infective forms of the parasite throughout the years. However, for subjects in several age groups, the present data were inconclusive because there was a limited number of samples in this study. Thus, further studies with more samples under controlled conditions are necessary to further define the potential morbidity associated with T. gondii infection.
Cats, definitive hosts for T. gondii, play a crucial role in transmitting T. gondii because they have the ability to discharge oocysts in their faeces [Reference Elmore22, Reference Du23]. In China, cats' excretion of T. gondii oocysts into the environment has been considered to increase the possibility of human infection [Reference Du23]. Casual contact with cats may not necessarily be a risk factor, but continuous exposure to feline faeces or neglect of preventive measures (i.e. not washing hands or wearing gloves) may increase the risk of infection to an appreciable level. In China, with the continuous development of society and the improvement of human wellbeing, more and more people are starting to keep pets, including cats and dogs. This, together with inadequate inspection and quarantine measures, could enhance the potential risk to pet owners of zoonotic hazards, such as Toxoplasma [Reference Wu24].
Several previous studies have demonstrated that humans can acquire T. gondii infection via ingesting tissue cysts in undercooked or raw meat from an infected intermediate host, or by ingesting oocytes via unwashed vegetables and fruit or drinking water polluted by the excrement of infected cats [Reference Jones25–Reference Daryani28]. Similarly, our study found that the consumption of raw/undercooked meat was highly associated with T. gondii seropositivity in the Han and Mongol groups. Meanwhile, the consumption of unwashed vegetables and fruits and the source of drinking water were also highly associated with T. gondii seropositivity in the Han group. Therefore, it is very important to publicize the knowledge of disease prevention to the public, with particular emphasis on the important role cats play in the transmission of T. gondii and the association between T. gondii infection and behavioural characteristics.
In contrast to other regions of the world, where the prevalence of T. gondii infection has been declining over the past several decades, in eastern and northeastern China the prevalence seems to be increasing [Reference Cong8, Reference Cong12–Reference Li18]. The increasing number of pets in recent decades may have contributed to an increased T. gondii prevalence. Thus, further studies should be conducted to investigate the possible sources of infection and the health burden that toxoplasmosis imposes on the population of China.
CONCLUSION
Using a cross-sectional design, the present study revealed that infection with T. gondii in Korean, Manchu, Mongol and Han ethnic groups is common in eastern and northeastern China. T. gondii seroprevalence in the Mongol, Korean, Manchu groups was higher than that in the Han group. Several behavioural characteristics were important risk factors for the acquisition of T. gondii infection, including having cats at home, the consumption of raw vegetables and fruits, the consumption of raw/undercooked meat and the source of drinking water. Considering the increasing number of pets over the past several decades, it is essential to implement integrated strategies with efficient management measures to prevent and control T. gondii infection in this region and elsewhere in China.
ACKNOWLEDGEMENTS
This work was supported by the national high-tech R&D programme of China (863 programme) (2013AA102806, 2011AA10A215), the National Natural Science Foundation of China (31272552, 31272541), Jilin Province Science and Technology Development Programme of China (20 111 816), Jilin Province Quality and Safety of Agricultural Products Programme by the World Bank (2011-Y07) and the Key Scientific and Technological Project of Jilin Province (20 140 204 068NY).
DECLARATION OF INTEREST
None.
