Seroprevalence and associated risk factors of Toxoplasma gondii infection in the Korean, Manchu, Mongol and Han ethnic groups in eastern and northeastern China

Summary

A cross-sectional study was conducted from June 2013 to August 2015 to determine the seroprevalence and possible risk factors for human Toxoplasma gondii infection in Korean, Manchu, Mongol and Han ethnic groups in eastern and northeastern China. A total of 1842 serum samples, including Han (n = 802), Korean (n = 520), Manchu (n = 303) and Mongol (n = 217) groups, were analysed using enzyme-linked immunoassays to detect IgG and IgM T. gondii antibodies. The overall T. gondii IgG and IgM seroprevalences were 13·79% and 1·25%, respectively. Of these groups, Mongol ethnicity had the highest T. gondii seroprevalence (20·74%, 45/217), followed by Korean ethnicity (16·54%, 86/520), Manchu ethnicity (13·86%, 42/303) and Han ethnicity (11·35%, 98/802). Multiple analysis showed that the consumption of raw vegetables and fruits, the consumption of raw/undercooked meat and the source of drinking water were significantly associated with T. gondii infection in the Han group. Likewise, having a cat at home was identified as being associated with T. gondii infection in the Korean, Manchu and Mongol groups. Moreover, the consumption of raw/undercooked meat was identified as another predictor of T. gondii seropositivity in the Mongol group. The results of this survey indicate that T. gondii infection is prevalent in Korean, Manchu, Mongol and Han ethnic groups in the study region. Therefore, it is essential to implement integrated strategies with efficient management measures to prevent and control T. gondii infection in this region of China. Moreover, this is the first report of T. gondii infection in Korean, Manchu, and Mongol ethnic groups in eastern and northeastern China.

INTRODUCTION

Toxoplasma gondii, an opportunistic protozoan parasite, can infect nearly all warm-blooded animals worldwide, including humans [1–5]. It is estimated that nearly a third of the human population in the world has been infected by T. gondii [1]. The seroprevalence of T. gondii is evolving worldwide, subject to complex environmental, socioeconomic and health-related practices [6]. 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 [4–7].

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 [8–10], female sterilized patients [11], psychiatric patients [12, 13], children [14, 15], and cancer patients [16, 17]. Moreover, a previous study reported T. gondii infection in the Bai and Han ethnic groups in southwestern China [18]. 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 [6, 8]. 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 [19], but lower than that in the Bai ethnic group, which presents a 32·3% seroprevalence [18]. 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 [18, 20, 21]. 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 [22, 23]. In China, cats' excretion of T. gondii oocysts into the environment has been considered to increase the possibility of human infection [23]. 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 [24].

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 [25–28]. 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 [8, 12–18]. 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.