Toxoplasma gondii can infect a wide range of hosts including wild mammals, birds, livestock, poultry and human beings, leading to zoonotic toxoplasmosis. Most T. gondii infections in immunocompetent individuals are asymptomatic; however, blindness and mental retardation can occur in congenitally infected children [Reference McLeod1–Reference Phan3], and the occurrence of toxoplasmosis in immunocompromised hosts may cause severe diseases or even death.
The prevalence of human toxoplasmosis ranges from 4% in Korea to 92% in Brazil [Reference Dubey4], and a mean prevalence of 7·9% has been estimated in China [Reference Xu5]. The different T. gondii prevalences may be due to different factors including sample size, age, occupation, serological techniques, and living customs and habits. Earlier studies indicated that the seroprevalence of T. gondii in slaughter pigs in Yunnan Province in China was 22·28% [Reference Zou6], and the genotype was Toxo DB#9 [Reference Zhou7], representing a widespread lineage from pigs, cats, and human patients in China. Dali Bai Autonomous Prefecture is located in the west of Yunnan Province, where 26 ethnic groups live, with the main ethnic groups being of Bai and Han ethnicity. The Bai ethnic group has the habit of consuming raw or undercooked animal meat and/or organs, which may represent a route of T. gondii transmission to humans, and thus stimulated the aims of the present study, which were to investigate T. gondii seroepidemiology and analyse the epidemiological correlation between T. gondii infection and risk factors in Bai and Han populations.
MATERIALS AND METHODS
A total of 555 and 595 blood samples were collected from Bai and Han groups in Dali urban and rural areas, respectively, by medically trained staff. The individuals' occupation and name were not available because of confidentiality. The information including age, sex, ethnicity, education, consumption of raw pork and/or liver and cat feeding information were obtained through questionnaires and recorded during the sample collection (March–December 2013). Blood was collected by venepuncture in plain tubes and centrifuged to remove blood cells. All sera were stored at −20°C after deactivation of complement at 56°C for 30 min before use. This study was approved by the Ethics Committee of Dali University.
All the serum samples were tested for T. gondii IgG antibodies using a commercially available enzyme-linked immunosorbent assay (ELISA) kit (Haitai Co. Ltd, China) according to the manufacturer's instructions. Positive and negative serum controls were included in every plate. The absorbance was measured at 450 nm using an ELISA reader (Bio-Tek ELISA × 808, USA). All samples were run in triplicate.
Between the two ethnic groups, the χ 2 test was used to analyse T. gondii prevalence relating to cat feeding (or not), level of education and various degrees of consumption of raw pork and/or liver by the subjects using SPSS software (release 16.0 standard version, SPSS Inc., USA). For all data, the variation in T. gondii seroprevalence (y) of humans of different ethnic groups (x1), sex (x2), cat-feeding behaviour (x3), education (x4), raw pork and/or liver consumption (x5) and age group (x6) was analysed by χ 2 test. Using multivariable regression analysis, each of these variables was included in the Binary Logit Model as an independent variable. The differences were considered as statistically significant between or among levels within factors and interactions when the probability (P) value was <0·05. Odds ratios (OR) with 95% confidence intervals (CI) were calculated by multivariate analysis using logistic regression.
The age and sex distribution of T. gondii seropositivity in Bai and Han ethnic groups are summarized in Table 1. In total, 248 (21·6%) of the 1150 serum samples were anti-Toxoplasma IgG positive. The overall seroprevalence of T. gondii was higher in the Bai group (32·3%, 179/555) than in the Han group (11·6%, 69/595) (P < 0·01). Within the Bai group, seroprevalence was significantly higher in males (37·8%) than in females (27·4%), but this difference was not significant between males (10·2%) and females (12·4%) within the Han group (P = 0·414). The infection risk was higher in males than in females in the Bai group (P = 0·008, OR 5·346, 95% CI 3·25–8·8). These results confirmed the highly significant difference in seroprevalence of both males and females between Bai and Han ethnic groups (P < 0·01).
OR, Odds ratio; CI, confidence interval.
P was estimated using the χ 2 test.
* P < 0·001, OR 5·346, 95% CI 3·25–8·8.
† P < 0·001, OR 2·654, 95% CI 1·777–3·962.
‡ P < 0·001, OR 3·629, 95% CI 2·668–4·936.
As shown in Table 1, the T. gondii seroprevalence in the <18 years age group was lower than other age groups in both Bai and Han populations. In the Han group, the seropositivity of T. gondii was 0% in the 1–18 years age group, 6·8% in the 18–40 years age group, 16·8% in the 41–65 years age group, and 15·9% in the >65 years age group. For the Bai group, the seroprevalence was also 0% in <18 years age group, but seroprevalence was significantly higher in the 18–40 years age group (34·1%, P < 0·001), 41–65 years group (33·1%, P < 0·001), and >65 years group (29·8%, P = 0·013) in the Bai group than in the Han group.
The results of characteristics of study subjects and risk factors associated with the seropositivity of T. gondii in Bai and Han groups are detailed in Table 2. The subjects were categorized into four education groups: primary school, junior high school, senior middle school and college. There was no significant difference in T. gondii seroprevalence within the Bai group of different education levels (P = 0·144), but the infection risk decreased with increasing level of education in the Han group, and the difference was significant (P = 0·01).
OR, Odds ratio; CI, confidence interval; Ref., used as the reference group.
* Estimated using the χ 2 test.
Consumption of raw pork and/or liver was divided into the following categories: never, occasionally (<6 times a year), not often (>6 to <12 times a year), often (⩾12 times a year). In contrast to the ‘never’ group, our statistical analysis showed that the increasing frequency of consumption of raw pork and/or liver is related to high T. gondii seroprevalence in both Bai and Han ethnic groups (P < 0·001). The ratio of people who ate raw pork and/or liver in the Bai group was far greater than in the Han group, but there was no significant difference in the ‘often’ group between the two ethnic populations (P = 0·257), and the seropositivity was 59% and 50% in Bai and Han ethnic groups, respectively.
There was no significant difference in T. gondii seroprevalence between cat feeding (33·3%) or non-cat feeding (32·1%) groups in the Bai groups. By contrast, cat feeding (or not) was significantly associated with T. gondii infection in the Han group (P < 0·001), and the seroprevalence was significantly higher in the cat-feeding group (34·3%, 35/102) than in the non-cat-feeding group (6·9%, 34/493).
The effects of ethnicity, sex, cat feeding, education level, consumption of raw pork and age group associated with T. gondii seropositivity in the present study were analysed using univariate analyses. Optimized by Fisher's scoring technique, forward stepwise logistic regression analysis was conducted to evaluate the risks of multiple variables relating to T. gondii infection. In the final model, two variables had effects on the infectious disease, described by the equation y = 4·12−1·52x3−x5. Not feeding a cat and consuming less raw meat had negative effects on the risk of T. gondii infection, for which the ORs were 0·22 (95% CI 0·134–0·355) and 0·37 (95% CI 0·315–0·428), respectively.
In China, T. gondii infection has not been given high priority owing to the limited knowledge of the route of transmission to humans, e.g. the infection risk of the well known food named ‘Heige’ (fresh pork or liver) in many restaurants in Yunnan Province. Therefore, the local cuisine including ‘Heige’ in Dali Bai Autonomous Prefecture, Yunnan Province may represent a potential risk of T. gondii infection, especially for the Bai group which has the habit of eating raw pork and/or liver. Significantly, this potential risk has led to 21·6% anti-T. gondii IgG positivity in Dali Bai Autonomous Prefecture, which is higher than that in northeast and the south of China (12·3%) [Reference Xiao8], and it is also much higher than the national average (7·9%) [Reference Xu5]. Although a previous study reported that the T. gondii seroprevalence was 25·4%, 25·3%, 17·1% and 16·7% in Miao, Buyi, Mongol and Zhuang ethnic groups, respectively [Reference Xu5], our study, focusing on the Bai ethnic group, showed the highest seroprevalence of 32·3%.
Some previous reports indicated that the seropositivity of T. gondii was related to age [Reference Markovich9, Reference Nowakowska10]. Consistent with these studies, the present study showed highly significant differences in T. gondii seropositivity in the different age groups (18–45, 45–65, >65 years) in the two ethnic populations. For subjects aged <18 years, there were a limited number of samples in this study, thus the data were inconclusive.
As the definitive hosts for T. gondii, cats play a key role in transmitting T. gondii due to their ability to pass oocysts in their faeces [Reference Elmore11, Reference Du12]. In China, it has been shown that excretion of T. gondii oocysts in the environment by cats leads to the increased possibility of human infection [Reference Du12]. The present study showed 33·3% (22/66) and 34·3% (35/102) seropositivity of T. gondii in Bai and Han ethnic groups, respectively, who feed cats. Although there was no significant difference in T. gondii seropositivity between the Bai groups that feed/do not feed a cat, a significant difference was found in T. gondii seropositivity between Han groups that feed cats and those that do not.
Previous studies have indicated that level of education was associated with T. gondii infection [Reference Jones13–Reference Babaie15]. Similarly, our study showed that T. gondii seroprevalence was significantly different in Han ethnic populations of different educational levels, showing that the proportion of subjects who have college education had lower T. gondii prevalence than others, although there was no significant difference in the Bai ethnic group for different education levels.
The present study revealed for the first time that human infection with T. gondii is common in Bai and Han ethnic groups in Dali Bai Autonomous Prefecture, Yunnan Province, southwestern China. The overall T. gondii seroprevalence in the Bai ethnic group was 32·3%, Eating raw pork and/or liver was the most important risk related to T. gondii infection in the Bai ethnic group. Cat feeding and raw meat consumption had positive effects on the risk of T. gondii in both Bai and Han ethnic groups in this area. Improved food habits and hygiene practices should be promoted to reduce the prevalence of T. gondii infection in humans in this unique region of the world.
Project support was provided, in part, by the State Key Laboratory of Veterinary Etiological Biology, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences (grant no. SKLVEB2013KFKT011), the Specialized Research Fund for the Doctoral Program of Dali University (grant no. KYBS201217), Application and Development Projects of Dali University (grant no. KYYY201302) and the Science Fund for Creative Research Groups of Gansu Province (grant no. 1210RJIA006).
DECLARATION OF INTEREST