INTRODUCTION
Toxoplasma gondii is an opportunistic protozoan parasite that can infect almost all warm-blooded animals throughout the world, including humans [Reference Dubey1–Reference Alvarado-Esquivel3]. It is estimated that T. gondii has infected about one-third of the human population in the world [Reference Dubey1]. Human infection is mainly by consumption of undercooked or raw meat containing tissue cysts from an infected intermediate host, as well as unwashed vegetables and fruit or drinking water containing oocysts, polluted by the faeces of infected cats [Reference Flegr4].
In most adults with a normal immune system, T. gondii does not cause serious diseases, and clinical symptoms are usually not apparent. Blindness and mental deficiency can be caused in congenitally infected children and the disease can be severe in immunocompromised individuals [Reference Torgerson5]. Moreover, reactivation and replication of T. gondii in immunocompromised individuals can cause neurological or psychiatric symptoms [Reference Hamidinejat6]. Studies performed in various countries showed increased seroprevalence of toxoplasmosis in psychiatric patients, and increasing evidence supports the hypothesis that psychosis may be related to T. gondii infection [Reference Zhu7].
Currently, several epidemiological investigations on the relationship between T. gondii infection and psychosis have been performed in different areas in China [Reference Zhu7], which were originally published in local Chinese journals (with limited accessibility to international readers). However, most of these investigations simply estimated the seroprevalence of T. gondii infection and did not evaluate the risk factors and possible contamination routes of T. gondii infection in psychiatric patients. Therefore, we conducted this case-control study in order to estimate the seroprevalence of T. gondii infection, and identify risk factors and possible contamination routes of T. gondii infection in a population of psychiatric patients in Shandong province, eastern China.
MATERIALS AND METHODS
Study site and study populations
The present investigation was conducted in Weihai, eastern China from June 2011 to July 2013. Weihai is located in a coastal region in the east of China (36° 41′–37° 35′ N, 121° 11′–122° 42′ E) at 200–300 m above sea level and has a typical temperate monsoon climate. Weihai has an average annual rainfall of 730·2 mm and an average annual temperature of 11·9 °C. Most inhabitants of Weihai are of Han ethnicity, and some residents are of Korean ethnicity. During the study period, 445 patients hospitalized for diacrisis or treatment were invited to participate in the study. All patients were included in the study based on the following inclusion criteria: (1) psychiatric patients, (2) aged ⩾16 years, and (3) agreed to participate in the study. Control subjects (n = 445), matched with psychiatric patients by age, gender, and residence, were included in the study. Serum samples were obtained from the general population of Weihai, eastern China.
Sample collection and transportation
About 5 ml of venous blood was drawn from each participant. Blood samples were kept overnight at room temperature to allow clotting and centrifuged at 1000 g for 10 min. The sera were collected in Eppendorf tubes and stored at 4 °C for 24–72 h until transportation in an icebox to State Key Laboratory of Veterinary Etiological Biology, Key Laboratory of Veterinary Parasitology of Gansu Province, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Lanzhou, Gansu Province where they were kept at −20 °C until tested.
Sociodemographic, clinical and behavioural data
Sociodemographic data including age, birth place, residence and race were obtained for all patients. Clinical data explored in patients included the type of the mental disorder. Behavioural data included animal contact, presence of cats and dogs at home, consumption of raw/undercooked meat, consumption of raw vegetables and fruit, source of drinking water and exposure to soil. These variables were selected based on the literature. Data were obtained from the patients, medical examination records, and informants. Patients were invited to provide veridical information and they were informed that data were used in a confidential manner. Classification of mental illnesses was performed according to the ICD-10 criteria [8].
Serological assay
Sera were analysed for the presence of IgG and IgM antibodies to T. gondii using commercially available enzyme immunoassay kits (Demeditec Diagnostics GmbH, Germany) [Reference Gebremedhin9] according to the manufacturer's instructions. Positive and negative serum controls were included in every plate.
Statistical analysis
Results were analysed with SPSS v. 19.0 software (SPSS Inc., USA). For comparison of the frequencies in the groups, the Mantel–Haenszel test, and when indicated Fisher's exact test, were used. Bivariate and multivariate analyses were used to assess the association between the characteristics of the subjects and T. gondii infection. Variables were included in the multivariate analysis if they had a P value of ⩽0·25 in the bivariate analysis. Adjusted odd ratios (ORs) and 95% confidence intervals (CIs) were calculated by multivariate analysis using multiple, unconditional, logistic regression. P ⩽0·05 was considered statistically significant.
Ethical consideration
This study was approved prior to commencement by the ethical committee of Weihaiwei People's Hospital, Wendeng Municipal Hospital and Wendeng People's Hospital. The purpose and procedures of the study were explained to the participants, and a written informed consent was obtained from all participants. The sera were collected with agreement from the volunteers.
RESULTS
In the present study, psychiatric patients were aged 16–91 years. There were 238 females and 207 males. All psychiatric patients resided in Weihai, eastern China. IgG antibodies to T. gondii were detected in 77 (17·30%) of the 445 psychiatric patients, and in 55 (12·36%) of the 445 controls. The difference in these seroprevalences was statistically significant (P = 0·038); IgM antibodies to T. gondii were found in 14 patients and in 10 controls (3·15% vs. 2·25%, respectively, P = 0·408). Table 1 shows the seroprevalences of latent T. gondii infection in the populations studied according to age group. Seroprevalence in patients and controls increased with age (Table 1). In addition, the seroprevalences of T. gondii infection in patients according to their psychiatric disorder are shown in Table 2. The highest prevalence of latent T. gondii infection was found in schizophrenia patients (11/34, 32·35%).
Seroprevalence of IgG antibodies to Toxoplasma gondii in psychiatric patients and controls according to age group
The types of psychiatric disorder and seroprevalence of IgG antibodies to Toxoplasma gondii antibodies in 445 psychiatric patients
* Compared to 12·36% seroprevalence of IgG antibodies to T. gondii antibodies in controls (55/445).
Bivariate analysis showed a number of sociodemographic characteristics and behavioural characteristics with P⩽0·25, including cats at home, dogs at home, contact with cats and dogs, consumption of raw vegetables and fruit, consumption of raw/undercooked meat and exposure to soil (Table 3). Multivariate analysis of these sociodemographic and behavioural characteristics showed that cats at home [adjusted OR (aOR) 5·060, 95% CI 2·814–9·100, P < 0·001] and consumption of raw/undercooked meat (aOR 2·438, 95% CI 1·167–5·096, P = 0·018) were significantly associated with T. gondii infection in psychiatric patients (Table 4). Other behavioural characteristics did not show an association with T. gondii infection.
Bivariate analysis of selected characteristics with Toxoplasma gondii infection in psychiatric patients and controls in China
Multivariate analysis of characteristics of the psychiatric patients and their association with Toxoplasma gondii infection
aOR, Adjusted odds ratio; CI, confidence interval.
* The variables included were those with a value of P < 0·25 obtained in the bivariate analysis.
† Adjusted by age and the rest of characteristics included in this Table.
DISCUSSION
According to prevalence projections conducted by some authoritative medical institutions, the number of psychiatric patients numbers more than 100 million in China [Reference Li10]. Psychiatric patients have become a serious social public problem in China. In recent years, a number of researchers showed interest in studying the effect of T. gondii infection on the cerebrum and in neuropsychiatric patients [Reference Zhu7]. T. gondii has a highly substantial connection with brain tissue where tachyzoites may develop into tissue cysts and persist for the individual's entire life. Some psychiatric clinical signs such as schizophrenia and mental deficiency may be motivated by T. gondii infection [Reference Flegr4, Reference Zhu7]. In this study, we found a 17·30% prevalence of T. gondii infection in psychiatric patients compared to 12·36% in controls. A statistically significant difference between psychiatric patients and controls was found in this study (P = 0·038), this difference in prevalence between psychiatric patients and controls might be caused, at least in part, by differences in the sanitary conditions between the groups. In fact most psychiatric patients were from the lower socioeconomic level. In addition, they had worse housing conditions and poorer sanitation compared to controls.
Regarding demographic characteristics, the T. gondii seroprevalence found in the present study was not significantly associated with any of the sociodemographic characteristics of the psychiatric patients. It has been reported that seropositivity to T. gondii is related to age [Reference Markovich11–Reference Li13]; however, in the present investigation, T. gondii infection in psychiatric patients was not significantly influenced by age. Although high seropositivity to T. gondii was found in the 70–79 and ⩾80 years age groups of psychiatric patients, the number of subjects in age groups 60–69, 70–79 and ⩾80 years was limited in this study. The difference was not considered to be statistically significant in age groups for psychiatric patients.
Of the types of psychiatric disorder, the highest prevalence of latent T. gondii infection was found in schizophrenia patients (11/34, 32·35%). Schizophrenia is a serious, chronic, and often debilitating neuropsychiatric disorder. Its causes are still poorly understood [Reference Torrey14]. Moreover, there are conflicting reports concerning the association of T. gondii infection and schizophrenia in humans. However, previous studies [Reference Alvarado-Esquivel3, Reference Torrey14–Reference Hinze-Selch17] and our case-control study provides evidence to clarify the controversial information on the association of T. gondii infection and schizophrenia.
With respect to behavioural characteristics, multivariate analysis showed that cats at home and consumption of raw/undercooked meat were associated with T. gondii seropositivity. As the definitive hosts for T. gondii, cats play a vital role in maintaining T. gondii in nature [Reference Dabritz18]. In China, cats are prevalently kept as pets because they are very easy to take care of and provide company that enriches people's home life. However, little attention has been focused on their potential to pollute the environment with T. gondii [Reference Du19]. In this study, we found that keeping cats at home was highly associated with T. gondii seropositivity (aOR 5·060, 95% CI 2·814–9·100, P < 0·001). Some previous reports have indicated that humans can become infected with T. gondii through ingesting tissue cysts in undercooked or raw meat from an infected intermediate host, or by ingestion of oocytes via unwashed vegetables and fruit or drinking water polluted by the faeces of an infected cat [Reference Flegr4, Reference Lopes20–Reference Daryani22]. Similarly, our study found that consumption of raw/undercooked meat was highly associated with T. gondii seropositivity (aOR 2·438, 95% CI 1·167–5·096, P = 0·018). 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.
CONCLUSION
Through a case-control study, the present study revealed, for the first time, that infection with T. gondii in psychiatric patient is common in Weihai, eastern China. Psychiatric patients (17·30%) have a significantly higher prevalence of T. gondii infection than controls (12·36%). Moreover, keeping cats at home and consumption of raw/undercooked meat are important risks for the acquisition of T. gondii infection in psychiatric patients. Considering that most psychiatric patients usually have lower cognitive functioning and additional transmission routes related to their inappropriate behaviours that could enhance the risk of infection, psychiatric patients should be considered as a specific group of T. gondii infection.
ACKNOWLEDGEMENTS
Project support was provided by the Science Fund for Creative Research Groups of Gansu Province (Grant No. 1210RJIA006) and the Agricultural Science and Technology Innovation Program (ASTIP) of Chinese Academy of Agricultural Sciences.
DECLARATION OF INTEREST
None.
