| Toxoplasma gondii is one of the most common intracellular parasites that can infect wild, domestic, and companion animals, and it also commonly infects humans. The importance of this parasite in animal husbandry, food safety and human health has been well recognized. It has been estimated that up to one third of the world's human has been infected by Toxoplasma gondii. The seroprevalence of T. gondii infection in human varies widely worldwide with endemicity from around 10% to 70% in most countries and the prevalence is higher in warm and humid areas in some related researchs. In immune competent individuals, Toxoplasma gondii preferentially infects tissues of central nervous systems, which might be an adding factor of certain psychiatric disorders. In several studies, patients with schizophrenia were found to have a higher tendency of T. gondii infection, but there has been no conclusive correlation between T. gondii infection and psychiatric disease. While in immune-compromised individuals, the parasite can cause life-threatening infections. Congenital transmission of T. gondii during pregnancy has been regarded as a risk factor for the health of pregnant women and newborn infants. Thus surveillance of T. gondii infection in the living environments has been regarded as an important measure to prevent the disease.Detection of Toxoplasma-specific antibodies has been a gold standard method for both epidemiological investigation and clinical diagnosis. Currently, toxoplasmosis is diagnosed primarily by demonstrating parasite-specific IgM or IgG antibodies in serum samples. Most of the commercially available tests in China only use T. gondii native antigens or RH strain proteins which may result variations in accuracy of detection. Recombinant antigens have been suggested as diagnostic reagents but their reliability may need extensive experimental test. Further, T. gondii remains in chronic infection in immune competent individuals, and tachyzoites and bradizoites do display different antigenic profiles. Thus it is critical to select accurate antigens to different diagnosis and epidemiological purposes. 2565 serum samples from clinically healthy individuals were collected in Changchun, Daqing and Shanghai areas from July 2006 to June 2008.541 serum samples were collected from patients with psychiatric disorders (including schizophrenia, mania and depression, etc) in Changchun region. Indirect ELISA assays were performed to measure the levels of anti-T. gondii IgG antibodies in the sera according to the standard protocol. In this study, we investigated more than 3000 Chinese individuals using crude antigens of RH strain (typeâ… ) and ME49 (Typeâ…¡) and the recombinant BAG1 (Typeâ…¡). Human sera which were previously confirmed T. gondii negative and positive by direct agglutination test using commercial agglutination kit (Toxo-Screen DA, BioMerieux, France) were included as negative control and positive control in every plate. This study aims to investigate the prevalence of T. gondii infection among clinically healthy individuals and patients with psychiatric disorders in China and to identify the potential risk factors related to the vulnerability of infection in the populations, compare the sensitivity and consistency in detection of T. gondii specific antibodies in the same set of samples. Prevalence of T. gondii infection in the Chinese population in respect of antigen, gender, age, residence, region and health status was systematically analyzed using the SPSS 15.0 software package.T. gondii tachyzoites (RH and ME49 strain respectively) were cultivated in BHK (baby hamster kidney) cell lines. Briefly parasites released from freshly host cells were harvested, purified by Percoll, washed in PBS and lysed by sonication. The insoluble component such as cell debris was eliminated by centrifugation and the soluble proteins, respectively termed RH-Ag and ME49-Ag were collected. To generate the recombinant BAG1 antigen, the coding sequence (XM002365075.1) of T. gondii ME49 bradyzoite antigen 1 (BAG1) was cloned into the plasmid pGEX-4T-1 to construct a recombinant plasmid pGEX-4T-1-BAG1, which was subsequently confirmed by sequencing. The plasmid pGEX-4T-1-BAG1 was transformed into BL21 competent cells and the recombinant BAG1-GST protein was expressed and purified by Glutathione Sepharose 4B according to standard protocol, finally the recombinant protein termed BAG1-GST-Ag was collected.The prevalence obtained with RH-Ag, ME49-Ag and BAG1-GST-Ag was 11.5% , 15.7% and 13.7% respectively in patients with psychiatric disorders population. No correlation was found between T. gondii infection and psychiatric disorders in this study. The prevalence obtained with these antigens was 12.2% ,11.3% and 14.9% respectively in clinically healthy population and the differences in sero-prevalence among Changchun, Daqing and Shanghai areas where the samples collected were similarly obtained using RH-Ag, ME49-Ag and BAG1-GST-Ag. Differences of seroprevalence suggested RH strain dominated in the healthy population while ME49 strain dominated in psychiatric population in this study. A significant higher prevalence of T. gondii infection was observed in female (14.1% ) than male (10.4% ) recognized by antigen collected from high virulent RH strain which dominated in the healthy population. An increased prevalence of T. gondii was observed in the age group over 50 years by ME49-Ag and BAG1-GST-Ag in patients with psychiatric disorders population, meanwhile an increased prevalence was observed in the age group over 50 years by BAG1-GST-Ag in the healthy population.The sera-prevalence with ME49-Ag was lower than that with RH-Ag (P>0.05), whereas the sera-prevalence with BAG1-GST-Ag was significantly higher than that with the other two antigens (P<0.05) in the healthy population. The data reflected the nature of immuno-recognition of T. gondii antigens and correlated well with the serum samples which were all collected from individuals with no sign of clinical T. gondii infection. Both RH-Ag and ME49-Ag were crude antigens which were consisted of different parasite components. However, they were generated from tachyzoites, while the BAG1 was a bradizoite-derived antigen. Thus immuno-recognition of BAG1 is likely more prominent than that of bradyzoite antigens during inapparent infection and chronic infection.Further analysis among the serum samples which were positive, it was surprisingly found that the samples were not all overlapped or consistent with the three antigens. Among the 789 positive samples detected with the three antigens,52 (6.6%) was detected by both RH-Ag and ME49-Ag,49 (6.2% ) was detected by both RH-Ag and BAG1-GST-Ag, and 55 (7.0%) was detected by both ME49-Ag and BAG1-GST-Ag. The samples that were positive in reactions with all three antigens were only 20, accounted 2.5% of all positive samples. Thus the overall sera-prevalence of anti-T. gondii IgG in the studied samples was 30.8% , which was significantly higher than that previously reported. This is obviously due to the antigens used in the investigations which have been mainly based on the antigens of RH strain, a laboratory-adapted strain commonly available worldwide.Differences of seroprevalence suggested RH strain dominated in the healthy population while ME49 strain dominated in psychiatric population in this study. The study has shown that women have a significantly higher risk of being infected by T. gondii with RH-Ag in the healthy popualtion, possibly due to more exposure to the infective sources. An increased prevalence of T. gondii was observed in the age group over 50 years by BAG1-GST-Ag in all populations, meanwhile an increased prevalence was observed in the age group over 50 years by ME49-Ag in patients with psychiatric disorders population, obviously due to more exposure to the T. gondii. The general infection rate of T. gondii in the healthy population was over 30% in our study. The data further supported that there is more genetic diversity among the T. gondii in China, which argued for the necessity of establishment of a method that can detect most, if not all, of the variant specific antibodies, and the infection status of Chinese population may be much more serious than we estimated.
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