Epidemiology of Toxoplasma gondii in the pig industry of Yucatan, Mexico

Abstract

Toxoplasmosis is a worldwide distributed parasitic disease caused by the zoonotic pathogen Toxoplasma gondii. Serological studies have estimated that more than 30% of the human population has been exposed to this protozoan. T. gondii is considered a leading cause of death attributed to foodborne illness. The consumption of infected pork meat is suggested to be an important source for human infection. However, the prevalence of T. gondii in pigs vary greatly between countries, the reasons for this heterogeneity has been addressed to the differences in climate distribution, environments, husbandry systems and fam management. The geographical location of this study is Yucatan, a state located in the south-east of Mexico. Yucatan is considered an endemic area of toxoplasmosis; the last National Seroepidemiological Health Survey (NSHS) revealed more than 70% of prevalence among the human population. Numerous studies suggested that the consumption of pork in this geographical area may be a major source of T. gondii. The aims of this study were to investigate the disease levels in the pig industry of Yucatan, assess an in-home ELISA widely used in this area (ELISA kit Human- GmbH, WB), study the risk factors associated with the disease in theses pig farms and evaluate the contamination with T. gondii in pork meat intended for human consumption. To do that, swine blood samples were collected through a cross-sectional age stratified opportunistic sampling of 12 farms across the state during the year 2014. Farm management and characteristics were obtained by interviewing farmers. In addition, meat and blood samples were collected from a local abattoir from 2013 to 2015. Anti - T. gondii IgG antibody levels were investigated with the well validated MAT (Modified Agglutination Test), with an ELISA (Enzyme-Linked Immunosorbent Assay) for use on pig serum (ID Screen ®, IDVet) and with the gold standard Dye test. The overall seroprevalence was 1.4% (95%CI: 0.6%-2.7%) among 632 pigs sampled. This seroprevalence increased with age (p < 0.05), reaching the 11.5% (95% CI: 2.5%-30%) in pigs older than 20 weeks. The seroprevalence was even higher, 17.8% (95% CI: 6%-37%), in slaughtered animals (n=34). In addition, T. gondii prevalence was investigated using a highly sensitive nested PCR protocol targeting the SAG1 gene. PCR diagnosis revealed the high frequency of 21.2% (CI: 18%-24.6%) of T. gondii DNA circulating in the blood of these pigs (n=632). Furthermore, MLST (Multi-Locus Sequence Typing) of four alleles (SAG1, SAG2, SAG3 and GRA6) allowed, for first time in Yucatan, genetic diversity to be assessed.Data revealed the presence of high genetic diversity among T. gondii strains of this geographical area with shared alleles to strains from both North and South/Central America origin. Moreover, a relatively high number of pigs presented multiple infections with different T. gondii strains suggesting high levels of T. gondii transmission on the intensive pig farms sampled. The frequency of T. gondii DNA was also investigated in pig tongues sampled at the abattoir of which 38.2% (95% CI: 22%-56.4%) were shown to harbour T. gondii DNA in their tissue. The viability of the parasite was also investigated in the tongues intended for human consumption and a total of 8.8% (95% CI: 1.8%-23%) were shown to have viable T. gondii using a mouse bioassay. However, the agreement between serology, PCR and mouse bioassay was low (k=0.12-0.29). Due to the risk of underestimating T. gondii infection by using solely one diagnostic method, a combination of indirect (serology) and direct methods (PCR or/and bioassay) is preferred for a robust diagnosis. This study was pioneering in using a serological test validated in pigs in the state of Yucatan and the data revealed a much lower prevalence than previously reported (95.8%-100%) in market age pigs (Ortega-Pacheco et al., 2013, Hernández- Cortazar et al., 2016a). Although a more optimistic view is obtained; due to the potential of T. gondii to lead to severe illness, measures to control the disease should still be taken. Furthermore, questionnaire results suggested the need for an improvement to the sanitary management of the pig farms of this geographical area not only to prevent T. gondii transmission but also that of other pathogens of zoonotic significance