Toxoplasma gondii in animal reservoirs and the environment

Abstract

Toxoplasma gondii is an important zoonotic parasite that is best known as a cause of abortion and abnormalities in newborns if a woman is primary infected during pregnancy, although ocular toxoplasmosis from acquired infection may contribute substantially to the disease burden. Infectious parasites are present as tissue cysts in undercooked meat, and as oocysts in cat faeces, contaminated soil, or water. In The Netherlands prevention focuses on educating pregnant women about the risks for infection. However, inclusion of other measures that also target the total population may be more effective. To set up effective prevention strategies the importance of the different sources needs to be known. Therefore this thesis focuses on the prevalence of infection in various animal reservoirs. First a method for sensitive PCR-based detection of tissue cysts (MC-PCR) was developed. There was a strong correlation between detection of antibodies by ELISA and parasites by MC-PCR in sheep. Therefore the high seroprevalence in sheep (27.8%) indicates that the consumption of undercooked mutton or lamb poses a risk of infection. It was difficult to develop a sufficiently discriminatory serological assay for cattle, and only two cattle, which were negative by ELISA, tested positive by PCR. This demonstrates that the seroprevalence in cattle does not give an indication of the risk of infection from beef. Whether the PCR prevalence gives an indication remains to be studied, as dead or non-infective parasites may have been detected. The seroprevalence in sheep, PCR prevalence in cattle, previously published seroprevalence data for pigs, meat consumption and preparation data, and literature data on T. gondii survival during processing were integrated in a quantitative microbial risk assessment (QMRA) model. Although the predicted number of infections was too high, the results demonstrate that beef can be an important source of infection even if the prevalence is low, and raw products contributed 40% of the total number of predicted infections. This thesis also includes a study on the seroprevalence of T. gondii in wild boar, and based on fitting compartmental infection models to the observed age-seroprevalence relation, we hypothesized that antibodies, and possibly the infection itself, do not persist lifelong in wild boar. Unfortunately no specific consumption data for wild boar were available; therefore these data could not be included in the QMRA. The last study focuses on T. gondii infection in the definitive host, the cat. Because domestic cats are the only definitive host of importance in The Netherlands, prevention of infection in cats (and thereby prevention of oocyst shedding) can be highly effective in decreasing both oocyst- and tissue cyst acquired infections in humans. In this study risk factors were identified by logistic regression analysis on serological results and questionnaire data. Hunting and feeding of raw meat were identified as risk factors that could be targeted by prevention measures. Finally, an overview of the various options for intervention in T. gondii transmission to humans is given and it is discussed how the presented results and the QMRA model can aid deciding between those options.