Evaluation and Modeling of Food Safety Risk Factors Associated with Toxoplasma gondii Infection in the Farm-to-Fork Framework

Abstract

Toxoplasma gondii is a widespread zoonotic parasite with a high seroprevalence in the human population and the ability to infect almost all warm blooded animals. Animal meat may contain viable T. gondii tissue cysts that can potentially cause infection if undercooked meat is consumed. The goal of this research is to estimate T. gondii distribution in animal meats by integrating experimental data with predictive modeling and statistical analyses to better understand the ecology of T. gondii infection and further evaluate mitigation methods to reduce the public health burden of toxoplasmosis.

To understand the infectivity and transmission of T. gondii from the environment to animals and thereafter humans, the formation and distribution of T. gondii tissue cysts was estimated in varying sizes (5 g, 10 g and 50 g) of animal muscle tissues. Experimentally and naturally infected pigs, lambs and goats were evaluated. The sensitivity and specificity of different diagnostic tests for detecting T. gondii were also evaluated using logistic regression modeling and meta-analysis. Bootstrap and Gibbs statistical sampling techniques were used to assess the complete inactivation of T. gondii in pork through cooking and freezing. Dynamic compartmental modeling was used to simulate one year on a hypothetical pig farm to understand T. gondii infection transmission via the environment and in multiple hosts such as cats, rats, pigs and humans. Further, this modeled analyzed some of the dynamical behaviors of the T. gondii infection in the definite (cat) and intermediate (e.g. rat, pig and human) host populations.

The results suggested that T. gondii tissue cysts can develop as early as 7 days after infection in experimentally infected pigs and are unevenly distributed in the muscle tissues of naturally infected lambs and goats based on bioassay in mice. Meat samples as small as 5 g have the potential to cause T. gondii infection if consumed raw or undercooked. The regression model predicted varying specificity and sensitivity for different sized meat samples with the highest sensitivity and lowest specificity for the largest samples (50 g). T. gondii tissue cysts in fresh pork were completely inactivated at or above 64°C (147.2°F) and below -18°C (0°F). Tissue cysts can remain viable in fresh meat for up to 30 days stored at 4°C (39°F). With the calculated predation rate of the hosts and the transmission rate of infection from environment, the T. gondii infection is expected to persist (R0 > 1) in all hosts over the simulation run of one year. This dissertation evaluated the T. gondii infection flow in different hosts, assessed mitigation strategies for food safety risks and estimated the distribution of the parasite in fresh cut meats of food animals.