Risk Identification for Toxoplasma Transmission in Pasture Raised Animals

Non-Technical Summary:<br/>Toxoplasma gondii is responsible for 24% of all deaths attributed to foodborne pathogens in the U.S. with an estimated 327 deaths per year. Of the major meat animal species, pigs and lambs are shown to frequently harbor T. gondii. Pigs and lambs that are reared with complete or partial pasture access have substantially greater opportunities for exposure to T. gondii as compared to animals managed in confinement systems that use stringent biosecurity practices. In recent years, meat from pasture and organically raised animals is increasingly popular both with producers who can demand premium prices for animals that are managed under these conditions and with consumers of organic or free range meats, which are perceived as healthier than meat from conventionally raised animals. Given the increasing consumer demand for organic and free-range meat and the high risk of pasture and organically raised pigs and lambs being infected with T. gondii, these meats pose a significant public health risk. Our project is thus aimed at (i) providing more insights into T. gondii infection in organic and free-range pig and lamb meats, available for purchase by consumers at the retail level, and (ii) understanding and assessing the public health risk that may arise because of consumption of such meats. The knowledge and information from this project will help us better understand foodborne human infection with T. gondii and facilitate implementation of practical approaches to control public health risks.
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Approach:<br/>
Objective 1. We will be collecting meat samples (organic, free-range pork and lamb) from a nationwide sampling of retail stores from 25 metropolitan statistical areas (MSA) in the U.S.; within each MSA, meat samples will be acquired from 10 supermarkets, grocery outlets, or wholesale club stores that will be randomly selected for sampling. Thus samples from 250 retail stores (3 samples for each meat category from each store) will be collected and tested for the sero- and tissue prevalence of T. gondii using ELISA and bioassay in mice, respectively. The ELISA will be performed using a commercial test kit (Toxoplasma Microwell Immunoassay Kit), which has been validated for use with pork samples. For testing lamb tissue fluids, kits will be modified by changing the species-specific conjugate. For bioassay of viable T. gondii, 50-gram portions of meat will be homogenized in 0.85% NaCl (saline), digested in pepsin, centrifuged, neutralized, suspended in antibiotic saline, and will be inoculated subcutaneously into 5 mice. Tissue impression smears from dead mice will be examined for T. gondii tachyzoites or tissue cysts. Survivors will be bled at 8 wk post-inoculation (PI), and a 1:25 dilution of serum from each mouse will be tested for T. gondii antibodies by modified agglutination test (MAT). Mice will be killed 8-10 wk PI and brain squashes from all mice will be examined microscopically for tissue cysts. Mice will be considered infected with T. gondii when tachyzoites or tissue cysts are microscopically demonstrable in tissue smears.
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Objective 2. We will develop a risk assessment framework that will be useful in performing quantitative risk assessment studies for T. gondii in organic, free-range pork and lamb. We will develop a probabilistic model for the exposure of T. gondii for human consumers due to the consumption of meat from organic and free-range pigs and lambs. We will employ a probabilistic modeling approach using @Risk as the software platform. The model will be developed in a modular fashion so that multiple steps can be incorporated simultaneously. Data from this project, from published literature and unpublished data will be compiled and used for this modeling study. In addition, expert opinions will be elicited where significant or key data are missing. By integrating the prevalence of T. gondii in organic and free-range pork and lamb, consumption data, preparation habits, and inactivation of tissue cysts due to the application of interventions, we will estimate the likelihood of T. gondii exposure to human consumers due to the consumption of organic and free-range pork and lamb, for the whole U.S. population and different population groups. Monte Carlo simulation will be performed using Latin Hypercube sampling technique to ensure that during simulations, samples are drawn randomly throughout the distribution of model parameter. The developed model will be used to perform sensitivity and scenario analysis to identify sensitive variables and evaluate potential intervention strategies, respectively.