2022-23 Rapid Ag: Developing Best Practices for the Detection and Decontamination of Chronic Wasting Disease Prions Associated with Venison Processing
Peter A. Larsen, Department of Veterinary and Biomedical Sciences
David M Seelig, Department of Veterinary Clinical Sciences
- Gage Rowden, Department of Veterinary and Biomedical Sciences
- Marc D. Schwabenlander, Department of Veterinary and Biomedical Sciences
- Jason Bartz, Creighton University School of Medicine
- Joel Pedersen, UW-Madison, Department of Soil Science
Chronic Wasting Disease (CWD) is an infectious, 100% fatal neurodegenerative disease of cervids (e.g., elk, deer). However, recent data show that unique, naturally- occurring CWD strains can infect a variety of mammals and can cause human prions to misfold in cell culture. Therefore, there are growing concerns that CWD-positive venison poses a risk to human health. CWD prions accumulate in infected deer tissues that commonly enter the human food chain and it is estimated that over 15,000 CWD positive deer are consumed in the USA annually. The FDA and USDA now formally consider CWD-positive venison as an adulteration (i.e., not fit for food). The magnitude of CWD prion accumulation and the degree to which such contamination occurs during routine venison processing is unknown. Absent this knowledge, potential risks to meat processors, deer hunters, and venison consumers are unknown. To address this knowledge gap, our team seeks to determine CWD prion burden, and its infectivity, in meat and on meat processing equipment used in venison processing. We will also explore prion decontamination strategies for this processing equipment. Our results will directly inform best practices to reduce or prevent the introduction of CWD prions into the human food chain.
Objectives and Goals
The primary goal of the proposed research is to develop best practices for the detection and mitigation of CWD prions associated with venison processing. These best practices will be shared directly with the Minnesota Department of Agriculture (MDA) to assist with the establishment of food-safety practices aimed at preventing CWD prions from entering the food chain. We hypothesize that CWD prions contaminate the surfaces of equipment during the processing of CWD positive deer. We further postulate that CWD prion accumulation will depend on the volume of processed material, with increased quantities of CWD positive meat leading to increased infectivity potential. If accurate, the development and implementation of prion decontamination strategies would be essential to reduce prion burden and would provide meat processing facilities with effective control measures to protect our food supply. To test these hypotheses, we propose the following specific objectives:
- Determine the CWD prion load on equipment used during, and foodstuffs resulting from, the processing of tissues from CWD-infected white-tailed deer.
- Characterize the infectivity of CWD prions found on equipment used during, and foodstuffs resulting from, the processing of tissues from CWD-infected white-tailed deer.
- Determine the efficacy of disinfection protocols in mitigating CWD prion load on equipment used during the processing of tissues from CWD-infected white-tailed deer.
CWD is an infectious, progressive, and uniformly fatal neurodegenerative disease of cervids that is characterized by efficient animal to animal transmissibility. The pathogenic agent of CWD is a misfolded prion protein, similar to the causative agent of “mad cow disease” in cattle (bovine spongiform encephalopathy; BSE) and Creutzfeldt-Jakob Disease in humans. A critical observation relevant to the proposed research is that misfolded, disease-causing prions are highly resistant to degradation and can remain infectious on both natural and man-made surfaces for months to years. To date, the natural transmission of CWD to a non-cervid species (including humans) has not been reported. However, recent studies show that there are compelling reasons to conclude that CWD poses and increasing risk to humans.[2,3] Firstly, challenge experiments using CWD-causing prions have shown that CWD can cause neurodegenerative disease in numerous mammals, including ferrets, mink, domestic cats, sheep, goats, cows, pigs and squirrel monkeys. Additionally, there is increasing recognition that CWD strains, which represent biochemically and pathologically unique conformations of the misfolded prion protein, also demonstrate unique infectivity profiles. In CWD, the impact of strain on zoonotic potential has been shown through the misfolding of human prion protein by elk-origin CWD strains, but not mule deer strains. This reveals the potential for CWD prions to convert human prions into a misfolded and potentially disease-causing form. Furthermore, studies have shown that non-neural prions, including those found in tissues involved in meat processing (e.g. lymphoid tissues), facilitate protein misfolding more readily than neural-origin prions (which are traditionally used to test the zoonotic potential of a prion disease). Finally, the well-documented 1980’s outbreak of BSE in Great Britain (~1 million cattle infected; ~178 human deaths) coupled with experiments showing the transmission of scrapie, a prion disease of sheep that has long been believed to be non-zoonotic, to genetically-altered “humanized” mice (i.e., human prion protein knock-in mouse strains) underscore the uncertainty of CWD’s risk to humans.[6,7]
In addition to the uncertain zoonotic potential of CWD, there is compelling evidence that the routine processing of hunter-harvested and farmed cervids for venison may inadvertently contribute to the introduction of infectious prions into animal (i.e., livestock and pets) and human food chains. First, CWD prions accumulate early in tissues that are likely to enter the food chain, including skeletal muscle, lymph nodes, and peripheral blood.[8–10] Second, a recent study has demonstrated the capacity for meat processing equipment to act as fomites for prion transmission through the ability of many man-made surfaces (e.g. stainless steel, plastic, and aluminum) to efficiently bind and release infectious prions. Moreover, meat lockers processing cervids for venison typically process a variety of meats in an assembly- line fashion, thus the cross contamination of CWD prions to other meat products via fomites is possible. In light of these concerns, the Food and Drug Administration (FDA) and United States Dept. of Agriculture Food Safety and Inspection Service (USDA; FSIS) now consider cervids that test positive for CWD to be adulterated (i.e., not fit for human or animal food) under the Federal Food, Drug and Cosmetic Act.[11,12] Despite these concerns, there are no guidelines regarding the processing of cervids or the prion decontamination of cervid processing equipment. As such, the potential risks to meat processors, deer hunters, and venison consumers remain unknown. The risk of CWD exposure is compounded by the realization that most venison processing occurs without knowledge of an animal’s CWD status. It is estimated that at least 15,000 CWD positive cervids are consumed in the USA annually, a number that is anticipated to grow by approximately 20% per year as CWD spreads. Underscoring this observation is a well-documented 2005 exposure event in which over 200 participants at a Sportsmen’s feast were exposed to CWD. Current estimates from southern Wisconsin indicate a 50% CWD infection rate in harvested white-tailed deer bucks, however only 1 out of 8 are tested for the disease. Such data indicate a significant emerging food-related risk to human-health.
In aggregate, these studies reveal a number of knowledge gaps being addressed by our study objectives: 1) the degree to which CWD contamination occurs during the routine processing of CWD+ cervids, 2) the infectious implications of CWD prion accumulation on meat processing equipment and in the resultant foodstuffs, and 3) the best practices for the disinfection of CWD prions from meat-production surfaces. Our study objectives will directly contribute to one of the long-term goals of the Minnesota Center for Prion Research and Outreach (MNPRO), which is to assist the state with efforts to identify and minimize the risks associated with CWD. Thanks to previous RARF funds and the MN Legislature, our MNPRO lab has a new ultrasensitive prion detection tool (RT-QuIC; see below) that can help the MDA with CWD surveillance efforts along the venison food-chain. The outcomes of the research proposed will directly support and enhance MDA efforts to minimize CWD prions from entering the food supply in our state. We anticipate our work will provide meat processing facilities with an array of new options that will help to maintain safe practices that are in line with FDA and USDA FSIS regulations.
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