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Author affiliation: The University of Texas Health Science Center at Houston, Houston, Texas, USA (R. Benavente, F. Bravo-Risi, P. Soto, R. Morales); University of Edinburgh, Scotland, UK (F. Brydon, M.A. Baria); Universidad Bernardo O’Higgins, Santiago, Chile (F. Bravo-Risi, P. Soto, R. Morales); Texas Park and Wildlife Department, Kerrville, Texas, USA (J.H. Reed, M. Lockwood); Colorado State University, Fort Collins, Colorado, USA (G. Telling)
Prion diseases cause various diseases that affect several animal species, including scrapie in sheep and goats (1), Creutzfeldt-Jakob disease (CJD) in humans (2,3), bovine spongiform encephalopathy (BSE) in cattle (4), and chronic wasting disease (CWD) in cervids (5). In the 1990s, several atypical CJD cases occurred among persons who ingested cattle-derived products infected with BSE. Those cases later were attributed to the emergence of a new human prion strain templated by BSE prions (6). Subsequent studies have been conducted to investigate the zoonotic potential of other prionopathies, including CWD (7,8). Although no cases of CWD transmission to humans have been reported, the potential for human infection is still unclear because contradictory results have been reported from studies in animal models, in vitro systems, and nonhuman primates (8,9).
CWD prions have been detected in the muscle of both farmed and wild deer (10), and at concentrations relevant to sustain disease transmission (11). CWD prions have also been identified across several cervid species and in multiple tissues, including lymph nodes, spleen, tongue, intestines, adrenal gland, eyes, reproductive tissues, ears, lungs, and liver, among others (12–14). Those findings raise concerns about the safety of ingesting processed meats that contain tissues other than skeletal muscle (15) (Appendix). In addition, those findings highlight the need for continued vigilance and research on the transmission risks of prion diseases and for development of new preventative and detection measures to ensure the safety of the human food supply. Considering that humans consume products from ≈7,000–15,000 CWD-infected cervids each year (Appendix), the need for clarification of transmission risk for prion diseases is imperative. We investigated detection of CWD prions in seasoned and unseasoned raw and cooked meats prepared from a hunter-harvested elk. We also assessed the potential for CWD prions within meat products to template the misfolding of human cellular prion protein (PrPC).
Figure 1
Figure 1. Flowchart of experimental strategy to detect chronic wasting disease prions in raw, processed, and cooked elk meat. We sampled ham, boneless steak, sausage, and hamburger meat from an elk that…
We obtained different unprocessed and processed meats from a hunter-harvested, CWD-infected elk that encoded both methionine and leucine polymorphic variations at prion protein (PrP) position 132. We used meat and meat-derived products, including filets, jerky steak cuts, hamburger meat, chili meat, sausage, ham, cutlets, and boneless steaks (Appendix Figure 1). The meat was prepared from a 5-year-old bull elk (Cervus elaphus nelsoni) that was positive for PrPSc (scrapie isoform of the prion protein) in the obex region of the brain; lymph node was not tested. The elk was harvested on December 10, 2020, and the sample was confirmed as CWD-positive January 8, 2021. The animal was collected in Medina County, Texas, on a private high-fenced hunting ranch.
Figure 2
Figure 2. Western blot of PMCA used for detection of chronic wasting disease prions in raw, processed, and cooked elk meat. A) Representative PMCA data of the different samples tested. The panels…
We tested raw meat samples for CWD prions by using the protein misfolding cyclic amplification (PMCA) technique in an elk substrate (Figure 1). We selected the elk PMCA substrate to maintain the prion protein sequence homology between PrPC and the suspected PrPSc in meats. In a first PMCA round, prion detection was negative for most of the raw meat samples, except the boneless steak, which had a positive PMCA signal in 1 of the replicates (Figure 2). To increase the sensitivity of prion detection, we performed 2 additional PMCA rounds (Appendix Figure 2). In a second PMCA round, we observed positive signals for additional specimens, including sausages and cutlets (Figure 2). The jerky sample also provided CWD prion signals in a third PMCA round (Figure 2). No other samples tested PMCA-positive in that analysis.
To test persistence of CWD prions in cooked meat products, we grilled and boiled different pieces of the meat types, mimicking a medium-well cooking status by considering the external and internal appearance and internal temperatures (Appendix Table). Of note, grilling substantially increased prion detection by PMCA, and 5 sample meat types, hamburger, chili meat, ham, cutlets, and boneless steak, were positive in a first PMCA round (Figure 2). We observed a positive PMCA signal for jerky meat in a second PMCA round and an increased number of positive replicates for the samples detected in the first round. At the third PMCA round, all grilled and boiled meats were positive for CWD prion in >1 replicate, strongly suggesting that grilling increased the availability of CWD prions for in vitro prion amplification (Figure 2). We observed similar results when we boiled different cuts of the same specimens. Of note, the water used to boil some of the meat samples was also positive by PMCA analyses (Figure 2; Appendix).
Figure 3
Figure 3. Western blot of PMCA to evaluate zoonotic potential of CWD-containing meats in a study of chronic wasting disease prions in raw, processed, and cooked elk meat. A) Ham; B) boneless…
Considering the presence of CWD prions in the previously tested edible products, and their persistence after processing and cooking, we evaluated the zoonotic properties of CWD prions by using the PMCA technique. PMCA previously has been reported to be useful in estimating zoonotic potential for multiple animal (i.e., nonhuman) prion isolates (Appendix). Here, we evaluated raw and cooked meats for their potential to template the misfolding of the human PrPC in a single PMCA round to avoid further adaptation of the agent; hence, the goal of our experiment was to mimic initial interspecies transmission events. We specifically used a PMCA protocol optimized for human PrP encoding methionine at position 129. We used that specific human PrP version as PMCA substrate because of its increased susceptibility for misfolding in the presence of BSE prions (Appendix). The results demonstrated that none of the meat samples tested were able to induce conversion of human PrPC to PrPSc, suggesting a limited zoonotic potential for such edible products (Figure 3). Of note, the results were the same regardless of the cooking status of the meat (Figure 3).
To validate the PMCA method we used for modeling cross-species prion transmission, we incorporated classic BSE and sheep scrapie samples into the experimental protocol. We found PMCA reactions seeded by BSE prions and a CWD isolate were able to induce the misfolding of human PrPC, as previously reported (Appendix), but that sheep scrapie was unable to do so.
Overall, our study results confirm previous reports describing the presence of CWD prions in elk muscles (13). The data also demonstrated CWD prion persistence in food products even after processing through different procedures, including the addition of salts, spices, and other edible elements. Of note, our data show that exposure to high temperatures used to cook the meat increased the availability of prions for in vitro amplification. Considering the potential implications in food safety and public health, we believe that the findings described in this study warrant further research. Our results suggest that although the elk meat used in this study resisted different manipulations involved in subsequent consumption by humans, their zoonotic potential was limited. Nevertheless, even though no cases of CWD transmission to human have been reported, the potential for human infection is still unclear and continued monitoring for zoonotic potential is warranted.
Ms. Benavente is a master’s student and graduate research assistant at the Department of Neurology, McGovern Medical School, The University of Texas Health Science Center in Houston, Texas, USA. Her research focuses on developing methods for prion detection and drug discovery, as well as risk factors contributing to the spread of CWD.
Author contributions: R.B. performed a single replicate elk-PMCA assay for all the samples tested in this study, prepared the final version of the figures, and wrote part of the original manuscript draft. F.B. executed the human-PMCA experiments. F.B.R. and P.S. performed replicates on the elk PMCA assay. J.H.R. and M.L. provided the meat samples. G.T. provided brains from Tg5037 mice to prepare the elk PMCA substrate. M.B. supervised the human PMCA experiments. R.M. supervised the whole execution of the project, wrote the first draft of the manuscript, and was responsible for funding. All the authors approved the final version of this article.
