CWD research update from NADC

Authors: Cassmann, Eric; Greenlee, Justin

Submitted to: North American Deer Farmer
Publication Type: Trade Journal
Publication Acceptance Date: 9/1/2020
Publication Date: 9/1/2020
Citation: Cassmann, E.D., Greenlee, J.J. 2020. CWD research update from NADC. North American Deer Farmer. p.72-73.

Interpretive Summary:

Technical Abstract: Since the discovery of chronic wasting disease (CWD) in mule deer in the late 1960’s, it has been increasingly identified throughout North America in wild and farmed cervids. The cause of CWD was determined to be an accumulation of a misfolded protein called the prion protein. In addition to cervids, prion diseases are also found in other species. In sheep, a prion disease called scrapie is quite similar to CWD. In both of these diseases, the abnormal prion protein is transmissible between animals, the peripheral distribution in lymphoid tissue is similar, and the prion protein amino acid sequence appears to affect the potential for misfolding. The amino acid variations (polymorphisms) at sites in the protein sequence (codons) are usually discussed using a single letter abbreviation that corresponds with the amino acid that is encoded. For example, sheep have amino acid polymorphisms in the prion protein at codons 136, 154, and 171; sheep with the genotype A136R154R171 are highly resistant to scrapie. The National Scrapie Eradication Program has been very successful in decreasing the prevalence of scrapie in the United States. The backbone for this success has been in effective traceback of affected flocks and genetic breeding strategies. Breeding strategies are focused on the selection of rams with naturally occurring scrapie resistant prion genotypes. Considering the success of controlling scrapie in sheep by breeding resistant animals, the application of a selective breeding strategy to combat CWD is a possible avenue to pursue in cervids. The genotype of an animal can be predicted by extracting and sequencing DNA (typically from a blood sample). White tailed deer have several polymorphic sites including codons 95, 96, and 226. Polymorphisms at codon 96 are the most common. In naturally infected CWD herds, there is a lower prevalence of positive tests from GS96 and SS96 deer. Deer that are SS96 still develop CWD. Less is known about polymorphisms at codon 95; although, it seems that H95 somehow affects how CWD accumulates in different parts of the body. The 226 polymorphism is rare. To this day, no one has identified a prion protein genotype that is completely resistant to CWD; however, less common genotypes and different combinations have not been fully characterized. At the National Animal Disease Center in Ames, IA, we recently started a study that investigates polymorphisms in the prion protein at codons 95, 96, and 226. Experimental groups include deer with H95G96K226 and Q95S96K226, and Q95G96K226. To imitate natural exposure, we inoculated a highly susceptible deer (wild type; GG96) to serve as the source of infection. Exposure of potentially resistant deer will occur through cohousing with infected deer. There are two main questions that we hope to address. (1) Are any deer resistant to CWD, and if not, (2) how long do these deer shed CWD prior to developing clinical disease? For the duration of the study, we are regularly collecting rectal biopsies, skin biopsies, feces, saliva, and blood from each deer. In future editions of the Deer Farmer Magazine, we will elaborate on the plans and usefulness of these samples.