Attempted transmission of the CWD agent to deer with potentially resistant prion protein genotypes

Authors: Greenlee, Justin; Cassmann, Eric

Submitted to: North American Deer Farmer
Publication Type: Trade Journal
Publication Acceptance Date: 10/29/2021
Publication Date: 3/20/2022
Citation: Greenlee, J.J., Cassmann, E.D. 2022. Attempted transmission of the CWD agent to deer with potentially resistant prion protein genotypes. North American Deer Farmer. p. 117.

Interpretive Summary:

Technical Abstract: In March 2020, we initiated an experiment to test whether there are white-tailed deer with prion protein (PRNP) genotypes that lend resistance to CWD infection after natural exposure. Other groups are doing complementary work to understand if there are genes outside of the prion protein that influence susceptibility to prion disease, but the purpose of our study is to determine if there are PRNP genotypes leading to resistance like the R171 codon in sheep scrapie. The inoculum used in this study originates from a pool of wild type (QQ95GG96QQ226) hunter harvested deer from Wisconsin but comes from a single wild type deer after experimental oronasal passage. This material was used to oronasally inoculate 10 white-tailed deer with wild-type prion protein that were housed in 10 separate rooms with two non-inoculated deer with potentially resistant PRNP genotypes per room. The inoculated deer in each room act as sources of CWD to the non-inoculated deer and shed CWD prions that mimic natural exposure. Groups of the non-inoculated deer have uncommon prion protein genotypes that are underrepresented in positive CWD cases and may represent potentially resistant genotypes. The genotypes we are testing in the current experiment have the following polymorphisms: GS96, SS96, QH95GS96, GS96QK226, or KK226. We sampled each of the animals enrolled in this study at baseline (prior to inoculation) and every three months post inoculation. Samples collected include rectal mucosal biopsies, blood, saliva, nasal swabs, feces, and skin (ear notch). Samples are used at the NADC and shared with numerous outside collaborators. We are monitoring the CWD status of the deer with immunohistochemical (IHC) staining of rectal biopsies. The first rectal mucosal biopsies were positive by immunohistochemistry at 3-month post-inoculation, and by the 15-month timepoint, we have confirmed 7/10 inoculated deer are positive. We suspect that the other 3 deer also are positive, but thus far there have been insufficient lymphoid follicles present in the biopsies from those deer to perform a complete assessment. To compare IHC with in vitro amplification methods, we also dedicate a small portion of rectal tissue for RT-QuIC analysis at a later date. So far, we do not have any inoculated animals with definitive clinical signs, but a single inoculated deer died from unrelated disease at 13-months post-inoculation. That deer had changes consistent with CWD such as loss of body condition present at the postmortem examination. The brain and retropharyngeal lymph node tested positive for CWD prions. Based on previous experiments, we expect to see the first clinical signs in the remaining inoculated deer later this year- about the time of our sampling timepoint in December 2021. The nasal swabs, saliva, and feces from inoculated deer will be tested by RT-QuIC to determine when shedding occurred during the study. In theory, this will approximate when the non-inoculated deer were exposed to CWD. We will continue to sample the non-inoculated deer to determine when and if they become rectal biopsy positive by IHC compared to RT-QuIC. This experiment will continue for several more years and eventually will yield information aimed to enhance CWD resistance in farmed white-tailed deer.