Management of chronic wasting disease in ranched elk: conclusions from a longitudinal three-year study

Abstract

Chronic wasting disease is a fatal, horizontally transmissible prion disease of cervid species that has been reported in free-ranging and farmed animals in North America, Scandinavia, and Korea. Like other prion diseases, CWD susceptibility is partly dependent on the sequence of the prion protein encoded by the host's PRNP gene; it is unknown if variations in PRNP have any meaningful effects on other aspects of health. Conventional diagnosis of CWD relies on ELISA or IHC testing of samples collected post-mortem, with recent efforts focused on antemortem testing approaches. We report on the conclusions of a study evaluating the role of antemortem testing of rectal biopsies collected from over 570 elk in a privately managed herd, and the results of both an amplification assay (RT-QuIC) and conventional IHC among animals with a several PRNP genotypes. Links between PRNP genotype and potential markers of evolutionary fitness, including pregnancy rates, body condition, and annual return rates were also examined. We found that the RT-QuIC assay identified significantly more CWD positive animals than conventional IHC across the course of the study, and was less affected by factors known to influence IHC sensitivity - including follicle count and PRNP genotype. We also found that several evolutionary markers of fitness were not adversely correlated with specific PRNP genotypes. While the financial burden of the disease in this herd was ultimately unsustainable for the herd owners, our scientific findings and the hurdles encountered will assist future CWD management strategies in both wild and farmed elk and deer.

Keywords: Prion; RAMALT; RT-QuIC; antemortem; elk.

Figure 1.

Overview of antemortem testing and post-mortem findings (where available) from study years two and three. Elk were grouped into categories based on testing results, including: (1) “too young to test’ (TYTT), (2) negative by both immunohistochemistry (IHC) and real-time quaking induced conversion (RT-QuIC), (3) IHC positive and RT-QuIC negative or suspect, (4) IHC positive and RT-QuIC positive, (5) IHC negative and RT-QuIC positive, and (6) IHC negative and RT-QuIC suspect. No animals testing positive by either antemortem assay were found to be negative post-mortem, with many not returning for sampling the following year.

Figure 2.

Age of first detection of CWD infection in elk, based on combined antemortem and post-mortem testing data. Antemortem testing combined information collected from both the real-time quaking induced conversion assay (RT-QuIC) and immunohistochemistry (IHC), while post-mortem data was based on IHC testing. Genotypes were confirmed by sequencing of the PRNP gene.

Figure 3.

Annual return rates for yearling and two-year-old elk calves. Return rates of young elk were calculated based on animals present at inventory across the 2016–2018 sampling periods. Genotypes were confirmed by sequencing the PRNP gene.