Almost half of Oregon elk population carries advantageous genetic variant against CWD, study shows

(Press-News.org) URBANA, Ill. – Chronic Wasting Disease, a prion protein disease that is fatal in elk, deer, and other cervids, has spread rapidly across the United States since it was first identified in 1967. CWD has now reached Idaho near the Oregon border, causing concern for the Columbian white-tailed deer, a rare subspecies found only in two regions in Oregon.

The deer have little genetic protection against CWD, but a new study from the University of Illinois Urbana-Champaign shows that about half of Oregon elk carry a gene that makes them less susceptible to the disease. That could help slow down CWD’s progress into the state, but it is still important to monitor and manage cervid populations.

“CWD has not been detected in Oregon yet, but it’s almost inevitable that it will happen. We are collaborating with colleagues in Oregon to examine how quickly CWD is likely to spread once it gets into the state. We are studying how common the advantageous variant of the prion gene is in Oregon’s elk population,” said lead and co-corresponding author Yasuko Ishida, senior research specialist in the Department of Animal Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at U. of I.

CWD is caused by abnormally folded proteins known as prions, and it is related to scrapie in sheep and Creutzfeldt-Jakob disease in humans. The disease leads to progressive neurological damage, and it is always fatal. CWD is spread both by direct contact and through the environment, and it readily crosses between cervid species such as elk, deer, and mule deer.

While normal prion proteins exist in all mammals, they can misfold when an infectious prion enters the host. That initiates a chain reaction causing normal prion proteins to change shape and go rogue. Past research has shown that some variants of the gene for prion protein synthesis (PRNP) are associated with lower susceptibility to the disease. For elk, a codon that encodes the amino acid leucine rather than methionine is more frequently found in CWD-negative than -positive elk, indicating a genetic advantage for those animals. 

“We previously tested a small number of Columbian white-tailed deer in Oregon’s Douglas County, and we found that they were genetically very disadvantaged when it came to CWD. We didn’t find the protective variant of the gene in a single deer,” said corresponding author Alfred Roca, professor of animal sciences at Illinois.

“That’s why we wanted to look at the Oregon elk population, because if the elk get CWD, it's quite likely that Columbian white-tailed deer will get it as well.”

The researchers sequenced the coding region of PRNP in 183 elk collected across Oregon, including 82 Rocky Mountain elk from the eastern part of the state, and 101 Roosevelt elk found west of the Cascade Mountains. This is the first study to genotype the Roosevelt elk for the prion gene, Roca noted.

Ishida and Roca found that 42% of Roosevelt elk and 49% of Rocky Mountain elk carried at least one copy of the allele that encodes for leucine. That is a relatively high proportion compared to elk populations nationwide, they said.

“Without advantageous genetic variants, a whole population can be wiped out fairly quickly. It’s good to know that at least some of the elk in Oregon will be less susceptible. It’s not enough to prevent the spread of CWD once it enters the state, but it may slow it down a bit,” Roca said.

The Oregon Department of Fish and Wildlife (ODFW) tests extensively for CWD in elk and deer populations. Anyone who hunts and harvests an animal or wants to claim a vehicle-struck carcass can submit the head to a testing facility.

If an infection were detected, the ODFW would work to reduce herd density, said Colin Gillin, ODWF state wildlife veterinarian and a co-author on the study.

“Depending on the species, we may reduce herds in affected areas to keep the nose-to-nose contact at a bare minimum. There's absolutely no evidence of CWD crossing into humans, but people may still be reluctant to hunt if the disease shows up in their area. If we can keep the disease at low prevalence and prevent it from spreading across the 98,000 square miles of the state, it's a big benefit biologically, socially, and economically.”

“These susceptibility studies we conduct with the University of Illinois are hugely important; they inform the timing and type of management actions we would use,” he said.

The paper, “PRNP variant frequencies in Roosevelt and Rocky Mountain elk (Cervus canadensis) from Oregon and their implications for chronic wasting disease,” is published in the Journal of Heredity [DOI: 10.1093/jhered/esaf096]. The funding was provided by the USDA’s Animal and Plant Health Inspection Service.

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