Studies to Date Show Chronic Wasting Disease Not Jumping to Cattle

Officials have been working to study, control and eradicate Chronic Wasting Disease since its discovery in the 1960’s. Researchers are now in the last years of three CWD transmissibility studies and findings so far indicate the disease is unlikely to spread to cattle.

The studies, which enter their sixth year in June, are examining transmissibility under three circumstances: healthy cattle living with infected cervids (deer), feeding healthy cattle CWD-infected tissue and injecting the brains of healthy cattle with CWD-infected tissue.

Chronic wasting disease belongs to a group of diseases called transmissible spongiform encephalopathies (TSEs). Bovine spongiform encephalopathy (BSE), commonly known as “mad cow disease,” and scrapie, which affects sheep, are also TSEs. Typically when diagnosing CWD, researchers examine brain tissue for PrPcwd, the protein marker for the disease.

Co-habitation

Wildlife veterinarians Mike Miller, Colorado Division of Wildlife, and Terry Kreeger, Wyoming Game and Fish, have just over four years remaining in their 10-year trial at a Fort Collins, Colo., facility that put healthy cattle in pens with healthy and CWD-infected deer. The study started with 12 young, healthy deer and cattle being placed in a series of pens with deer that were naturally infected with CWD. The setting was the most natural possible with the animals sharing a common water source and moving in and out of each other’s domain.

One calf was lost to health problems unrelated to CWD soon after the study onset, but all of the other cattle are healthy. Out of the 12 healthy deer, however, only one remains; most of the rest became infected with, and died from, CWD. 

Oral exposure

Elizabeth Williams, veterinary pathologist and wildlife disease specialist at the University of Wyoming, is heading up a study in which healthy cattle were orally inoculated with tissue from mule deer naturally infected with chronic wasting disease. With just over four years remaining in this 10-year trial, all of the 12 cattle that started the study are healthy. So far, it does not appear CWD will spread to cattle through oral exposure. This is different from BSE, which seems to be transmitted via ingestion of prion-infected tissue. 

Brain inoculation

At the USDA National Animal Disease Center in Ames, Iowa, 13 calves were intracerebrally inoculated in 1997 with brain tissue from mule deer naturally infected with CWD. Drs. Amir N. Hamir and Janice M. Miller are the lead researchers on this 10-year study of CWD. During the first five years of the study, eight animals were euthanized because of health concerns. Although examination of the brain tissue did not show classical lesions of a TSE, an abnormal prion protein (PrPres) was detected. Two more cattle have died in the last year with one testing positive for PrPres. Three cattle remain entering the final four years of the study.

This study used an unnatural exposure route that presented the most aggressive and severe challenge to transmission. In this case, researchers judge ease of transmission, which they determine by considering the number of animals affected and the disease incubation period. Since CWD does not appear to be easily transmitted to cattle through brain inoculation, compared to CWD to sheep and scrapie to cattle, for example, it is likely that transmission by a more natural route, such as oral exposure, would be much more difficult to accomplish. The research at Wyoming backs up that statement, since all cattle inoculated orally with CWD continue to be healthy.