The discovery of bovine spongiform encephalopathy in a cow from northwestern Alberta has renewed speculation that chronic wasting disease can be passed from deer to cattle.
But researchers say the speculation is unfounded.
“Under any condition that even could be considered approaching natural transmission, researchers have been completely unsuccessful in passing CWD onto cattle,” said Murray Woodbury of the Western College of Veterinary Medicine.
“In short, cattle don’t catch chronic wasting disease from elk or deer out in the real world,” said Woodbury, speaking to cattle producers May 8.
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CWD is a member of a group of diseases called transmissible spongiform encephalopathies, which includes BSE in cattle, scrapie in sheep and Creutzfeldt-Jakob disease in humans.
Rob McNabb of the Canadian Cattlemen’s Association said his organization accepts there is no evidence BSE can be passed to cattle from deer and elk infected with CWD.
“I know there are folks out there that speculate about the origins of BSE coming from deer, but the science doesn’t support this. In fact, it seems to rule it out.”
The Canadian Food Inspection Agency confirmed May 22 that the BSE-infected cow near Wanham, Alta., had a typical BSE and not a form of CWD.
CWD was first found in 1967 and identified as a TSE about 10 years later. Researchers including Woodbury believe it was likely around for decades before that.
“Other prion diseases such as (CJD) in humans occur spontaneously in one in every million or so people. It is quite possible and very likely that CWD was around and the same spontaneous development happens in cervids. But at some point the disease is passed from cervid to cervid,” he said.
“It is quite likely that we are finding CWD because we are looking for it. It is still exceptionally rare.”
Beth Williams of the Wyoming State Veterinary Laboratory in Laramie, a leading CWD researcher, agrees that CWD in deer and elk and BSE in cattle are different diseases.
“The Lego pieces of deer and cattle don’t fit together,” she said.
Because of the different chemical and molecular structures of the prion mutation in the deer disease, CWD doesn’t readily infect bovines.
Williams said that in five years of tests at the University of Wyoming and in Colorado, scientists have been unable to transmit the disease from mule deer to cattle, other than when injecting infected CWD material directly into cattle brains.
Trials included cattle living in the same pastures and corrals with seriously infected elk and deer for six years. Cervids continued to die from the disease but cattle remained healthy.
Prion proteins, unlike viruses or bacteria, are not living organisms. Prions cannot reproduce and can be destroyed only by incinerating them at high temperatures or by using chemicals.
Infected prion proteins, or PnP, mutate neighbouring healthy PnP. Appearing only in the brain, lymph and bone tissues, CWD-infected PnP do not appear able to cross species barriers without significant assistance from scientists.
BSE was in the news in Great Britain because that prion disease can cross species barriers and infect humans as new variant CJD, as well as other animals.
Woodbury and Williams said because BSE and CWD are both in the family of TSE diseases and because BSE does appear to jump the species barrier, that is likely why the rare CWD cases have garnered so much attention from federal and regional governments.
Seven cases of CWD have been found in the wild population in Saskatchewan and none in Alberta or Manitoba.
