After 23 years since the first Alberta case, an infectious disease fatal to cervids is continuing to spread throughout the province.
Chronic wasting disease (CWD) targets species such as deer, moose, elk, reindeer, and caribou. The first Alberta case appeared in 2002, popping up in a farmed elk and a farmed white-tailed deer. Since then, case numbers have exploded. In 2023, the province found that nearly 20 per cent of the wild cervids tested were positive for CWD.
“Wherever CWD arrives, it will increase in abundance,” says Judd Aiken, a former professor at the University of Alberta’s Environmental Sciences Centre for Prions and Protein Folding Diseases.
Unlike other diseases, CWD isn’t caused by viruses or bacteria. Instead, CWD is a result of prions, misfolded proteins that cause fatal brain damage. Infected animals can survive for close to a year with the disease and don’t start showing symptoms until their final days. Symptoms include drastic weight loss, lack of coordination, drooling, excessive thirst, and a lack of fear of humans.
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The issue, Aiken points out, is that by the time these symptoms start showing, the animal has likely spread the disease to others.
“Three hundred nanograms of infectious agent is all that’s required to initiate an infection,” he says. “Three hundred nanograms is less than a grain of sand.”
CWD is spread through saliva, blood, urine, or feces. And Aiken says that the prions can exist in the environment for years, if not decades, clinging to vegetation and soil. “These infectious agents are exceptionally resistant to their destruction,” he says.
Researchers discovered the first ever case of CWD in a captive deer in Colorado in 1967. Aiken says scientists still argue over what caused this first case, whether it was naturally occurring or contracted through transmission.
From Colorado, CWD spread to surrounding states, such as Wyoming. The disease then jumped up to Canada in 1996 through farmed elk transported from South Dakota to Saskatchewan. CWD has since taken root in Saskatchewan and Alberta, with a few cases popping up in Quebec, B.C., and Manitoba.
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“On its own, this disease should have spread animal to animal, with a gradual spread from Colorado and Wyoming,” Aiken says. “It’s where we get these jumps of territory, where animals or animal carcasses have been vehicularly moved that we see the disease spread to new areas.”
At this stage, there’s no evidence that CWD can jump to species outside of cervids, such as livestock. Although, many livestock have their own version of a prion disease, such as mad cow disease in cows or scrapie in sheep. There’s also no evidence that the disease can jump to humans. However, the World Health Organization recommends that people shouldn’t knowingly consume meat from an animal with the disease.
Currently, there’s no cure for CWD. Once a cervid has contracted it, it’s fatal. Aiken says labs are working on a vaccine but it’s difficult because all the animals share the same protein sequence, prions just happen to be folded differently. “It’s very different than if you’re trying to make an antibody to the measles virus or something that is unique to the infected animal,” he says.
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Instead, Alberta is focusing on slowing the disease down. This is done through targeted hunts where experts reduce the deer population, trimming the edges of the disease to prevent it from spreading out. But this technique tends not to be popular with the public.
“Otherwise,” Aiken says, “we don’t have a good way of eliminating it once it’s here.”
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