A ‘Zombie Disease’ Is Killing Off Canadian Deer
Credit to Author: Lori Fox| Date: Wed, 30 Mar 2022 15:34:49 GMT
A strange, debilitating, and highly communicable infection is spreading throughout deer populations in Canada’s West.
Chronic wasting disease (CWD) is an alarming phenomenon which could have ecological and conservation impacts in the near future, experts say.
“There’s an epidemic of CWD in Alberta and Saskatchewan—and it’s already underway,” Margo Pybus, a wildlife disease specialist with the Alberta government’s fish and wildlife division and a researcher at the University of Alberta, told VICE World News. “This epidemic is raging through deer in the prairies and parklands.”
Originally detected in captive deer at a research facility in the late 1960s and later in wild populations in Colorado in 1981, CWD has since been found in 26 states. It’s now considered endemic to Colorado, Oklahoma, Kansas, Nebraska, Minnesota, Wisconsin, South Dakota, and Montana.
In Canada, the disease first emerged in 1996 on an elk farm in Saskatchewan, then spread into wild populations. Alberta confirmed its first case in December 2005 in a wild deer, a game animal taken near the Saskatchewan border, said Pybus. CWD was detected through a sample submitted under the hunter surveillance program, in which hunters provide samples of harvested animals to check for the disease.
“We’ve been doing steady surveillance since 2005,” said Pybus. “And now we’re looking at CWD encroaching on the eastern edges of Edmonton, Red Deer, and Calgary.”
Although the majority of infections have been observed in Alberta and Saskatchewan, Manitoba reported its first documented wild case in late 2021.
CWD belongs to a unique class of pathogens called prions, the same class of diseases to which bovine spongiform encephalopathy (BSE), more commonly called mad cow disease, belongs, as well as scrapie, which infects sheep and goats, and variant Creutzfeldt-Jakob disease (vCJD), which sickens humans.
Unlike bacteria and viruses that hijack host cells, no genetic information is involved in a prion disease infection; prions don’t contain genes and are instead composed of amino acids like any other protein, said Dr. David Westaway, the Canada research chair in prion diseases and director of the Centre for Prion and Protein Folding Diseases.
Deer are the most common host for the disease in North America, but CWD is capable of infecting any cervid, including elk, moose, and caribou. Infected animals eventually sicken, growing thin and weak. They may lose their fear of humans and other predators, and exhibit drooling, stumbling, poor coordination, depression, behaviour changes, and paralysis. These outward symptoms have led some lay people to refer to CWD as “zombie disease,” a moniker which is particularly apt because deer can transmit the illness through animal-to-animal contact, especially in urine and saliva.
CWD is a particularly “insidious” disease, said Pybus, as it can take infected deer up to two years to show outward symptoms, which really only appear “in the later stages, maybe the last month or so,” of infection. Prior to this, the infected deer behave and look normal, eating, moving, and even successfully mating as healthy animals do—all the while constantly shedding infectious prions. These prions don’t become inert once outside the host body, but remain infectious, building up in the environment and becoming a new vector for transmission over time.
“We believe that in the early stages of this disease getting into a new area, the primary [method of] transmission is direct, from one infected individual [deer] to an uninfected individual,” said Pybus. “But the longer CWD stays in an area and the more infected deer you have, you start to see a buildup of prions in the soil and eventually over time the environment starts to play a bigger role in infecting deer.”
To date, there have been no recorded cases in humans, but the U.S. Center for Diseases Control and Prevention “strongly recommends” having deer harvested from areas where CWD is known to be present before consuming it, and to not eat the meat if it tests positive.
Some of this concern comes from previous cases of prion diseases jumping the species barrier, such as the outbreak of mad cow disease in the 80s and 90s that devastated the European and North American beef industries and killed at least 232 people. When it infects people, BSE causes vCJD, a fatal illness that causes behaviour changes, tremors, dementia-like symptoms, and eventually death in humans unlucky enough to contract it, said Dr. Valerie Sim, a specialist in prion diseases with the University of Alberta.
Both human hunters and eaters can be exposed to CWD, either through improper handling of the carcass, which might allow blood or spinal and brain matter to enter the body, or through the simple consumption of the flesh—the prion that causes CWD doesn’t break down and remains infectious when cooked.
The data indicates that it’s cervids, not humans, that we need to be worried about.
“The really concerning thing about chronic wasting disease is the long-term effect it has on deer populations,” said Pybus. Deer that are born in areas where the disease is already established are more likely to be infected, and therefore die early, further shortening the population's reproductive capacity, Pybus said. In other words, the deer are dying before they have time to make more deer to replace themselves.
“That’s the other insidious component of this disease—it can take a long time before you start to see that declining reproduction actually set in,” said Pybus.
Although the precise effects of both the die-off and reproductive decline are difficult to document in the field, research has shown that some populations in Colorado and Wyoming (places where the disease has been well established) found around a 19 percent increase in mortality, Pybus said.
Attempting to curb the spread of the disease is complicated. Deer are constantly moving around and coming in contact with one another and introducing the disease to new areas, especially as climate change causes the ranges of some species to shift and overlap. Manitoba, which has just identified its first case through an already existing monitoring program, will hopefully be able to “hit it aggressively” and prevent it from spreading further. Alberta was able to do that for about three years, Pybus said, before becoming a “victim of their own success.”
“We lost the support for that program. If we could have retained that program, I firmly believe we could have kept CWD bottled up,” she said.
Ideally, though, you do whatever you can to keep it from spreading into new jurisdictions, Pybus said, particularly by thorough and consistent monitoring of wild populations.
“Preferably, you prevent it from getting there,” she said. “But when you first find it, hit it hard and hit it fast.”
Follow Lori Fox on Twitter.