By Russ Daly
Special to the Farm Forum
Lately I’ve been noticing one particular animal disease popping up more frequently and over widening areas in the U.S. – chronic wasting disease.
Chronic Wasting Disease (CWD) is a fascinating animal disease. It shares similarities with bovine spongiform encephalopathy (BSE, or “Mad Cow” disease) in cattle and scrapie in sheep. These diseases are caused by the most vexing of all pathogens – the prion. Prions are more basic than the simplest germs. They are simply protein molecules folded in a certain configuration. When pathogenic prions get into an animal, they interact with similar normal proteins, transforming them into the disease-causing version. Much of this takes place in the animal’s brain, spinal cord, and nerve tissue. After enough prions accumulate there, they create “holes” in those organs. Not a good thing to have happen in a brain!
The structure of prions makes them incredibly hard to inactivate. They survive rendering, disinfection, and surgical sterilization processes. This makes CWD control and prevention extremely challenging compared to other animal diseases.
Chronic wasting disease only affects deer and elk (and an occasional moose). It has shown up in free-ranging animals as well as those raised in captivity. Unlike BSE and scrapie prions, which are only found in brain and nerve tissue in their target species, CWD prions can be also found in an infected animal’s saliva, manure, and urine. Their hardiness in the environment means they can stay around a long time to possibly infect another animal. Deer or elk probably become infected when they lick or otherwise ingest contaminated feed sources or soil in their environments.
The U.S. geography of CWD is expanding. Even though testing’s been carried out for roughly 20 years now, CWD has just now been found in Montana, and is spreading in areas of southeastern Minnesota and southern Wisconsin, among other areas. In South Dakota so far, CWD has only been found in free-ranging deer and elk in the Black Hills (we’ve had a few captive herds with it, but they have long been cleaned up).
The signs of CWD progress slowly in an infected deer or elk and are usually only seen in adults. Those signs can be very subtle and are not unique to CWD: progressive weight loss, separation from the rest of the herd, listlessness, excessive salivation, and teeth grinding. In elk, hyperexcitability and nervousness have been noted. These are all due to the slowly progressing but relentless effects of the prion on the brain. There is no cure for CWD – it’s always fatal.
Because it’s a prion disease like BSE, CWD has caused worry about people becoming ill from eating deer or elk meat from infected animals. There’s no evidence that people have ever been affected with CWD. An experiment reported this past summer, however, revealed that monkeys fed meat from a CWD deer over a long period of time developed problems with CWD. Does that mean we should worry about our own health if we eat deer or elk meat?
A definite answer to that question hasn’t been determined yet. One would have to say the possibility is there. But we should consider all the facts at hand.
Remember that CWD is still very rare in the U.S. deer and elk population. While we’ll never do enough testing to find every CWD case, we know that it’s more prevalent in certain parts of the country than others. Accordingly, South Dakota deer outside of the Black Hills would appear to present a very low risk.
Also, just like any disease, the numbers of germs (prions in this case) are tremendously higher in sick animals compared to animals who may be exposed, but still act healthy. Therefore, the common sense notion of not eating meat from an animal that was acting “off” or in poor condition should prevail. Similarly, because we know the prion prefers nerve tissue, care should be taken to make sure that meat is boned out and that brain, spinal cord, and similar tissues aren’t consumed.
Chronic wasting disease appears to be expanding its geography as a threat to deer and elk. So far that threat hasn’t extended to people, but we all would be better served by more research into the disease, and more importantly, how its spread can be prevented.
Russ Daly, DVM, is the Extension veterinarian at South Dakota State University. He can be reached via e-mail at [email protected] or at 605-688-5171.