Still much to learn about chronic wasting disease

Happily, after recently testing 1,180 deer near Pine Island, the Department of Natural Resources found no additional whitetails afflicted with chronic wasting disease (CWD) in southeastern Minnesota, near where one was killed by an archer last fall.

Still, CWD looms as a potential threat in Minnesota, and there is much we do not understand about the disease. This could hinder our response if ever it does become prevalent.

What do we know about CWD? We're confident that people can't become infected. And while saliva and contaminated soil are suspected agents in the transmission of CWD from deer to deer, it's not clear how deer become infected in the wild. Finally, captive deer and elk farms often are blamed for introducing CWD to an area. But even this is uncertain in all cases.

The (nonliving) beast

Most scientists think CWD is caused by a naturally occurring protein that undergoes a change in shape and accumulates in the brain (and elsewhere), causing a neurological meltdown. These misshapen proteins are called prions, which physically contact normal versions of the protein and cause them to go bad. A deer can be infected for several years before showing outward symptoms of CWD. During this time it can shed prions, much as we shed flu virus via sneezing before we feel sick.

People and CWD

There is no known instance of people getting CWD from eating infected deer or elk. But we remain curious, and some answers might be forthcoming: In 2005, a fire company in upstate New York hosted a fundraiser at which venison from infected deer was unknowingly served. Scientists are monitoring 81 people who ate the venison or were in some way exposed, and there should be some results soon.

How do deer in wild get CWD?

We suspect a deer can get infectious prions from another deer or the soil. But a deer might also develop them spontaneously. Cattle and humans develop prion disease spontaneously; the frequency for humans is about 1 in a million. If that was true for deer, with several million in the upper Midwest, a few could get the disease without contacting an infected animal.

Feeding and baiting deer could cause prions to be transmitted via saliva left in, say, a corn pile. Another route for CWD transmission is via soil that is contaminated by deer urine and feces. Both routes have been established experimentally in captives, but in the case of saliva, the report added that researchers had to use "likely unrealistic doses to be acquired in a natural setting."

In another study on captive deer, bedding was inoculated with CWD prions, and the two exposed deer developed CWD within 19 months. However, the deer used the same bedding for 570 consecutive days, and the bedding was refreshed daily with prions. This establishes the potential for environmental contamination but does not clarify what is going on in the wild. A genetic study showed that related deer were more likely to get CWD, suggesting that deer infect each other via direct contact rather than through the soil. And while some whitetails are genetically less likely to get CWD, there appears to be no resistant type.

Captive herds and CWD

Counterintuitively, perhaps, if a captive herd develops CWD, it might indicate that CWD is in the local wild herd. If, say, wild deer contact captive deer at a fence, CWD might be transferred to the captive herd, where it will be noticed. Deer are abnormally concentrated and live longer in game farms, both factors that lead to discovery of infected animals -- wild deer might be harvested before showing outward symptoms.

Future concerns

In parts of the CWD core area in southern Wisconsin, infection rates of 20 percent have been reported. We do not yet know the population-level consequences of CWD. A worst-case scenario would be a drastic reduction in deer numbers, when the rate of spread would decline. Perhaps then a more resistant deer will emerge.