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Inside the Outdoors: Deer disease is a less familiar kind of invader

Minnesotans have become all too familiar with what are known as "invasive species." Some are plants, like Eurasian watermilfoil, which can take over shallow water environments, choke out native vegetation and make navigation and recreational use next to impossible. Others are animals, like the nonnative zebra mussel, which consumes so many minute organisms that it can drastically alter a lake's food chain; to say nothing of their razor-sharp shells attaching by the many thousands to underwater surfaces, including docks, boat lifts, outboard motors, water intakes and more. We know about tent caterpillars, sometimes mistakenly called "army worms," which can defoliate many thousands of trees during an outbreak.

There are quite a few others, on land and water, species that include, fish, plants, worms, beetles, tiny plankton and so on. But even the tiny ones are visible to the naked eye at some stage of their life cycle, and despite some outlandish alien shapes — the spiny water flea, for instance — we can relate because they have recognizable physical form. As difficult to control as some of these have proven to be, we're inclined to think there must be some way to control something we can see and touch, considering how good our species has become at controlling living things that compete or interfere with our priorities.

A very different kind of threat, potentially affecting one of our most treasured outdoor traditions, is back in the news. The invader can't be seen, or held in the hand, which makes it all the more mysterious compared to nonnative plants and animals we are trying to control. The timing of this latest news is no coincidence. It comes on the heels of the firearms deer season, following the testing of almost 3,000 hunter-harvested deer for chronic wasting disease — CWD — in Southeastern Minnesota. Two of those deer, harvested near Lanesboro, were found in Minnesota Department of Natural Resources (DNR) testing to have CWD.

If you're a deer hunter of any experience or seriousness, you know about CWD, and probably by initials more than by its full name. For the unfamiliar, it is an incurable, inevitably-fatal brain-destroying disease that infects members of the biological family that includes deer, elk and moose. CWD essentially turns the brain of its victim into a sponge, the destruction of vital neurological tissue resulting in losing normal bodily functions and body weight, abnormal behavior and eventual death. It is believed that signs of physical deterioration in deer are likely to occur within a year and a half of contracting CWD.

CWD in the deer family is not unique in the animal kingdom. A similar brain-destroying neurological disease, known colloquially as "mad cow disease," affects cattle. Mink raised in captivity on fur farms can contract their own version of it. Rare, but real, is the similar-in-effect Creutzfeldt-Jakob disease in humans, said to occur in about one in every one million persons worldwide. A mutation or variant of the human form was speculated as being related to an epidemic of mad cow disease in Great Britain in the early 1990's. Generally, however, the human and animal forms of this degenerative brain disease are not thought to be transmitted between our species.

On the other hand, there are mixed messages from the public health community. It is often stated that there has been found to be no "causal link" between eating a CWD-infected deer and developing the human version of the disease, Creutzfeldt-Jakob. Nonetheless, both health and wildlife management officials encourage hunters not to consume venison from an animal known to be infected. Further, hunters are advised to use caution in field dressing or processing deer harvested in an area with CWD-infected deer, precautions that would prevent direct contact with disease-carrying tissue or fluids.

The spread of the disease from one deer to another is believed to be by contact, perhaps such incidental contact as rubbing noses and passing the pathogen via fluid from mouth or nasal passages. Feces or urine are also thought to be possible vectors for disease transmission. The microscopic culprit is resistant to breaking down in the environment, so areas frequented by diseased animals might remain infectious and an indirect source of infection for healthy animals.

If you have a mind for statistics, you might be asking how big a problem it is when two deer out of almost 3,000 tested this fall were found to have CWD. One need only to look across the border from the Southeastern corner of Minnesota to Iowa, and across the river to Wisconsin. Both states have deer herds in which CWD has proliferated and is now common.

Neither Iowa nor Wisconsin has taken the kind of steps to control the disease that are planned by the Minnesota DNR. When a single whitetail shot near Pine Island — also in Southeast Minnesota — was tested and found to have CWD in 2011, the Minnesota DNR took drastic population control measures, hoping to eliminate any other deer that might have contracted the disease. Some 4,000 deer were "culled" in that effort.

The DNR is now planning a similar population control effort in the Lanesboro area, including a special winter hunt that will involve the public, landowners and potentially even government "sharpshooters." There will likely also be a ban on recreational feeding of deer, a practice that unnaturally concentrates them, making the spread of disease more possible. Unfortunately, landowners can accept or decline participation in the DNR's intensive harvest plan, which would seem to potentially undermine its effectiveness.

If the expected intensive harvest in the Lanesboro area happens, the deer population in that area will certainly suffer for several years at least, as was the case with Pine Island. One especially painful side-effect is the fact that special antler-point harvest restrictions intended to boost the number of mature "trophy" deer seemed to be paying off there, and a population control effort now will certainly set that program back.

One of the uncertainties is just where the infection came from. There are a substantial number of captive deer and elk farms in that part of Minnesota, some very close to Lanesboro and Pine Island. CWD has been found among animals on similar farms in other parts of the country, and deer are shipped periodically from one property to another. There are procedures in place intended to identify causes of deer dying unnaturally on these farms, or escaping from them, but some of this involves self-reporting.

Many are convinced that these game farms are the culprit, despite assurances from the State Board of Animal Health — which has official oversight of them — that there is no direct evidence in this latest outbreak. The fact that deer can swim rivers, in this case the Mississippi, and can cross the arbitrary boundary with Iowa — both states with unsolved CWD problems — suggests that there might continue to be a source of CWD infection just beyond Minnesota's borders. Between these two potential sources of infection, wildlife managers who hope to keep Minnesota deer CWD-free certainly have their work cut out for them.

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