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SPEAKER: Well, Saturday is the opening of Minnesota's rifle deer hunting season. Hunters across broad swaths of Minnesota will be required to have their deer tested for Chronic Wasting Disease, or CWD. There's growing concern that the disease is about to explode in Minnesota and cause long-term population declines. Two deer have already tested positive just in this year's early archery season. John Enger has more.

JOHN ENGER: First off, let's nail down what chronic wasting disease really is. All mammals, including humans, have what are called prion proteins. They're a basic building block of nerve and brain cells, harmless in their regular form. But sometimes those prions can mutate. And when they do, things get really ugly, according to CWD expert Bryan Richards.

BRYAN RICHARDS: It begins a domino-like effect inside the body, converting normal, healthy prions that you and I produce over into this disease-associated form, resulting in neurodegeneration and death.

JOHN ENGER: Richards says CWD is one of a group of diseases known as transmissible spongiform encephalopathies. Think mad cow disease, and you're getting close. In a CWD-infected deer, prions gather in the brain and form a sort of starchy paste, which eats away at the neurons.

BRYAN RICHARDS: So microscopically, we do see vacuolization or very sponge-like appearance. And each one of those vacuoles or holes is in association with where a neuron used to reside.

JOHN ENGER: In totally nonscientific terms, the deer's brain becomes like Swiss cheese. Within just a few years, a once-healthy deer becomes skinny and confused, wandering around with strings of infectious drool running from its mouth.

It's early yet in Minnesota. Only 20 wild deer have tested positive. But I wanted to get a look at what may be coming. What does CWD do to a deer herd over the long term?

So I went to Wisconsin, where in certain areas, half the deer are infected. And that's where I met Bryan Richards. He's the Emerging Disease Coordinator for the Wildlife Division of the US Geological Survey in Madison. It's like the CDC for animals.

BRYAN RICHARDS: I mean, we as USGS are kind of big on these fact sheet kind of things.

JOHN ENGER: He slides a glossy pamphlet across the table, then spends the next hour and a half laying out the state of CWD in Wisconsin, and it's not good. What started as a fairly localized outbreak near Madison 16 years ago has spread across the lower third of the state.

BRYAN RICHARDS: We now have evidence that when CWD is there long enough, it impacts at the population level.

JOHN ENGER: CWD deer die young. Richards estimates they have one fawn, instead of six. Studies show the disease will eventually shrink the herd.

BRYAN RICHARDS: So we know this occurs. We know what the likely outcome of disease is.

JOHN ENGER: This is what Minnesota has to look forward to, he says, if something dramatic isn't done. And that's what worries the Minnesota DNR's Michelle Carstensen. She runs the agency's disease management effort.

CWD was first found in wild deer in Southeastern Minnesota two years ago. The next winter, her team launched a special hunting season, hoping to reduce deer numbers so much the disease couldn't spread. They even brought in federal sharpshooters and killed 1,400 deer.

MICHELLE CARSTENSEN: And so we were aggressive right away, trying to get additional animals harvested and basically try to define our area of infection.

JOHN ENGER: But after just one year of intensive management, they ran out of money. And the effort petered out. She says she and her team are working on a new course of action.

Back in Wisconsin, Richards says sooner is always better because those faulty prions that cause CWD have staying power. Unlike a virus or bacteria, there's nothing to degrade. It's just protein.

So say, a hunter kills an infected deer and guts it in the woods. Inevitably, a scavenger, like a coyote, will eat those guts then travel 5 or 10 miles and relieve itself. The scat disintegrates in the grass over the next year or maybe five. And after all that, if another deer eats that grass, there's a good chance it just contracted CWD. Richards says to control that disease, Minnesota will have to do more than one year of sharpshooting. John Enger, Minnesota Public Radio News, Madison.