Deer probably spread a brain-destroying illness called chronic wasting disease through their saliva, concludes a study that finally pins down a long-suspected culprit.

The key was that Colorado researchers tested some special deer.

Chronic wasting disease is in the same family of fatal brain illnesses as mad cow disease, properly known as bovine spongiform encephalopathy, and its human equivalent, variant Creutzfeldt-Jakob disease.

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There is no evidence that people have ever caught chronic wasting disease from infected deer or elk.

But CWD is unusual because, unlike its very hard-to-spread relatives, it seems to spread fairly easily from animal to animal.

[Mad cow disease is thought to be transmitted when cattle on industrial farms, which are routinely fed the ground-up remains of other cattle, eat the brain matter of infected individuals. The brain matter contains prions, rogue proteins which reproduce themselves and slowly cause massive brain damage.

Variant Creutzfeldt-Jakob disease, first noticed in Britain in the late 1980s, is thought to be a result of eating beef from infected cattle. A related disease, kuru, was once common in a single tribe in New Guinea that practiced ritual cannibalism.]

Scientists were not sure how, primarily because studying large wild animals is a logistical nightmare. The sheer stress of researchers handling a deer caught in the wild could kill it.

Likewise, animals deliberately exposed to infections must be kept indoors so as not to spread disease, another stress for deer used to roaming.

So Colorado State University researcher Edward Hoover turned to fawns hand-raised indoors in Georgia, which has not experienced chronic wasting disease.

"This allows you to do this safely so the deer aren't freaking out," explained Hoover, who reported the first evidence of saliva's long-suspected role in Friday's edition of the journal Science. "These deer are calm and approachable."

Hoover took saliva from wild Colorado deer found dying of CWD, and squirted it into the mouths of three of the healthy tame deer — about 3 tablespoons worth.

Additional tame deer were exposed to blood, urine and feces from CWD-infected deer.

He housed the newly exposed deer in a specialized lab for up to 18 months, periodically checking tonsil tissue for signs of infection and eventually autopsying their brains.

All of the saliva-exposed deer got sick.

So did deer given a single transfusion of blood from a CWD-infected deer — not a surprise, as blood is known to transmit this disease's cousins. But it does reinforce existing warnings to hunters in states where CWD has been found to take precautions in handling their kills.

The three deer exposed to urine and feces didn't get sick. That doesn't rule out those substances, Hoover cautioned; he simply may not have tested enough animals.

Proving that saliva is able to spread CWD is important, so that scientists next can determine exactly how that happens in the wild, said Richard T. Johnson, a Johns Hopkins University neurology professor who headed a major report on prion science.

"You can move deer out of a pasture, put other deer into the pasture, and they'll come down with the disease. It's not even casual contact, it's contact with the pasture," Johnson said. "It must be something in their secretions."

Is it spread through shared salt licks? Or by drooling onto grass or into streams? Studying environmental contamination by the prions thought to cause CWD is among Hoover's next steps.

"It's very likely they could be shedding a lot of saliva" shortly before death, noted Richard Race, a veterinarian who studies CWD at the National Institutes of Health's Rocky Mountain Laboratories. "Saliva's a good bet."