Researchers still have much to learn about chronic wasting disease spread
LAURAN NEERGAARD AP medical writer with correspondent reports | Posted: Friday, October 6, 2006 12:00 am
WASHINGTON - 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, found in some deer, elk and moose in Wyoming, is in the same family of fatal brain illnesses as mad cow disease and its human equivalent. 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.
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 the latest 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.
Hoover explained that the likeliest sources for CWD prions - a type of protein - in the saliva were either nerve tissue in the animals' tongues or tonsils, where the disease is concentrated or amplified. Prions could easily be sloughed off those tissues into the saliva, he said.
But determining which tissue was ultimately or primarily responsible for transmission of the disease would be so difficult as to be impossible, Hoover said.
"If you find an infection in the blood, where does it come from?" he asked.
Currently, the only test that can be taken on a living subject is a tonsil biopsy, Hoover said. That's difficult to do, and fairly traumatic for the animals. Far better, he said, is to develop a test that could detect CWD prions in bodily fluids, such as saliva or blood.
"That'll be the holy grail," Hoover said. Such a test is off in the future, he said, because CWD prions are so diluted in deer saliva that they can't be found in current tests. One line of research he and his colleagues want to pursue is to concentrate the saliva and then see if those CWD prions can be more easily detected.
Of the 17 authors of the study, none were from Wyoming, he said, save Mike Miller of the Colorado Division of Wildlife, who works with Wyoming CWD researchers.
Terry Kreeger, CWD researcher for the Wyoming Game and Fish Department, said Hoover's research confirms something he's suspected for years - that saliva was a transmission route for CWD.
"It makes sense to me," said Kreeger, agreeing with Hoover that there may be other transmission routes yet to be discovered.
Kreeger said that in final stages, CWD infected animals become very thirsty, reluctant to leave a water source, and drool saliva. He speculated that stagnant water sources could prove to be yet another transmission route of the disease, if shared by sick and healthy animals.
Kreeger also noted that healthy animals will approach sick ones, to sniff and lick the sick animals out of curiosity.
Lloyd Dorsey, Wyoming representative for the Greater Yellowstone Coalition, also agreed that it made sense to identify saliva as a transmission route for the disease.
"I'm afraid this does not bode well for dense concentrations of deer and elk," specifically artificial feeding situations for deer or the feedgrounds for elk in northwest Wyoming, Dorsey said. In either case, dense concentrations of deer and elk simply provide more opportunities for the transmission of CWD.
"We've even seen in Colorado that healthy animals can get the disease simply by foraging in a pasture once used by sick animals. These prions are remarkably persistent in the environment," he said.