Scientists prove environmental transmission of CWD

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A new study provides the first solid evidence that chronic wasting disease can be transmitted through environments contaminated by carcasses or excrement of infected animals, in addition to spreading through direct interactions between animals. The study appears in the June issue of the journal Emerging Infectious Diseases.

"The experimental findings show that we need to consider several potential exposure routes when attempting to control this disease," said Dr. Michael Miller, a veterinarian at the Colorado Division of Wildlife who was involved with the study. The study also involved researchers from Colorado State University and the University of Wyoming.

Chronic wasting disease was first identified in Colorado in 1967. Since then, the fatal neurologic illness has spread to wild and captive deer and elk in Illinois, Kansas, Minnesota, Montana, Nebraska, New Mexico, Oklahoma, South Dakota, Utah, Wisconsin, and Wyoming.

For their study, the researchers confined healthy deer in three sets of separate paddocks. In the first set, healthy deer were exposed to another deer already infected with CWD. In the second, deer were exposed to carcasses of deer that had died of CWD. In the third set, deer were confined in paddocks where infected deer had previously been kept. The researchers found that a few of the healthy deer contracted CWD in each exposure scenario over the course of one year.

"There has been some pretty good anecdotal evidence for quite some years," said Dr. Margaret Wild, a wildlife veterinarian in the biological resources management division at the National Park Service, Fort Collins, Colo. "Finally, we have some experimental data too. It shows that environmental contamination could be playing a role. How big a role? It's not clear at this point."

Previous disease models have been based on animal-to-animal contact as the sole source of infection and the premise that disease prevalence would decline as the number of infected animals is reduced. "Our findings that contaminated environments can cause transmission means that these declines in infection rates may be much slower than would be predicted by models that only consider animal-to-animal transmission," said Thomas Hobbs, PhD, a senior scientist from Colorado State University who was also involved in the study.

Dr. Wild says that the Park Service is vigilant about removing and testing all dead cervid carcasses in parks where the disease is known to occur, and that should help reduce disease transmission. Managing the disease in the wild is a different story.

"Realistically, when you are dealing with free-ranging populations, it is extremely difficult from a management approach to do anything about environmental contamination, so that makes approaches for attempting to eliminate the disease even more complex," she said.