New Research Supports Theory That Indirect Transmission Of Chronic Wasting Disease Possible In Mule Deer

The research confirms long-held theories that CWD can be indirectly spread through environmental sources, in addition to direct interactions between infected and healthy mule deer.

Funded by the National Science Foundation (NSF) and the National Institutes of Health, the study results were published on-line last week in the journal Emerging Infectious Diseases. The authors are Colorado Division of Wildlife (DOW) veterinarians Michael Miller and Lisa Wolfe, Colorado State University (CSU) scientist Thomas Hobbs and University of Wyoming scientist Elizabeth Williams.

"Diseases like CWD are poorly understood and of rising concern," said Sam Scheiner, program director in NSF's division of environmental biology, which funded the research. "This study provides significant new information showing the potential for transfer of the infection through the environment after many months. The knowledge will substantially alter how we manage the disease in wild and domestic animals."

Based on anecdotal observations, "we have long suspected that CWD could be transmitted when healthy deer were exposed to excreta and carcasses of mule deer that had the disease," said Miller. "Our findings show that environmental sources of infection may contribute to CWD epidemics, and illustrate how potentially complex these epidemics may be in natural populations."

Added Williams, "We've had a great deal of circumstantial evidence suggesting that indirect transmission occurs. The experimental findings show that we need to consider several potential exposure routes when attempting to control this disease."

Hobbs said the research could be important in helping to slow the spread of CWD.

"Ultimately, we want to develop models that predict the behavior of the disease," Hobbs explained. "For example, we would like to predict how prevalence changes over time in different areas of Colorado."

Hobbs said previous disease models have been based on animal-to-animal contact as the sole source of infection and that disease prevalence was expected to 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."

Miller said that while the research shows environmental contamination is possible in a captive setting, the impacts in the wild are still unknown.

The research 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 set, 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.

A few of the healthy deer contracted CWD under all three exposure scenarios over the course of one year.

Chronic wasting disease is a fatal neurological ailment of elk, white-tailed deer and mule deer. Most researchers believe the disease is caused by an aberrant prion protein that misfolds in the brain, destroying brain tissues as it progresses. Clinical signs include lethargy, excessive salivation, loss of wariness of predators and slowly deteriorating body condition. The disease is always fatal and there is no known cure or treatment to prevent CWD.

Federal and state health officials have found no connection between CWD and human health. As a precaution, officials recommend that the meat of animals infected with CWD should not be eaten.

"Although live deer and elk still seem the most likely way for CWD to spread geographically, our data show that environmental sources could contribute to maintaining and prolonging local epidemics, even when all infected animals are eliminated," Miller said. He said the appropriateness of various culling strategies may depend on how quickly the CWD agent is added to or lost from the environment.