A recent study shows chronic wasting disease may prove especially slippery to eradicate. Scientists discovered that CWD, a disease affecting elk, white-tale and mule deer, can be passed on to uninfected animals up to two years after sick animals are gone. The June paper appeared on-line in the journal "Emerging Infectious Diseases" published by the Centers for Disease Control and Prevention.
"The traditional view of the spread of disease among people or wildlife is that there has to be some contact (among individuals)," said Thompson Hobbs, senior research scientist at Colorado State University and one of the paper's authors. "This research shows that that contact is not necessary."
Scientists placed uninfected mule deer in enclosures under different conditions. The results showed that animals can acquire the disease from grazing in an area where infected animals lived or carcasses decomposed approximately two years before.
Elizabeth Williams, professor at the University of Wyoming and co-author of the paper, said the results merely confirmed what scientists have suspected for a long time. It doesn't answer the question of how likely it is to happen, just that it can happen, she said.
Decaying carcasses and feces fertilize the surrounding area making for lush vegetation that deer and elk may be attracted to, Hobbs said. If animals are attracted to an infected area, they may be more likely to contract the disease, he said. Hobbs described one of the test sites where there was a "halo of green grass" around a decaying carcass.
The biggest implication of the research results may be for wildlife managers whose goal has been to eradicate or slow the spread of the disease by culling sick animals.
"Transmission by environmental contamination makes it a lot tougher job to do. It's not impossible, just more difficult," Hobbs said.
Most scientists believe misshapen proteins, called prions, are to blame for the disease. CWD can lay dormant for years, but is eventually fatal after onset and is characterized by the progressive deterioration of the nervous system. Symptoms include lethargy, excessive salivation, weight loss and changes in behavior or temperament. These same proteins exist naturally on the surface of cells, and only cause disease when they are folded in an unusual way, said Williams. Prions are thought to infect animals by reshaping normal proteins into the unusual pattern.
Prions are more concentrated in the brain, eyes, spinal cord and in lymph nodes, tonsils, pancreas and spleen than other areas of the body and have not been found in muscle tissue of deer or elk, according to the Colorado Division of Wildlife.
In the related disease in cattle, bovine spongiform encephalopathy or mad cow disease, prions were found in muscle tissue and the disease is believed to have infected humans, according to studies published in 1996 and 2002. Although CWD-causing prions have been shown to reshape normal human prions in a test tube, no human cases of a related disease have been linked to CWD, according to another June study published by the CDC.
Wildlife and health officials still recommend that people not eat infected meat. Hunters can have their animals tested for a $15 fee at state-sponsored testing facilities (the fee may be waived in some parts of the state). More than 45,000 animals have been tested in Colorado over the past two years, stated the state wildlife department.
Officials are clear, however that the testing should not be considered a food safety test. Positive results help officials determine where the disease is and how it has spread. A negative result does not guarantee that a carcass is not contaminated.
"Indeed, at this time there is no test that can be used reliably on individual animals to determine whether that animal is free from CWD," according to a U.S. Department of Agriculture report issued in February 2004. n