Chronic Wasting Disease (CWD) is related to a group of diseases known as transmissible spongiform encephalopathies (TSEs). TSEs include such diseases as scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle (aka Mad Cow Disease) and Creutzfeldt-Jakob disease of humans and are diseases of the nervous system that result in distinctive lesions in the brain.
The causative agent is believed to be a modified protein (prion). These modified proteins are typically found in nervous and lymphatic tissues, but recent experimental evidence shows prions can occur in muscle tissue of mice.
CWD infects elk, white-tailed deer, and mule deer, but is not known to infect livestock or humans at the present time. No treatment is known and the disease is typically fatal. The mode of transmission of CWD between animals is not known, although direct contact between infected and non- infected animals via saliva, urine and feces is the most likely route of transmission.
Contamination of soil by excreta from infected animals is thought to be another route of transmission, particularly among captive herds of deer and elk. However, the implication of environmental contamination in free-ranging animals is not clearly understood. Infected deer and elk can appear robust and healthy in the early stages of CWD and may take many years before they show clinical signs of the disease.
The clinical signs are not unique to the disease and can be due to other conditions such as malnutrition. Currently all testing for CWD requires the microscopic examination of a specific portion of the brain. Recently, a biopsy technique for tonsilar tissues from live deer has been developed; however, this test seems to only work for white-tailed deer and mule deer but not for elk.
The incidence of CWD in wild animals is of great concern. The disease was originally described in captive animals 35 years ago in Colorado. However, over the last five years, CWD has been found in wild herds in several surrounding states and Canada. In early 2002, CWD has been detected in wild deer in South Dakota, Wisconsin and New Mexico. Researchers speculate that CWD could have been transported long distance as a result of interstate shipment of infected animals.
The recent detection of CWD in the wild white- tailed deer herd in Wisconsin is of particular concern. White-tailed deer appear more susceptible than mule deer and elk to CWD with a greater percentage of the herd becoming infected. Until now, CWD was found in white-tailed deer herds in Colorado, Wyoming, and Nebraska where deer occur at densities of approximately 2-5 deer per square mile. In contrast in Wisconsin, deer are found at 75+ animals per square mile (conservative estimate by Wisconsin Department of Natural Resources). No one knows how rapidly CWD will infect white-tailed deer at these densities or what long term affect this disease will have on a herd of this size (approximately 1.6 million animals).