Chronic wasting disease (CWD) is a fatal, neurological illness that affects cervids, such as deer, elk, and moose. CWD is caused by abnormal proteins called prions that accumulate in the brain and other tissues of infected animals.
It can be transmitted through direct contact with bodily fluids or tissues of infected animals, or indirectly through contaminated environments or feed.
Currently, CWD has no cure or vaccine, and poses a serious threat to the health and welfare of cervids in North America.
CWD transmission from farmed to wild deer
JEAN-CHRISTOPHE VERHAEGEN/AFP via Getty Images
CWD was first discovered in 1967 in a captive mule deer herd in Colorado. Since then, CWD has spread to 24 states and two Canadian provinces, affecting both farmed and wild cervid populations.
One of the main sources of CWD transmission is the movement of infected farmed deer to other farms or to the wild.
According to a study by University of Minnesota researchers, 32% of CWD-infected herds in Minnesota, Pennsylvania, and Wisconsin experienced direct contact with imported deer from other herds that were later found to be CWD-positive.
This poses a substantial risk of introducing CWD to susceptible farmed deer, as well as to wild deer that may come into contact with escaped or released farmed deer.
To prevent this transmission pathway, several regulations have been implemented to control the movement and testing of farmed deer.
For example, the United States Department of Agriculture (USDA) requires that all farmed cervids be identified with official ear tags and that all deaths be reported and tested for CWD.
Additionally, some states have banned or restricted the importation of live cervids or cervid products from CWD-positive areas.
However, these regulations are not always enforced or complied with, and some gaps and loopholes still exist.
For instance, some states do not require testing of all farmed cervids, or allow exemptions for certain types of movements.
Another source of CWD transmission is the indirect contact between infected wild deer and farmed deer through shared environments or vectors.
The same study by University of Minnesota researchers found that several factors were associated with the risk of CWD transmission from wild deer to farmed deer, including:
The proximity of CWD-positive wild deer to farmed deer. The study found that the risk of CWD introduction increased by 7% for every kilometer closer a farm was to a CWD-positive wild deer within a five-kilometer radius.
The presence of scavengers inside or near the perimeter fence. Scavengers, such as coyotes, crows, or raccoons, can carry CWD prions from infected carcasses or environments and deposit them on farms where they can infect farmed deer.
The use of single perimeter fencing. Single fencing can allow direct or indirect contact between farmed and wild deer through gaps, holes, or overhangs. Double fencing can provide a buffer zone that reduces the likelihood of such contact.
The use of practices that could attract scavengers through the fence. These include placing water sources, feeders, mineral licks, or carcass disposal sites near the outer perimeter fence. These practices can lure scavengers and potentially infected wild deer closer to the farm boundary where they can transmit CWD prions.
To prevent this transmission pathway, several biosecurity measures have been recommended for farmed deer owners. These include:
Monitoring and managing the local wild deer population and their movements around the farm. This can be done by collaborating with wildlife agencies, hunters, and landowners to conduct surveillance and control programs for CWD in wild deer.
Installing and maintaining double perimeter fencing around the farm. This can create a physical barrier that prevents direct or indirect contact between farmed and wild deer. The buffer zone between the fences should be kept clear of vegetation and debris that could facilitate contact or attract scavengers.
Implementing proper sanitation and disposal practices on the farm. This can reduce the exposure of farmed deer to CWD prions from contaminated environments or materials. For example, water sources, feeders, mineral licks, and carcass disposal sites should be placed away from the outer perimeter fence and cleaned regularly.
The University of Minnesota researchers have developed a CWD Biosecurity Assessment tool that deer farmers can use to evaluate and mitigate the risks specific to their farms.
The tool is based on the results of their study and other research and provides information and recommendations on biosecurity practices for CWD prevention.
