Fact Pack

Spread of CWD

The unusual biological features of CWD pose significant challenges for wildlife managers attempting to control or eradicate the disease. Because agents of transmissible spongiform encephalopathies (TSEs) are extremely resistant in the environment, transmission may be both direct (from animal to animal) and indirect (from contaminated soil).(1)(2) Concentrating unnaturally large numbers of deer or elk in captivity or supplemental feeding of wild cervids increases the likelihood of direct and indirect transmission of CWD between individuals.(3) Contaminated pastures are thought to have served as sources of infection in some CWD epidemics; similar phenomena have been suspected in some outbreaks of sheep scrapie.(4) The apparent persistence of the CWD agent in contaminated environments represents a significant obstacle to eradication of CWD from either farmed or free-ranging cervid populations.(4)

The incidence of CWD can be remarkably high in captive cervid populations. In one infected research facility, more than 90% of mule deer over 2 years died or were euthanized due to illness from CWD.(5) Recently, a high CWD prevalence (about 50%) has been demonstrated in white-tailed deer confined at an infected Nebraska elk farm.(6) Among captive elk, CWD was the primary cause of adult mortality (five out of seven, 71%; four out of 23, 17%) in two research herds and a high prevalence (59%) was detected in a group of 17 elk slaughtered from an infected game farm herd.(7)

The potential for density-dependent disease transmission is greater among animals in captivity than in free-ranging wildlife. Captive animals are often held at higher than natural densities and thus are more frequently in direct contact and are more consistently stressed.(2)(3) Their repeated exposure to the same (potentially contaminated) soil may exacerbate effects of density on captive cervids.

CWD may be transmitted between captive and wild cervid populations, in either direction, and there is concern that transmission between cervids and cattle is possible, but this has only been demonstrated experimentally.(8) To date, cattle have rarely become infected when experimentally inoculated with CWD via intracerebral injection.(8)

Types of CWD Outbreaks

Three types of CWD outbreaks occur in the United States and Canada:

Endemic – CWD was first found in free-ranging deer and elk of southeast Wyoming and contiguous northeast Colorado. This endemic area is the only place CWD is known to be established as a self-maintained disease, with a prevalence rate less than 15 percent. The spread of CWD outward from this endemic area is very slow.(9)

Game farms – Once introduced into a commercial captive farm, CWD appears to readily become established, although it may be years before it is detected. The disease can spread over great distances via intrastate and interstate trade of affected animals. CWD can also move from farmed cervids to free-ranging deer near enclosures.(9)

Hot spots – Commercial transport has contributed to new foci of CWD outside of the endemic Wyoming and Colorado area. Controlling these hot spots, such as the occurrence in Southern Wisconisn, is critical to preventing the disease’s establishment and spread.(9)

Fatality of Disease and Prevention

CWD is a fatal disease that cannot be treated or vaccinated against. The disease’s modes of transmission and environmental contamination are poorly understood. Live-animal tests are not reliable at this time, though extensive effort is being focused on their design. Diagnosis and treatment are further complicated by CWD’s long incubation periods, subtle early clinical signs, and extremely persistent infectious-like agent.(4)

Colorado, Connecticut, Indiana, Idaho, North Carolina, Nebraska, New York, Texas, Virginia, and Wisconsin have ended imports of some cervids. Eight states have banned canned hunts – one of the primary reasons for cervid imports – completely.(4)

Some states, such as Idaho and Wisconsin, have taken steps to strictly regulate game farms and establish more sophisticated monitoring, tracking, and herd certification programs.(9)