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) |