| Chronic wasting disease
(CWD) is a transmissible spongiform encephalopathy transmissible spongiform
encephalopathy (TSE) of deer and elk. To date, this disease has been
found only in cervids (members of the deer family). First recognized
as a clinical "wasting" syndrome in 1967 in mule deer in a wildlife research
facility in northern Colorado, it was identified as a TSE in 1978.
CWD is typified by chronic weight loss leading to death. There is
no known relationship between CWD and any other TSE of animals or people.
In the mid-1980s, CWD was detected in free-ranging deer and elk in contiguous
portions of northeastern Colorado and southeastern Wyoming. In May
2001, CWD
was also found in free-ranging deer in the southwestern corner of Nebraska
(adjacent
to Colorado and Wyoming) and later in additional areas in western Nebraska.
The
limited area of northern Colorado, southern Wyoming, and western
Nebraska in which
free-ranging deer and/or elk positive for CWD have been found is referred
to as the
endemic area. Soon after diagnosis of the disease as a TSE, Colorado
and Wyoming
wildlife management agencies stopped the movement of deer and elk from
their
research facilities; wild cervids have not been translocated from the endemic
area. In
2002, CWD also has been detected in wild deer in south-central Wisconsin,
southwestern South Dakota, the western slope of Colorado, southern New
Mexico,
and northern Illinois.
CWD also has been diagnosed in farmed elk and deer herds in a number of
States and
in two Canadian provinces. The first positive farmed elk herd in
the United States was
detected in 1997 in South Dakota.
Since then, 25 additional positive elk herds and two positive farmed deer
herds have
been found: South Dakota (7), Nebraska (4), Colorado (10), Oklahoma (1),
Kansas (1),
Minnesota (1), Montana (1), and Wisconsin (2). As of October 2002,
three of these 27
positive herds remain under State quarantine. Twenty-three of the
herds have been
depopulated or have been slaughtered and tested, and the quarantine has
been lifted
from one herd that underwent rigorous surveillance with no further evidence
of disease.
CWD also has been found in farmed elk in the Canadian provinces of Saskatchewan
and Alberta and in free-ranging mule deer in Saskatchewan. For more
information on
CWD in Canada, visit the Canadian Food Inspection Agency Web site at
www.inspection.gc.ca/english/anima/heasan/disemala/cwdmdce.shtml.
Species that have been affected with CWD include Rocky Mountain elk, mule
deer,
white-tailed deer, and black-tailed deer. Other ruminant species,
including wild
ruminants and domestic cattle, sheep, and goats, have been housed in wildlife
facilities in direct or indirect contact with CWD-affected deer and elk
with no evidence
of disease transmission. There is ongoing research to further explore
the possibility of
transmission of CWD to other species.
Causative Agent
The agent responsible for CWD (and other TSEs, such as scrapie and bovine
spongiform encephalopathy) has not been completely characterized.
There are three
main theories on the nature of the agent that causes CWD: (1) the
agent is a prion,
an abnormal form of a normal protein, known as cellular prion protein,
most commonly
found in the central nervous system. The abnormal prion protein “infects”
the host
animal by promoting conversion of normal cellular prion protein to the
abnormal form;
(2) the agent is an unconventional virus; (3) the agent is a virino, or
“incomplete” virus
composed of nucleic acid protected by host proteins. The CWD agent
is smaller than
most viral particles and does not evoke any detectable immune response
or
inflammatory reaction in the host animal. Based on experience with
other TSE
agents, the CWD agent is assumed to be resistant to enzymes and chemicals
that
normally break down proteins, as well as resistant to heat and normal disinfection
procedures.
Clinical Signs
Most cases of CWD occur in adult animals. The disease is progressive
and always
fatal. The most obvious and consistent clinical sign of CWD is weight
loss over time.
Behavioral changes also occur in the majority of cases, including decreased
interactions with other animals, listlessness, lowering of the head, blank
facial
expression, and repetitive walking in set patterns. In elk, behavioral
changes may also
include hyperexcitability and nervousness. Affected animals continue
to eat grain but
may show decreased interest in hay. Excessive salivation and grinding
of the teeth
also are observed. Most deer show increased drinking and urination.
Diagnosis
Research is being conducted to develop live-animal diagnostic tests for
CWD.
Currently, definitive diagnosis is based on postmortem examination (necropsy)
and
testing. Gross lesions seen at necropsy reflect the clinical signs
of CWD, primarily
emaciation. Aspiration pneumonia, which may be the actual cause of
death, also is a
common finding in animals affected with CWD. On microscopic examination,
lesions
of CWD in the central nervous system resemble those of other TSEs.
In addition,
scientists use a technique called immunohistochemistry to test brain tissue
for the
presence of the abnormal prion protein to diagnose CWD.
Epidemiology
The origin and mode of transmission of CWD is unknown. Animals born
in captivity
and those born in the wild have been affected with the disease. Based
on
epidemiology, transmission of CWD is thought to be lateral or from animal
to animal;
although maternal transmission may occur, it appears to be relatively unimportant
in
maintaining epidemics.
Surveillance
Surveillance for CWD in free-ranging deer and elk in Colorado and Wyoming
has been
ongoing since 1983 and has helped define the core endemic areas for the
disease in
those States. CWD in free-ranging deer in Nebraska was detected in
2000/2001; more
intensive surveillance to better define the prevalence and distribution
of the disease in
free-ranging deer in Nebraska is underway. In addition, an extensive
nationwide
surveillance effort was started in 1997-98 to better define the geographic
distribution of
CWD in free-ranging cervids in the United States. This surveillance
effort is a
two-pronged approach consisting of hunter-harvest cervid surveys conducted
in many
States, as well as surveillance throughout the entire country targeting
deer and elk
exhibiting clinical signs suggestive of CWD. Over 15,000 harvested
free-ranging deer
and elk have been tested to date, including over 13,000 animals harvested
from outside
of the endemic area. The recent finding of CWD in wild deer in areas
far removed from
the known endemic area has resulted in the development of plans for intensive
surveillance in the 2002-2003 hunting season to better define distribution
of the
disease in wildlife in the United States.
Surveillance for CWD in farmed elk began in 1997 and has been a cooperative
effort
involving State agriculture and wildlife agencies and the U.S. Department
of
Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS).
Farmed
cervid surveillance has been increasing each year since 1997 and will be
an integral
part of the USDA program to eliminate CWD from farmed elk.
Management
In each State where CWD has been detected in wildlife, State wildlife agencies
have
enacted response and/or management plans. APHIS has provided assistance
to
State officials in diagnosing CWD and in monitoring international and interstate
movements of animals to help prevent further spread of CWD. Also,
APHIS is
developing a program to eliminate CWD from farmed elk. In addition,
many State
animal health regulatory agencies have instituted CWD programs for farmed
elk. All of
these agencies are committed to limiting the distribution of the disease
in free-ranging
deer and elk to the current localized area and decreasing its occurrence
in both the
free-ranging and farmed deer and elk populations.
National CWD Program
In May 2002, Congress requested that USDA and the Department of Interior
develop a
plan to assist State wildlife management and agriculture agencies with
CWD
management. A CWD task force was formed to ensure that Federal and State
agencies cooperate in the development and implementation of an effective
national
CWD program. The task force delivered the Plan for Assisting States, Federal
Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive
Cervids to Congress in June 2002 and is currently developing an implementation
document for the plan. The plan addresses CWD diagnostics, communication,
information dissemination, management, research, and surveillance.
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