Frequently asked questions concerning Chronic Wasting Disease (CWD)
What is CWD?
Chronic wasting disease (CWD) was first recognized in 1967 as a clinical syndrome of unknown etiology
among captive mule deer Odocoileus hemionus at wildlife research facilities in
Colorado (Williams and Young 1992). The disease was diagnosed in 1978 as a spongiform
encephalopathy by histopathologic examination of CNS from affected animals. Shortly
afterward CWD was recognized among captive deer in Wyoming (Williams and Young
1980). Diagnosis of CWD in Rocky Mountain elk from these same facilities quickly
followed (Williams and Young 1982). Deer and elk in a few zoological gardens in
the United States and Canada were identified with CWD in subsequent years (Williams
and Young 1992). Apparently it did not persist in these locations. Chronic wasting
disease has recently become a concern to the game farm industry following its
diagnosis in elk in Saskatchewan, Canada, and in South Dakota, Nebraska, Montana,
Colorado, and Oklahoma.
In 1981, CWD was recognized in a free-ranging elk in
Colorado (Spraker et al. 1997). Subsequently, it was found in free- ranging
elk in Wyoming, and in free-ranging mule deer [1985 (M.W. Miller unpublished)]
and white-tailed deer Odocoileus virginianus [1990 (E.S. Williams unpublished)]
in both states. The known distribution of CWD currently includes captive and
free-ranging cervids in southeast Wyoming and north-central and northeast Colorado
(Miller et al. 2000) and several game farms in the United States and Canada.
- What causes CWD?
The origin of CWD is
not known. Spontaneous development of PrPres might have occurred in deer,
with subsequent transmission to other deer and elk. An alternate explanation is=
that CWD is actually scrapie occurring in cervids. Chronic wasting disease could
also have originated by infection with an as-yet-unrecognized prion.
Characteristics of the agent causing CWD are poorly
understood, but the agent is presumed to be a prion. Based on mouse strain typing,
it appears to differ from the BSE agent (Bruce et al. 1997), many strains of scrapie,
and the TME agent (M.E. Bruce personal communication).
The marked similarity of CNS lesions and epidemiology strongly suggests CWD agent
is the same in captive and free-ranging deer and elk.
- What is the epidemiology of CWD?
The mode of transmission
of CWD is unknown. There is no evidence that CWD is a food- borne disease associated
with rendered ruminant meat and bonemeal as was the case in BSE (Wilesmith et
al. 1988). Occurrence of the disease among captive deer and elk, many of which
were acquired as neonates, fawns, or adults, provides strong evidence of lateral
transmission (Williams and Young 1992; Miller et al. 1998; Miller et al. 2000).
Maternal transmission may also occur; however, this has not been definitively
determined. It is likely transmission occurred from mule deer to elk.
The scrapie agent is found in many lymphoid tissues,
including those of the digestive tract (Hadlow et al. 1980, 1982), suggesting
the agent may be shed through the alimentary tract. Lymphoid tissues of affected
deer and elk contain PrPres; thus, alimentary tract shedding may
also occur in CWD. The TSE agents are extremely resistant in the environment
(Brown and Gajdusek 1991); pasture contamination has been suspected of being
the source of scrapie agent in some outbreaks of sheep scrapie (Greig 1940;
Pálsson 1979). Concentration of deer and elk in captivity or by artificial
feeding may increase the likelihood of transmission between individuals.
The youngest animal diagnosed with natural CWD was 17
months of age, suggesting this as an approximate minimum incubation period;
however, without knowledge of when the animal was infected, it is impossible
to accurately determine the incubation period. Maximum incubation periods are
not known. Most cases of CWD among deer and elk residing in facilities with
a long history of CWD are in 3 7-year-old animals. The age of onset of clinical
signs is variable in animals brought into facilities as adults or among animals
in herds newly recognized to have CWD. For example, one elk in a presumed newly
infected herd was more than 15 years old. It is not known when during the course
of infection an animal may be infectious. In one study, more than 90% of mule
deer residing on a premises for more than 2 years died or were euthanized due
to CWD) (Williams and Young 1980). Chronic wasting disease was the primary cause
of adult mortality [5 (7 1%) of 7 and 4 (23%) of 23] in two captive elk herds
(Miller et al. 1998).
Relatively little is known about the epidemiology of
CWD in free-ranging cervids. In addition to necropsy and examination of brains
from animals showing clinical signs suggestive of CWD to determine its distribution
(targeted surveillance), brains from deer and elk harvested by hunters in the
CWD-endemic area have been used to estimate prevalence. Within endemic areas,
prevalence of preclinical CWD, based on histopathology and/or immunohistochemistry
for PrPres, is estimated at less than 1%-8% (Miller et al. 2000).
Chronic wasting disease has not been found in cervids outside the endemic areas.
Preliminary modeling suggested lateral transmission
is necessary to maintain CWD at the prevalence observed in surveillance programs.
Maternal transmission may occur, but in the model this route of transmission
alone was not adequate to maintain the disease at observed levels (Miller et
- What are the clinical signs of CWD?
The most striking
clinical features of CWD in deer and elk are loss of body condition and changes
in behavior. Clinical signs of CWD may be more subtle and prolonged in elk than
in mule deer. Affected animals may increase or decrease their interaction with
handlers or other members of the herd. They may show repetitive behaviors, such
as walking set patterns in their pens or pastures, show periods of somnolence
or depression from which they are easily roused, and may carry their head and
ears lowered. Affected animals continue to eat, but they consume reduced amounts
of feed, leading to gradual loss of body condition. As the disease progresses,
many affected animals display polydipsia and polyuria; increased salivation
with resultant slobbering or drooling; and incoordination, particularly posterior
ataxia, fine head tremors, and wide-based stance. Esophageal dilatation, hyperexcitability,
and syncope are rarely seen. Death is inevitable.
In captive herds newly experiencing CWD, sporadic cases of prime-aged animals
losing condition, being unresponsive to symptomatic treatment, and death from
pneumonia are common. Aspiration pneumonia, presumably from difficulty swallowing
and hypersalivation, may lead to misdiagnosis of the condition if the brain
is not examined.
The clinical course of CWD varies from a few days to a year, with most animals
surviving a few weeks to 3 4 months. Although a protracted clinical disease
is typical, occasionally acute death may occur in white-tailed deer (M.W. Miller
unpublished). Caretakers familiar with individual animals often recognize subtle
changes in behavior well before those not familiar with the particular animal
detect abnormalities or serious weight loss occurs. Also, those who have seen
clinically affected animals are more astute at detecting early behavioral changes
than naive observers.
The clinical course of CWD in free-ranging deer and elk is probably shorter
than in captivity. Wild cervids must forage, find water, and are susceptible
to predation, all factors affecting longevity of sick animals in the wild.
- How is CWD diagnosed?
Clinical signs of
CWD are not specific, and currently diagnosis is based on examination of the
brain for spongiform lesions and/or accumulation of PrPres. The parasympathetic
vagal nucleus in the dorsal portion of the medulla oblongata at the obex is
the most important site to be examined for diagnosis of CWD (Williams and Young
1993) and should be submitted for histopathologic examination on every animal
suspected of having CWD. The whole head or whole brain can be submitted to the
diagnostic laboratory to ensure that the correct portion of the brain is examined.
Supplemental tests include negative-stain electron microscopy for SAF or Western
blotting for detection of PrPres in brain (Williams and Young 1992;
Spraker et al. 1997).
PrPres in lymph nodes, tonsil, and conjunctival lymphoid tissues
is useful in antemortem diagnosis of sheep scrapie (Ikegami et al. 1991; Schreuder
et al. 1996, 1998; O'Rourke et al. 1998a). These techniques are currently being
tested in deer and elk to determine their sensitivity and specificity.