In 1967, a syndrome known as chronic wasting disease was
first described in several wildlife facilities in Colorado
and Wyoming.
These facilities used deer from captive does, captured pregnant
does, and orphaned fawns for nutritional, metabolic and
disease studies. In a 12 year period, 53 mule deer and
one blacktail deer in these facilities showed signs of chronic
wasting disease. The affected animals were young adults
that had been within captivity for 2.5-4 years. These animals
had minimal contact with wild ruminants, domestic ruminants
or free ranging small mammals such as rodents and small
furbearers.
Chronic wasting disease is now found in captive and wild
elk and deer in both the United
States and Canada.
The disease has been demonstrated in RockyMountain
elk, mule deer, white-tailed deer, and black-tailed deer.
On many occasions, affected cervids have been in direct
or indirect contact with other ruminants such as cattle,
sheep, and goats, with no evidence of disease transmission
to these other ruminant species. There is also no evidence
of CWD being related to Transmissible Spongiform Encephalopathies
in humans. For years, the disease has occurred endemically
in free ranging elk and deer in northern Colorado,
southern Wyoming,
and southwestern Nebraska.
No animals in these areas are permitted to be moved to other
research facilities or translocated for repopulation. Recently,
the disease has also been found in free ranging deer and/or
elk in Wisconsin,
South Dakota,
and New Mexico.
Numerous positive animals have also been identified on farm
raised elk herds. They include farms located in South
Dakota, Nebraska,
Colorado,
Oklahoma,
Montana, Kansas
and the Canadian province
of Saskatchewan.
These farms were quarantined and many have been depopulated,
slaughtered and tested.
The signs of CWD are numerous and vary in severity and
time of onset. Males, females and castrates are affected
equally. The onset of signs can occur during any season
of the year. The signs may start with listlessness, progressive
weight loss, and depression that lasts over a 2 week –
8 month period of time. Many deer develop signs of polyuria,
polydipsia, excess salivation, odontoprisis, flaccid facial
muscles, drooping ears, lowering of the head, and terminal
anorexia. Many animals have changes in behavior which may
include a lack of awareness that is seen by a lowered head
and fixed stare for a period of minutes, followed by a return
to a normal state of consciousness. These signs progress
until the animal develops secondary complications, such
as pneumonia, that lead to the death or euthanasia of the
animal. Clinical pathological findings are non-specific
and are generally related to the secondary infections.
Many pathologic changes are seen on necropsy. Gross
pathological changes are varied and often non-specific.
The most common gross finding is severe emaciation; however,
some animals show no abnormalities at all. Many other findings
are due to secondary complications such as pneumonia, abscesses,
enteritis, and parasitism. Histopathologic changes of the
central nervous system are the most specific finding in
affected animals. These changes include microcavitation
(spongiform transformation) of the neuropil, intracytoplasmic
vacuoles in neuronal perikaryons and neuronal degeneration.
The spongiform transformation was seen in the gray matter
of the spinal cord, medulla oblongata, pons, mesencephalon,
thalamus, hypothalamus and cerebral cortex. The histologic
lesions are most consistently found in the “obex”
region of the medulla oblongata (caudal brain stem), and
it is this portion of the brain that is routinely tested
for CWD.
Because of the similar lesions, CWD was linked to a category
of other transmissible spongiform encephalopathies including
scrapie, bovine spongiform encephalopathy, transmissible
mink encephalopathy, Kuru and Creutzfeldt-Jakob disease.
These diseases are caused by unique agents called prions.
The incubation period of the prions is from weeks to years;
for this reason, they are known as a “slow virus”
disease. However, prions are an unconventional agent in
that they differ from viruses in many ways. They facilitate
the use of normal cellular pathways for replication. However,
no evidence of nucleic acid has been found in conjunction
with any prions. They are very resistant to many agents
that are used to neutralize many viruses such as ultraviolet
radiation, ionizing radiation, ultrasonication, proteases,
nucleases, heat, formaldehyde, chloroform and ether. It
is believed that prions are an abnormal protein made from
a normal protein-making gene. The normal gene, PrP gene,
encodes the formula for the production of a normal protein
called the “prion protein”, PrP. The protein
attaches to the outer layer of the plasma membrane lipid
bilayer. The function of the “prion protein”
is not known. The “prion protein” is normally
in an a-helix formation. The abnormal isoform forms a b-pleated
sheet that is protease resistant and, therefore, forms the
characteristic protein plaques that are often seen ultrastructurally
in many of these diseases. How the ingestion of an abnormal
protein can facilitate the transformation of a normal gene
to make an abnormal protein is not yet known.
Because of its similarity to bovine spongiform encephalopathy
and its apparent spread to wild deer populations outside
of the endemic area, there is increasing public concern
about CWD. Because of this concern, Indiana
now requires all captive elk herds to be registered with
the state and a number of restrictions have been placed
on the movement of captive elk. If you have any questions
about the management or transport of elk you are encouraged
to contact the State Veterinarian’s office (317-227-0300).
To date, there is no evidence that CWD exists in Indiana.
To give us greater confidence that Indiana
is free of CWD, the ADDL will cooperate with the State Veterinarian’s
office and the Indiana DNR this fall in a survey of hunter-killed
deer. It is hoped that we will continue to find no evidence
of CWD in Indiana;
however, if we do, it is important to keep in mind that
this disease is not known to pose a threat to domestic cattle
or humans.
If you have questions about the fall deer survey, you
are again encouraged to contact the Office of the State
Veterinarian. If you would like to learn more about chronic
wasting disease, an excellent review has been made available
by the Southeastern Cooperative Wildlife Disease Study (SCWDS)
on the internet at www.uga.edu/scwds/briefs.htm.
After accessing the site, click on Volume 18, issue 1.
-by Jill Rietdorf, Class of 2002
-edited by Dr. Duane Murphy, ADDL Pathologist
References:
1. Cotran R, Kumar V, Collins T: 1999. Transmissible
spongiform encephalopathies (prion diseases). Robbins Pathologic
Basis of Disease pp 1323-1326.
2. Jones TC, Hunt RD,
King NW: 1997. Prions. Vet Path pp 360-366.
3. USDA, APHIS. October, 2001. www.aphis.usda.gov/pa/pubs/fscwd.html
4. Williams ES, Young S: 1980. Chronic wasting disease
of captive mule deer; a spongiform encephalopathy. J Wildl
Dis 16:89-98.
5. Williams ES, Young S: 1982. Spongiform encephalopathy
of RockyMountain
elk. J Wildl Dis 18:465.471.
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