- Abstract and Introduction
-
- Abstract
-
- Chronic wasting disease (CWD) of deer and elk is endemic
in a tri-corner area of Colorado, Wyoming, and Nebraska, and new foci of
CWD have been detected in other parts of the United States. Although detection
in some areas may be related to increased surveillance, introduction of
CWD due to translocation or natural migration of animals may account for
some new foci of infection. Increasing spread of CWD has raised concerns
about the potential for increasing human exposure to the CWD agent. The
foodborne transmission of bovine spongiform encephalopathy to humans indicates
that the species barrier may not completely protect humans from animal
prion diseases. Conversion of human prion protein by CWD-associated prions
has been demonstrated in an in vitro cell-free experiment, but limited
investigations have not identified strong evidence for CWD transmission
to humans. More epidemiologic and laboratory studies are needed to monitor
the possibility of such transmissions.
-
- Introduction
-
- Chronic wasting disease (CWD) is classified as a transmissible
spongiform encephalopathy (TSE), or prion disease, along with other animal
diseases, such as scrapie and bovine spongiform encephalopathy. The only
known natural hosts for CWD are deer (Odocoileus species) and Rocky Mountain
elk (Cervus elaphus nelsoni).[1,2] CWD and other TSEs are believed to be
caused by a pathogenic effect on neurons of an abnormal isoform of a host-encoded
glycoprotein, the prion protein. The pathogenic form of this protein appears
to be devoid of nucleic acids and supports its own amplification in the
host. TSEs in animals primarily occur by transmitting the etiologic agent
within a species, either naturally or through domestic husbandry practices.
In contrast, most such encephalopathies in humans occur as a sporadic disease
with no identifiable source of infection or as a familial disease linked
with mutations of the prion protein gene.[3] A notable exception among
the human TSEs is the variant form of Creutzfeldt-Jakob disease (vCJD),
which is believed to have resulted from the foodborne transmission of bovine
spongiform encephalopathy (BSE) to humans.[4,5]
-
- CWD was first identified as a fatal wasting syndrome
of captive mule deer in the late 1960s in research facilities in Colorado
and was recognized as a TSE in 1978.[6,7] Subsequently, this wasting disease
was identified in mule deer in a research facility in Wyoming and in captive
elk in both the Colorado and Wyoming facilities.[6ñ8] The disease
was first recognized in the wild in 1981, when it was diagnosed in a free-ranging
elk in Colorado.[1,9] By the mid-1990s, CWD had been diagnosed among free-ranging
deer and elk in a contiguous area in northeastern Colorado and southeastern
Wyoming, where subsequent surveillance studies confirmed it to be endemic.[10]
Epidemic modeling suggested that this wasting disease might have been present
among free-ranging animals in some portions of the disease-endemic area
several decades before it was initially recognized.[10] On the basis of
hunter-harvested animal surveillance, the overall prevalence of the disease
in this area from 1996 through 1999 was estimated at approximately 5% in
mule deer, 2% in white-tailed deer, and <1% in elk.[10] In 2000, surveillance
data indicated that the disease-endemic focus extended eastward into adjacent
areas of Nebraska,[1,11] and ongoing surveillance continues to redefine
the limits of this focus.
-
- Clinical manifestations of CWD include weight loss over
weeks or months, behavioral changes, excessive salivation, difficulty swallowing,
polydipsia, and polyuria.[1,6ñ8] In some animals, ataxia and head
tremors may occur. Most animals with the disease die within several months
of illness onset, sometimes from aspiration pneumonia. In rare cases, illness
may last for >/=1 year. In captive cervids, most cases occur in animals
2ñ7 years of age; however, the disease has been reported in cervids
as young as 17 months and as old as >15 years of age.[1] This disease
can be highly transmissible within captive deer and elk populations. A
prevalence of >90% was reported among mule deer in facilities where
the disease has been endemic for >2 years.[2,6,7,12] The mode of transmission
among deer and elk is not fully understood; however, evidence supports
lateral transmission through direct animal-to-animal contact or as a result
of indirect exposure to the causative agent in the environment, including
contaminated feed and water sources.[12]
-
- Geographic Distribution of Chronic Wasting Disease
-
- The geographic extent of CWD has changed dramatically
since 1996.[2] Two largely independent and simultaneous epidemics, one
in free-ranging deer and elk and another in the captive elk and deer industry,
appear to represent the main framework for explaining the disease's current
distribution.[2] More extensive and coordinated surveillance has provided
a clearer picture of its distribution over the last few years. Since 2000,
the disease in free-ranging cervids has been increasingly identified outside
of the original CWD-endemic areas of Colorado and Wyoming (Figure). The
observed distribution seems to be related in part to natural movement of
deer and elk and to commercial movement of infected animals to areas far
from the disease-endemic zone. Considerable attention has been given to
recent increases in the geographic spread of the disease, which in some
areas is likely a result of increased surveillance rather than evidence
of explosive geographic spread.
-
- No single original event or source links all wasting
disease foci documented to date. Given the disease's insidious nature and
the apparent duration (at least several decades) of epidemics among captive
and free-ranging cervids, gaps in knowledge about its spread and distribution
are not surprising, particularly within the captive deer and elk industry.
However, our current knowledge cannot explain some of the distinct foci
of CWD among free-ranging animals (e.g., in New Mexico and Utah). Thus,
unidentified risk factors may be contributing to the occurrence of CWD
among free-ranging and captive cervid populations in some areas.
-
- Chronic Wasting Disease in Free-ranging Deer and Elk
-
- In 2000, surveillance of hunter-harvested deer first
detected the occurrence of CWD in counties in southwestern Nebraska, adjacent
to the previously recognized areas of Colorado and Wyoming that are endemic
for this disease (Figure).[1,11] It was reported subsequently in other
Nebraska counties, including among deer and elk in a commercial, large
enclosure surrounded by a fence in northwestern Nebraska, where the prevalence
of CWD was >50%.[11] Free-ranging deer from areas surrounding the enclosure
also tested positive for the disease but at substantially lower rates.
In 2001, CWD in a free-ranging deer was identified in the southwestern
part of South Dakota along the Nebraska border close to an area where the
disease had been reported among captive elk.[13] Since then, additional
CWD-positive free-ranging deer and elk have been identified in southwestern
South Dakota.
-
- CWD in free-ranging cervids was first reported east of
the Mississippi River in Wisconsin among white-tailed deer harvested in
the 2001 hunting season.[14] Subsequent surveillance indicated that this
CWD epidemic focus was limited to several counties in the south-central
region of Wisconsin, although a second focus spanning the Illinois border
was also detected.[15] The absence of evidence for a widespread occurrence
of CWD and its low prevalence, despite a highly dense deer population,
indicate that the disease probably was recently introduced into Wisconsin.
Because the distance from the CWD-endemic area of Colorado-Wyoming effectively
precludes eastward migration of animals as a logical source of infection,
CWD in Wisconsin was more likely introduced by an imported infected cervid
or some other unidentified source.[14] The proximity of the Wisconsin-Illinois
focus to a white-tailed deer farm with infected animals appears to support
this explanation, as highlighted by the report of CWD in a previously captive
white-tailed deer approximately 7 months after it escaped into the wild
in southern Wisconsin.[14] The disease among the captive deer herd from
which the white-tailed deer escaped was demonstrated earlier, when a still-captive
deer tested positive for the disease. The captive source herd was held
in a facility 30ñ50 km from the Illinois location where CWD was
recently identified in a free-ranging deer.[16] In 2002, the Wisconsin
Department of Natural Resources launched an ambitious culling program by
providing special hunting permits to eliminate the disease in a designated
"eradication zone" around the areas where it was detected.[15,17]
Whether such aggressive management will succeed in eliminating free-ranging
foci of CWD remains to be determined.
-
- In Colorado, the Continental Divide initially appeared
to have prevented natural expansion of CWD into the western part of the
state. However, in 2002, the disease was confirmed for the first time in
several free-ranging deer harvested in western Colorado in an area surrounding
a commercial enclosure, where entrapped mule deer tested positive for CWD.
Aggressive culling of deer and elk surrounding the enclosure was initiated
to prevent further spread of the disease in the western slope of Colorado.
Through the 2002 hunting season, CWD-positive deer and elk continued to
be identified outside of the previously defined disease-endemic area, primarily
in northwestern Colorado.[18] This northwestern focus appears to be discontinuous
from the previously identified CWD-endemic area, although surveys conducted
in 2002 demonstrated that the western and southern boundaries of that area
were wider than previously believed. The ultimate source of this wasting
disease in northwestern Colorado remains unidentified.
-
- In 2002, samples from an emaciated, free-ranging mule
deer found in White Sands, New Mexico, tested positive for CWD.[1,19] No
cervids have been held in captivity close to the area where the New Mexico
deer was found, and the origin of the disease in this deer remains unknown.
In addition, CWD-positive, free-ranging deer have been identified in Wyoming
to the west over the Continental Divide from the known CWD-endemic zone.[20]
In 2003, a mature buck deer harvested in the fall of 2002 in northeastern
Utah tested positive for the disease[21]; additional cases have since been
found in central and eastern Utah (Figure). These cases provide additional
evidence for the potential spread of this wasting disease in the wild.
-
- In Canada, CWD was first detected in free-ranging cervids
(two mule deer) in 2001 in Saskatchewan; a few additional deer tested positive
in 2002 and 2003.[22] Saskatchewan Environment has implemented a herd-reduction
program using "control permits" to prevent further spread of
the disease among free-ranging cervids.
-
- Chronic Wasting Disease in Captive Deer and Elk
-
- CWD was first recognized in the captive elk industry
in Saskatchewan in 1996, but subsequent investigations indicated that the
most likely source of Canadian cases was captive elk imported from South
Dakota prior to 1989.[2,22] Since 1996, surveillance has detected infected
animals on more than 25 elk farms in Colorado, Kansas, Minnesota, Montana,
Nebraska, Oklahoma, South Dakota, and Alberta, Canada, and the Republic
of Korea.[1,14,23,24] CWD in most of these farms was identified in the
past 5 years. In 2002, the disease was detected in white-tailed deer on
farms in Alberta and Wisconsin.[23,25] More extensive and uniform surveillance
in captive white-tailed deer is needed to determine the full extent of
the disease in this industry.
-
- Captive herds with a CWD-infected cervid are often depopulated
both in Canada and the United States. Carcasses of depopulated animals
are incinerated or buried in accordance with local regulations. Meat from
depopulated animals has not been allowed to enter the human food and animal
feed supply.
-
- Transmission to Other Animals
-
- Concerns have been raised about the possible transmission
of the CWD agent to domestic animals, such as cattle and sheep, which may
come in contact with infected deer and elk or CWD-contaminated environments.
If such transmissions were to occur, they would potentially increase the
extent and frequency of human exposure to the CWD agent. In addition, passage
of the agent through a secondary host could alter its infectious properties,
increasing its potential for becoming more pathogenic to humans. This phenomenon
may have occurred with BSE when a strain of scrapie, a possible original
source of the BSE outbreak, changed its pathogenic properties for humans
after infecting cattle. However, the exact origin of BSE remains unknown.
-
- Although CWD does not appear to occur naturally outside
the cervid family, it has been transmitted experimentally by intracerebral
injection to a number of animals, including laboratory mice, ferrets, mink,
squirrel monkeys, and goats.[1,26] In an experimental study, the CWD agent
was transmitted to 3 of 13 intracerebrally injected cattle after an incubation
period of 22 to 27 months.[27] The susceptibility of cattle intracerebrally
challenged with the agent of this disease was substantially less than that
observed after intracerebral scrapie challenge: nine of nine cattle succumbed
to scrapie challenge after intracerebral injection.[28] In ongoing experimental
studies, after >6 years of observation, no prion disease has developed
in 11 cattle orally challenged with the CWD agent or 24 cattle living with
infected deer herds (E.S. Williams and M.W. Miller, unpub. data).[1] In
addition, domestic cattle, sheep, and goat residing in research facilities
in close contact with infected cervids did not develop a prion disease.
-
- Analysis by immunohistochemical studies of the tissue
distribution of prions in CWD-infected cervids identified the agent in
the brain, spinal cord, eyes, peripheral nerves, and lymphoreticular tissues
(Table 1).[29,30] Distribution of the CWD agent outside of the brain seems
to be less widespread in elk than in deer.[2] Involvement of the tonsils
and peripheral nerves early in the course of experimental and natural prion
infection suggests the possible involvement of the lymphoreticular and
peripheral nervous systems in the pathogenesis and transmission of the
disease.[2,12,30,31]
-
- Risk for Transmission to Humans
-
- Epidemiologic Studies
-
- The increasing detection of CWD in a wider geographic
area and the presumed foodborne transmission of BSE to humans, resulting
in cases of vCJD, have raised concerns about the possible zoonotic transmission
of CWD.[32] In the late 1990s, such concerns were heightened by the occurrence
of CJD among three patients 30 years of age who were deer hunters or ate
deer and elk meat harvested by family members (Table 2). However, epidemiologic
and laboratory investigations of these case-patients indicated no strong
evidence for a causal link between CWD and their CJD illness.[33] None
of the patients were reported to have hunted deer or eaten deer meat harvested
in the CWD-endemic areas of Colorado and Wyoming. Such a history in unusually
young CJD patients, if present, would have supported a causal link with
CWD. Moreover, the testing of brain tissues from >1,000 deer and elk
harvested from areas where the patients hunted or their venison originated
did not show any evidence of CWD.[33] In addition, the lack of homogeneity
in the clinicopathologic manifestation and codon 129 of the prion protein
gene among the three patients suggested that their illnesses could not
be explained by exposure to the same prion strain. In vCJD, homogeneity
of the genotype at codon 129 and the clinical and pathologic phenotype
were attributed to the patients' exposure to the same prion strain, the
agent of BSE.
-
- In 2001, the case of a 25-year-old man who reportedly
died of a prion disease after an illness lasting ≈22 months was
investigated (Table 2). Although this man had hunted deer only rarely,
his grandfather hunted deer and elk throughout much of the 1980s and 1990s
and regularly shared the venison with the case-patient's family. The grandfather
primarily hunted in southeastern Wyoming, around the known CWD-endemic
area. The case-patient's illness began with a seizure and progressed to
fatigue, poor concentration, and depression. Memory loss, ataxia, speech
abnormalities, combative behavior, and recurrent seizures also developed.
Histopathologic, immunohistochemical, and Western blot testing of brain
autopsy samples confirmed a prion disease diagnosis. Analysis of the prion
protein gene indicated a P102L mutation coupled with valine at the polymorphic
codon 129 in the mutant allele, confirming a diagnosis of Gerstmann-Strussler-Scheinker
syndrome (GSS). This case-patient was unusually young even for a person
with a GSS P102L mutation. It remains unknown whether the possible exposure
of the case-patient to CWD-infected venison potentially contributed to
the early onset of his prion disease.
-
- In 2001, two additional CJD patients 26 and 28 years
of age were reported from a single state (Table 2).[34] The patients grew
up in adjacent counties and had illness onset within several months of
each other. As a result of this fact and their unusually young age, a possible
environmental source of infection, including exposure to CWD-infected venison,
was considered. One of the patients died after an illness lasting 5ñ6
months that was characterized by progressive aphasia, memory loss, social
withdrawal, vision disturbances, and seizure activity leading to status
epilepticus and induced coma. Histopathologic, immunohistochemical, and
Western blot testing of brain biopsy and autopsy samples confirmed a CJD
diagnosis. The patient's disease phenotype corresponded to the MM2 sporadic
CJD subtype reported by Parchi et al.[35] This patient did not hunt, and
family members provided no history of regularly eating venison. The patient
may have occasionally eaten venison originating from the Upper Peninsula
of Michigan while away from home during his college years. However, ongoing
surveillance has not detected CWD in Michigan deer.[36]
-
- The second patient died from an illness lasting ≈16
months. The patient's illness began with behavioral changes, including
unusual outbursts of anger and depression. Confusion, memory loss, gait
disturbances, incontinence, headaches, and photophobia also developed.
Western blot analysis of frozen brain biopsy tissue confirmed a prion disease
diagnosis. Immunohistochemical analysis of brain tissue obtained after
the patient's death showed prion deposition consistent with GSS. A prion
protein gene analysis could not be performed because appropriate samples
were lacking. However, prion protein gene analysis of a blood sample from
one of the patient's parents indicated a GSS P102L mutation. The patient
did not hunt but may have eaten venison from Michigan once when he was
1ñ2 years old. The GSS diagnosis greatly reduced the likelihood
that the two patients reported from adjacent counties had disease with
a common origin.
-
- Recently, rare neurologic disorders resulting in the
deaths of three men who participated in "wild game feasts" in
a cabin owned by one of the decedents created concern about the possible
relationship of their illnesses with CWD (Table 2).[37] Two of the patients
reportedly died of CJD, and the third died from Pick's disease. More than
50 persons were identified as possibly participating in these feasts; the
three patients were the only participants reported to have died of a degenerative
neurologic disorder. Reanalysis of autopsy brain tissues from the three
patients at the National Prion Disease Pathology Surveillance Center indicated
that two of them had no evidence of a prion disease by immunohistochemical
analysis. CJD was confirmed in the third patient, who had clinicopathologic,
codon 129, and prion characteristics similar to the most common sporadic
CJD subtype (MM1/MV1).[35] This patient participated in the feasts only
once, perhaps in the mid-1980s. In addition, the investigation found no
evidence that the deer and elk meat served during the feasts originated
from the known CWD-endemic areas of Colorado and Wyoming.
-
- In 2003, CJD in two deer and elk hunters (54 and 66 years
of age) was reported.[38] The report implied that the patients had striking
neuropathologic similarities and that their illness may represent a new
entity in the spectrum of prion diseases. A third patient (63 years of
age), who was also purported to have been a big game hunter, was subsequently
reported from the same area. However, none of the three patients were reported
to have eaten venison from the CWD-endemic areas of the western United
States. The 66-year-old patient hunted most of his life in Washington State.
Although information about the 54-year-old patient was limited, there was
no evidence that he hunted in CWD-endemic areas. The third patient was
not a hunter but ate venison harvested from Pennsylvania and Washington.
The neuropathologic changes, Western blot profile, and genotype at codon
129 of the three patients each fit the MM1, VV1, or VV2 sporadic CJD subtype,
indicating absence of phenotypic similarity among the cases or atypical
neuropathologic features.[35]
-
- To date, only two nonfamilial CJD cases with a positive
history of exposure to venison obtained from the known CWD-endemic areas
have been reported. One of the patients was a 61-year-old woman who grew
up in an area where this disease is known to be endemic, and she ate venison
harvested locally. She died in 2000, and analysis of autopsy brain specimens
confirmed that the patient's CJD phenotype fit the MM1 subtype, with no
atypical neuropathologic features. The second patient was a 66-year-old
man who was reported to have eaten venison from two deer harvested in a
CWD-endemic area. Both deer tested negative for CWD, and the patient's
illness was consistent with the MM1 CJD phenotype.
-
- Despite the decades-long endemicity of CWD in Colorado
and Wyoming, the incidence of CJD and the age distribution of CJD case-patients
in these two states are similar to those seen in other parts of the United
States. From 1979 to 2000, 67 CJD cases from Colorado and 7 from Wyoming
were reported to the national multiple cause-of-death database. The average
annual age-adjusted CJD death rate was 1.2 per million persons in Colorado
and 0.8 in Wyoming. The proportion of CJD patients who died before age
55 in Colorado (13.4%) was similar to that of the national (10.2%). The
only CJD case-patient <30 years of age in Colorado had iatrogenic CJD
linked to receipt of human growth hormone injections. CJD was not reported
in persons <55 years of age in Wyoming during the 22-year surveillance
period.
-
- Laboratory Studies
-
- The possible interspecies transmission of prions can
be assessed with laboratory methods. In BSE and vCJD, several laboratory
studies provided crucial evidence that helped establish a causal link between
the two diseases.[39ñ41] These studies characterized the molecular
similarities of the agents causing BSE and vCJD and determined the lesion
profile and incubation period patterns of different panels of mice inoculated
by the two agents. Limited laboratory studies have been performed to molecularly
characterize CWD-associated prions and to compare them with prions from
human case-patients and other species. Strain typing studies involving
wild-type inbred mice indicated that the CWD agent from a mule deer produced
incubation-period and brain-lesion profiles different from those produced
by the agents causing BSE and scrapie.[39,42] These same strain-typing
techniques had identified the similarities of the etiologic agents of BSE
and vCJD, providing strong laboratory evidence for a link between the two
diseases.
-
- In human prion diseases, two major types of the proteinase-Kñresistant
prion protein fragment have been identified on the basis of their molecular
size by one-dimensional immunoblot analysis: type 1 migrating at 21 kDa
and type 2 at 19 kDa.[35] N-terminal protein sequencing indicated that
the cleavage site of the type 1 fragment is generally at residue 82 and
that of type 2 is at residue 97.[43] Prion strain diversity is believed
to be encoded in the three-dimensional conformation of the protein, which
determines the cleavage site and molecular size of proteinase-Kñtreated
prion fragment, indicating that the difference in molecular size may correlate
with strain differences. However, one-dimensional immunoblot analysis may
not identify more subtle differences that may influence the conformation
of different prion strains. Analysis of the glycoform ratios of prion fragments
and application of a two-dimensional immunoblot may help further identify
these subtle differences. On one-dimensional immunoblot analysis, the prion
fragment from several CWD-infected deer and elk migrated to 21 kDa, corresponding
to the type 1 pattern. This specific type has been identified in most cases
of sporadic CJD in the United States. However, the deer and elk prion fragment
differs from that in sporadic CJD cases in the glycoform ratio. In the
CWD-associated prion fragment, the diglycosylated form was predominant,
but in the CJD-associated prions, the monoglycosylated form was predominant.
Preliminary analysis using two-dimensional immunoblot indicated that the
CWD-associated prion fragment exhibited patterns different from that of
the CJD-associated prion fragment from a human patient with the type 1
pattern (S. Chen, pers. comm.). Although analysis of more samples from
cervids and humans is needed before meaningful conclusions can be made,
these molecular techniques could potentially be used to study the similarities
or differences in prion strains from cervids and humans with possible exposure
to CWD.
-
- The likelihood of successful interspecies transmission
of prion diseases is influenced by the degree of homology of the infecting
prion with that of the host endogenous prion protein. Such observations
have given rise to the concept of a "species barrier," which
would need to be overcome before an infecting prion strain caused disease
in a recipient host. In vitro cell-free conversion reaction experiments
have been developed to assess the degree of molecular compatibility of
disease-associated prions from one species with normal prion protein obtained
from a different species.[44,45] Such experiments specifically assess the
likelihood that an infecting prion would potentially initiate the formation
and propagation of pathogenic prions if it came in contact with normal
prion protein. A cell-free conversion experiment indicated that CWD-associated
prions can convert human prion protein into its abnormal conformer, albeit
at a very low rate.[44] The efficiency of this conversion was >14-fold
weaker than the homologous conversion of cervid prion protein and >5-fold
weaker than the homologous conversion induced by CJD-associated prions.
A similar low efficiency conversion of human prion protein by bovine- and
scrapie-associated prions was also reported.[44,45] Although a high level
of compatibility of prions in in vitro conversion reactions is believed
to correlate with in vivo transmissibility of the agents, the threshold
of compatibility efficiency below which no natural transmission should
be anticipated is unknown. A low level of compatibility of infecting prions
and host prion protein does not necessarily rule in or out natural interspecies
transmission of prion diseases. However, the comparably low-level in vitro
conversion of bovine prion protein by CWD-associated prions is consistent
with the relative in vivo resistance of cattle to CWD under all but the
most extreme experimental challenges. In addition, several other factors
may determine the in vivo transmission of disease-associated prions, including
dose, strain of the agent, route of infection, stability of the agent inside
and outside the host, and the efficiency of agent delivery to the nervous
system.[44,46]
-
- Conclusions
-
- The lack of evidence of a link between CWD transmission
and unusual cases of CJD, despite several epidemiologic investigations,
and the absence of an increase in CJD incidence in Colorado and Wyoming
suggest that the risk, if any, of transmission of CWD to humans is low.
Although the in vitro studies indicating inefficient conversion of human
prion protein by CWD-associated prions raise the possibility of low-level
transmission of CWD to humans, no human cases of prion disease with strong
evidence of a link with CWD have been identified. However, the transmission
of BSE to humans and the resulting vCJD indicate that, provided sufficient
exposure, the species barrier may not completely protect humans from animal
prion diseases. Because CWD has occurred in a limited geographic area for
decades, an adequate number of people may not have been exposed to the
CWD agent to result in a clinically recognizable human disease. The level
and frequency of human exposure to the CWD agent may increase with the
spread of CWD in the United States. Because the number of studies seeking
evidence for CWD transmission to humans is limited, more epidemiologic
and laboratory studies should be conducted to monitor the possibility of
such transmissions. Studies involving transgenic mice expressing human
and cervid prion protein are in progress to further assess the potential
for the CWD agent to cause human disease. Epidemiologic studies have also
been initiated to identify human cases of prion disease among persons with
an increased risk for exposure to potentially CWD-infected deer or elk
meat.[47] If such cases are identified, laboratory data showing similarities
of the etiologic agent to that of the CWD agent would strengthen the conclusion
for a causal link. Surveillance for human prion diseases, particularly
in areas where CWD has been detected, remains important to effectively
monitor the possible transmission of CWD to humans. Because of the long
incubation period associated with prion diseases, convincing negative results
from epidemiologic and experimental laboratory studies would likely require
years of follow-up. In the meantime, to minimize the risk for exposure
to the CWD agent, hunters should consult with their state wildlife agencies
to identify areas where CWD occurs and continue to follow advice provided
by public health and wildlife agencies. Hunters should avoid eating meat
from deer and elk that look sick or test positive for CWD. They should
wear gloves when field-dressing carcasses, bone-out the meat from the animal,
and minimize handling of brain and spinal cord tissues. As a precaution,
hunters should avoid eating deer and elk tissues known to harbor the CWD
agent (e.g., brain, spinal cord, eyes, spleen, tonsils, lymph nodes) from
areas where CWD has been identified.
-
- References: http://www.medscape.com/content/2004/00/47/95/479556/479556_ref.html
-
- © 2004 Centers for Disease Control and Prevention
(CDC) http://www.medscape.com/viewarticle/479556_1
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