Chronic Wasting Disease in Free-Ranging Wisconsin White-Tailed Deer
Damien O. Joly,* Christine A. Ribic,* Julie A. Langenberg,† Kerry
Beheler,† Carl A. Batha,† Brian J. Dhuey,‡ Robert E. Rolley,‡ Gerald
Bartelt,‡ Timothy R. Van Deelen,§; and Michael D. Samuel*¶
States Geological Survey-Wisconsin Cooperative Wildlife Research Unit,
University of Wisconsin-Madison, Madison, Wisconsin, USA; †Wisconsin
Department of Natural Resources, Madison, Wisconsin, USA; ‡Wisconsin
Department of Natural Resources, Monona, Wisconsin, USA; §Wisconsin
Department of Natural Resources, Rhinelander, Wisconsin, USA; and ¶United
States Geological Survey–National Wildlife Health Center, Madison,
Suggested citation for this article: Joly DO, Ribic CA,
Langenberg JA, Beheler K, Batha CA, Dhuey BJ, et al. Chronic wasting
disease in free-ranging Wisconsin white-tailed deer. Emerg Infect Dis
[serial online] 2003 May [date cited];8.
Deer shot within 5 km during the 2001 hunting season in Wisconsin tested
positive for chronic wasting disease, a prion disease of cervids.
Subsequent sampling within 18 km showed a 3% prevalence (n=476). This
discovery represents an important range extension for chronic wasting
disease into the eastern United States.
Chronic wasting disease (CWD) is degenerative and usually considered to
be fatal in White-tailed Deer (Odocoileus virginianus), Mule Deer
(O. hemionus), and Elk (Cervus elaphus) associated with the
presence of transmissible protease-resistant prion proteins
Although the transmission route of PrPcwd is unknown, it may be
transmitted in deer and elk by direct contact or indirectly from the
environment (1,2). In
experiments, clinical signs have appeared as early as 15 months after
exposure (1) and
include weight loss, anorexia, repetitive behaviors, hyperesthesia, and
intractability. Signs progress to severe emaciation, extreme behavioral
changes, excessive salivation, tremors, and mild ataxia (1,2). CWD
was first recognized in captive Mule Deer in Colorado (3) and
subsequently described in the free-ranging cervid populations of Colorado
and Wyoming (1);
prevalence in these disease-endemic areas varies spatially and among the
three sympatric cervid species (4). Before
its discovery in Wisconsin, CWD was detected in captive cervid farms in
Colorado, Nebraska, South Dakota, Oklahoma, Kansas, Montana (USA), as well
as Alberta, Saskatchewan (Canada), and South Korea (1). Apart
from the contiguous areas of Colorado, Wyoming, and Nebraska, CWD had
previously only been detected in two free-ranging Mule Deer from
Saskatchewan, one Mule Deer from South Dakota, and in a number of Mule
Deer from the western slopes region of Colorado (1).
Previously, no cases of CWD were reported east of the Mississippi;
however, subsequent to our research, CWD-positive cervids were found in
Minnesota (captive Elk), Wisconsin (captive White-tailed Deer and Elk),
and Illinois (free-ranging White-tailed Deer). Further, west of the
Mississippi, the following CWD-positive animals have been found: Mule Deer
in New Mexico and Utah; free-ranging Mule and White-tailed Deer in
Saskatchewan, Canada; and captive Elk and White-tailed Deer in Alberta,
In autumn of 1999 and 2000, the Wisconsin Department of Natural
Resources (WDNR) submitted to the National Veterinary Services
Laboratories (NVSL) (Ames, Iowa) brain material (obex) from 657
hunter-killed White-tailed Deer registered at hunter check stations across
the state. None came from the study area we describe. Samples were tested
for CWD prion by immunohistochemistry (IHC) (5). Prion
was not detected in any samples. However, 3 of 445 White-tailed Deer shot
in autumn of 2001 were positive for CWD. These deer were males, 2.5 years
of age, and were shot within 5 km in south-central Wisconsin. WDNR
subsequently conducted a sampling program to assess the distribution and
prevalence of CWD in the vicinity of these three positive deer. We report
the results of this sampling program.
Samples were collected from 500 adult (>1 year of age) White-tailed
Deer within an approximate 18-km radius, and all samples were tested for
CWD. Deer were submitted by hunters who were issued scientific collection
permits, collected at roadside after vehicular collison, or collected by
WDNR or U.S. Department of Agriculture sharpshooters. Data from collected
deer included the geographic location based on the Wisconsin Public Land
Survey System (township-range-section), sex, and age (estimated by using
tooth eruption and tooth wear patterns ). Location of kill was
indicated on a map by hunters during interviews by DNR staff. Samples of
brain stem (obex) and retropharyngeal lymphatic tissue were fixed in 10%
buffered formalin and submitted to NVSL for testing using IHC. We
considered a deer to be CWD positive if either obex or retropharyngeal
samples were IHC positive (1).
We used the spatial scan statistic provided by Kulldorff and Nagarwalla
(program SaTScan available from: URL: http://www3.cancer.gov/prevention/bb/satscan.html)
to assess the presence and location of CWD clusters within the
surveillance area. Location data were collected to the survey unit
“section” (approximately 2.6 km2). We pooled locations into 4X4
section quadrats for analysis to compensate for sections from which no
deer were collected. In a separate analysis, sex and age were assessed as
predictors of CWD status by using logistic regression (function glm in
program R v. 1.5.0; available from: URL: http://www.cdc.gov/ncidod/EID/vol9no5/disc08.htm)
Model selection uncertainty was incorporated into the odds ratio (OR)
estimates by using model averaging (9).
From March 2 to April 9, 2002, samples were collected from 505 deer;
however, 29 deer were not included in the analysis because of sample
autolysis, inappropriate tissue submission, or lack of availability of
appropriate tissues (e.g., deer with no intact cranium or those shot in
the head). Of the remaining 476 deer (87 males, 386 females, and 3 for
which sex was not recorded), 15 (3.2%; 95% confidence limit [CI] 1.7% to
5.1%) were IHC positive, 11 in both obex and retropharyngeal lymph node
samples and 4 from lymph nodes only. We inferred that deer that were only
lymph node positive were in the earlier states of infection (1).
Estimated prevalence varied spatially within the surveillance area. A
cluster of higher than expected prevalence was detected in the
north-central region of the sampling area (prevalence 9.4%; 95% CI 5.0% to
16.0%; p=0.003; n=127) (Figure).
Prevalence did not vary by sex (males: 3.4%, 95% CI 0.1% to 9.7%, n=87;
females: 3.1%, 95% CI 1.6% to 5.3%, n=386; male vs. female OR 1.1, 95% CI
0.56 to 2.19), a pattern consistent with Mule Deer sampled in Colorado and
Increasing prevalence with age was suggested, although we could not
distinguish whether the OR differed from 1 (OR 1.13, 95% CI 0.93 to 1.39).
We had a small sample (n=32) of older animals (>5 years of age), which
weakened our ability to detect an increase in prevalence with age
statistically. Miller et al. (4) found
that CWD prevalence increased with age in male Mule Deer and then abruptly
declined in older age classes. We did not have a sufficient sample size to
evaluate a sex difference in prevalence by age.
The known range of CWD was extended by its detection in Wisconsin,
which is the first report of the disease east of the Mississippi
River. Although we do not know how the free-ranging deer population
of Wisconsin became affected by CWD, the most commonly suggested
hypothesis is that CWD in Wisconsin may have emerged through importing of
an affected cervid. The current enzootic of CWD in free-ranging deer and
elk is paralleled by an enzootic in the captive cervid industry, and the
relationship between CWD-affected elk farms and recent (2000–2002)
diagnoses of CWD in free-ranging deer in Nebraska, South Dakota, and
Saskatchewan remains under investigation (1).
Elk were imported to Wisconsin from CWD-affected herds in Colorado during
the 1990s, and recently (September and October 2002) captive White-tailed
Deer were found to be positive on two separate farms in central and
southern Wisconsin (10).
Furthermore, during epidemiologic investigations of these positive farms,
WDNR discovered that deer had escaped in March 2002 from one of these
farms, one of which was later shot and found to be CWD positive (9). We
stress that these positive captive deer are likely not the source of CWD
in this free-ranging White-tailed Deer outbreak because of the captive
deer’s distance from the area where the CWD-positive free-ranging deer are
(approximately 130 km). No direct evidence exists that CWD came to
Wisconsin by the captive cervid industry. However, further investigation
on possible links between CWD cases in captive and free-ranging cervids in
Wisconsin is ongoing.
The state of Wisconsin is undertaking an integrated research,
surveillance, and management program to determine the distribution of CWD
in the Wisconsin free-ranging deer population and eventually eliminating
the disease from the known affected area of south-central Wisconsin (10,11). As of
March 2003, a total of 39,636 deer had been sampled statewide for CWD as
part of this surveillance and management program (data are available from:
Computer simulation of CWD dynamics in western cervid populations (12)
indicated that CWD could severely reduce deer numbers. Disease
transmission may occur at a greater rate and consequently have a larger
impact on the population in the eastern United States, where White-tailed
Deer densities are typically an order of magnitude larger than western
deer and elk populations (e.g., deer densities in the CWD-affected area
are estimated to be currently >20 deer per km2) (WDNR,
unpub. data). Deer and deer-related activities, such as hunting, wildlife
viewing, and other social factors, are an important component of the
Wisconsin culture and economy (approximately $1 billion/year) (13),
prompting an aggressive research and management strategy to combat CWD in
Wisconsin’s free-ranging deer population.
We thank the staff of the Wisconsin Departments of
Natural Resources, Agriculture, Trade and Consumer Protection, and
Health and Family Services under the direction of the Wisconsin
Interagency Chronic Wasting Disease (CWD) Task Force; the landowners and
hunters who provided samples and participated in CWD management; the
National Veterinary Services Laboratories (Ames, Iowa) for conducting
disease testing; and B.R. Patterson, D. Grear, C. Yahnke, and J. Ahumada
for critical reviews.
Dr. Joly’s research fellowship was supported by funding
from the U.S. Geological Survey-National Wildlife Health Center and the
Wisconsin Department of Natural Resources.
Dr. Joly is a research associate with the United States
Geological Survey-Wisconsin Cooperative Wildlife Research Unit,
University of Wisconsin-Madison. His research interests include the
ecology of infectious diseases of wildlife.
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