Questions and

Answers About

Chronic Wasting

Disease

Q. What is chronic wasting disease?

A. Chronic wasting disease (CWD) is a fatal, 

neurological disease of farmed and wild deer and elk.

The disease has been identified in wild and captive

mule deer, white-tailed deer and North American elk,

and in captive black-tailed deer. CWD belongs to the

family of diseases known as transmissible spongiform

encephalopathies (TSEs). TSEs include a number of

different diseases affecting animals or humans

including bovine spongiform encephalopathy (BSE) in

cattle, scrapie in sheep and goats, and CreutzfeldtJacob disease (CJD) in humans. Although CWD

shares certain features with other TSEs, it is a 

distinct disease affecting only deer and elk. CWD is

a progressive, fatal, degenerative disease. Clinical

signs in affected animals include loss of body 

condition, behavioral changes, excessive salivation,

increased drinking and urination, depression, and

eventual death. CWD is always fatal. There is no

known treatment, vaccine, or live animal test for

CWD.

Q. What causes this disease?

A. The agent that causes CWD and other TSEs has

not been completely characterized. However, the

theory supported by most scientists is that TSE diseases are caused by little understood proteins called

prions. Prions are a form of protein normally found in

the cells of the nervous system and other body tissues. Stanley Prusiner, a Nobel Prize winning neurologist, first described an abnormal form of prion

resistant to enzymes that break down normal proteins. These abnormal, protease resistant prions are

referred to as PrPres. PrPres have the ability to

transform normal prions into this abnormal state. As

the disease progresses, PrPres accumulate in the

brain and lymphoid tissues (lymph nodes and tonsils). Accumulation of these abnormal PrPres produce tiny sponge–like holes in the brain that are visible microscopically. The word “spongiform” in TSEs

describes the sponge–like condition of brain tissue

found in infected animals. As the disease progresses, the affected animal loses its basic physical and

mental abilities.

Q. What is the history of CWD?

A. The following is a brief chronology of CWD:

• CWD was first described clinically as a wasting 

syndrome in captive deer belonging to Colorado 

research facilities in 1967. A few years later it 

was described in a Wyoming research facility.

• CWD was first determined to be a TSE in 1978 

by Dr. Elizabeth Williams of the University of 

Wyoming.

• The first cases of CWD in wild deer and elk were

diagnosed in 1981 in Colorado and 1985 in 

Wyoming.

• Beginning in the 1980s, the distribution of CWD 

in wild deer and elk in Colorado and Wyoming 

was determined through surveillance by wildlife 

agencies in those States. Through their efforts, 

an endemic area for the disease in wildlife in their

States was described. This area includes much 

of northeastern Colorado and southeastern 

Wyoming.

• In 2001, discovery of a positive wild mule deer in 

neighboring Kimball County, NE, extended the 

endemic area into southwestern Nebraska.

• From 1996 to June 2002, CWD was diagnosed in

farmed elk herds in Colorado, Kansas, Montana, 

Nebraska, Okalahoma, South Dakota, and the 

Canadian Provinces of Alberta and 

Saskatchewan.

• From 2000 to June 2002, CWD has also been 

found in wild deer in northwestern Nebraska, 

southern New Mexico, southwestern South 

Dakota, south–central Wisconsin, northwestern 

Colorado, and the Canadian Province of 

Saskatchewan.

Q. What are the symptoms of CWD?

A. CWD is a slow and progressive disease.

Because the disease has a long incubation period,

elk and deer with CWD may not produce any visible

symptoms of the disease for a number of years after

they become infected. As the disease progresses,

deer and elk with CWD show changes in behavior

and appearance. These clinical signs may include

progressive weight loss, stumbling, tremors, lack of

coordination, blank facial expressions, excessive 

salivation, loss of appetite, excessive thirst and 

urination, listlessness, teeth grinding, abnormal head

posture, and drooping ears. Because of effects on

the central nervous system, animals can have 

difficulty in swallowing, resulting in pneumonia

caused by aspiration of food or saliva. Clinical signs

of CWD are usually present a few weeks to several

months before the animal dies. Unfortunately, these

Factsheet

 

 
 

APHIS
signs are not specific to CWD and can occur with

other diseases or malnutrition.

Q. How is CWD transmitted?

A. The exact mechanism of transmission is unclear.

Evidence suggests CWD is transmitted directly from

one animal to another (lateral or horizontal 

transmission). The route by which the agent is shed

from the animal’s body is unknown. However, 

experimental and circumstantial evidence suggests

that indirect transmission from an environment 

contaminated with the agent appears to be possible.

Transmission of CWD has not been associated with

any particular feeding practice or regimen in farmed

elk or deer. Supplemental feeding of wild elk and deer,

however, concentrates the animals and may 

contribute to disease spread.

Q. Is there any way to destroy the infectious

agent?

A. A characteristic of all TSE agents is their 

resistance to conventional disinfectants, high 

temperatures, and enzymes that normally break down

proteins. Recommendations for disinfection of areas

in which infected animals have resided are still being

developed.

Q. Is the disease transmissible to humans?

A. The Centers for Disease Control and Prevention

has issued this statement: “It is generally prudent to

avoid consuming food derived from any animal with

evidence of a TSE. To date, there is no evidence that

CWD has been transmitted or can be transmitted to

humans under natural conditions. However, there is

not yet strong evidence that such transmissions could

not occur. To further assess the possibility that the

CWD agent might occasionally cause disease in

humans, additional epidemiologic and laboratory 

studies could be helpful. Such studies include 

molecular characterization and strain typing of the

agents causing CWD in deer and elk and CJD in

potentially exposed patients. Ongoing national 

surveillance for CJD and other neurological cases will

remain important for continuing to assess the risk, if

any, of CWD transmission to humans.”

Q. Is the disease transmissible to domestic 

livestock?

A. During the approximately two decades of 

monitoring, researchers have not found any evidence

that CWD can be transmitted to domestic cattle under

natural conditions. Ongoing experiments involving

oral exposure and contact exposure on heavily CWD

contaminated sites have not resulted in infection of

cattle. These experiments, however, require additional

time before they are completed. CWD has been

experimentally transmitted by artificial means to mice,

ferrets, mink, goats, squirrel monkeys, and calves.

Q. How is CWD diagnosed?

A. Currently, CWD is diagnosed by examining brain

and lymphoid tissue (lymph nodes and tonsils) from a

dead animal. Tests to confirm CWD are performed in

a laboratory, using brain tissue. Immunohistochemical

(IHC) staining is the most commonly accepted method

of detection and is the standard test used by USDA’s

National Veterinary Services Laboratories. IHC 

staining is an antibody-based test. Antibodies bind to

abnormal PrPres in the tissue on a slide. Additional

steps in the test allow a colored agent to be bound to

the abnormal PrPres-antibody complex.

Accumulations of color indicate the presence of the

abnormal PrPres when the slide is examined microscopically. A CWD-positive animal is one in which the

presence of abnormal PrPres has been confirmed in

the brain or lymphoid tissues.

A research team in Colorado has recently 

developed a live animal test for CWD based on the

collection of tonsil biopsies for microscopic 

examination. This test seems to work well in mule

deer, but not in elk, and its application may be limited

to special circumstances. Scientists are continuing to

work on a number of approaches that may provide a

rapid postmortem or live animal test for both deer and

elk.

Q. Why is it so important that the sample 

collected for testing include the obex portion of

the brainstem?

A. Studies on the distribution of abnormal PrPres in

CWD–affected deer and elk have shown that the obex

portion of the brainstem is the first place that the

abnormal PrPres can be detected in the brain. As the

disease progresses, the abnormal PrPres can be

detected in multiple locations and, finally, throughout

the brain. Because of this, it is necessary to test the

obex to detect CWD in animals that are in the early

stages of the disease. It is possible that other parts of

the brain may test negative for the presence of disease while the obex would test positive. For whitetailed deer and mule deer (but not elk) some lymphoid

tissues from the head (tonsils and retropharyngeal

lymph nodes) become positive before the obex does,

so these tissues will also be useful in surveillance and

monitoring efforts in deer.

Q. What does a negative IHC test mean? 

A. A negative test is one in which there is no

detectable IHC staining of abnormal PrPres. The

interpretation of a negative test depends on the

species and the tissue tested. In elk, if the obex is

negative, the animal is most likely not infected with

the CWD agent. There is the possibility, however, that

the animal is infected but the disease process is so

early that the abnormal PrPres is not detectable with

the current IHC test. Similarly, in white–tailed deer

and mule deer if the obex and/or the lymphoid tissue
from the head are IHC–negative, the animal is most

likely not infected with CWD. There is the possibility,

however, that the disease process is so early that the

abnormal PrPres is not detectable by the current IHC

test.

Q. What is the USDA doing about CWD?

A. The USDA has taken the following steps to help

control the spread of CWD:

• During 1997, USDA began supporting 

surveillance for CWD in farmed and wild elk and 

deer in cooperation with State agriculture and 

wildlife agencies. Farmed elk herds that tested 

positive for the disease were put under State 

quarantine.

• In September 2001, $2.6 million in Commodity 

Credit Corporation (CCC) emergency funds were 

transferred to USDA to increase the effort to 

eliminate CWD in farmed elk. These funds paid 

for enhanced surveillance as well as depopulation

of farmed CWD-positive, exposed, and suspect 

animals with compensation to the owners. In 

addition, the funds allowed USDA to provide 

assistance in cleaning and disinfecting premises 

where positive and exposed animals resided.

Factsheets were developed to disseminate CWD 

information.

• In February 2002, an additional $12.2 million of 

CCC funding was transferred to APHIS to 

continue this effort. These funds will also be used

to support surveillance and diagnostics in wild elk 

and deer.

• In April 2002, USDA agreed to purchase farmed 

elk herds in the endemic area of Colorado where 

wild animals have tested positive for the disease.

Sixteen ranches with about 1,350 animals 

accepted the offer to purchase their animals for 

95 percent of their appraised value.

• USDA continues to provide diagnostic and 

surveillance support to States with active CWD 

surveillance and control programs for farmed elk 

and deer. USDA has proposed a 

Federal/State/industry certification program for 

farmed elk and deer.

• USDA has supported efforts to control CWD in 

wild populations through assistance with 

diagnostic testing and research.

• In May 2002, USDA and the U.S. Department of 

Interior formed a joint working group on CWD.

The purpose of the group is to ensure a 

coordinated and cooperative Federal approach to 

assisting States, Tribes and Federal land 

management agencies with CWD response 

efforts. In June 2002, the working group 

presented a plan to the Congress.

Q. What can elk and deer farmers do?

A. Elk and deer farmers are strongly urged to enroll in

State CWD surveillance and control programs. They

should only purchase animals that have been enrolled

in a State program or are otherwise known not to

have been exposed to CWD.

Q. What precautions should hunters of deer and

elk follow?

A. Hunters should be vigilant for identifying elk or

deer displaying CWD symptoms. They should report

suspected cases to authorities immediately. Several

States have issued specific guidelines for hunters.

Q. What is the USDA’s official position on CWD

sample testing?

A. Official diagnosis of CWD should be performed

exclusively by Federal and State regulatory agency

laboratories. The international credibility of the U.S.

animal health system is in large part predicated on

having an established set of government laboratories

with the expertise to accurately conduct diagnosis not

only for CWD, but also BSE, avian influenza, foot-andmouth disease, and a host of other diseases of 

concern. The system is designed not only to ensure

consistency and accuracy but also to preserve

domestic and international market confidence in U.S.

agricultural commodities. Indeed, a “false positive” for

any disease could result in unnecessary public concern and costly regulatory action. And in the case of a

disease like BSE, a false positive could be 

devastating, costing the U.S. economy billions of 

dollars in unnecessary domestic and international

market disruption from which it could take years to

recover.

Q. Can this test be used to determine if an animal

is safe for human consumption?

A. Because of the limitations of currently available

tests for CWD, testing serves purely as a surveillance

tool to determine the geographic parameters and

prevalence of the disease in the United States. A 

positive test result can be used as reliable information

that the disease has spread into a given area.

However, a negative test result is not necessarily a

reliable indicator that an animal is free of the disease.

Indeed, at this time no test that can be used reliably

on individual animals to determine whether that a

nimal is free from CWD and whether the meat is safe

to eat. This is because the disease has a very long

incubation period, which leads to a high “false 

negative” rate during early infection. In addition, 

relatively little is known about the distribution of the

CWD agent, so an animal whose brain and nervous

system tissue tests negative might actually be 

carrying the infective agent in its muscle or other 

tissues.
Q. What test is used as the official test for CWD

surveillance?

A. In order to ensure the integrity of the U.S.

surveillance effort, USDA has designated an official

test for CWD surveillance: the IHC assay as 

performed by APHIS’s National Veterinary Services

Laboratories (NVSL) and State/university laboratories

with which NVSL has contracted. Employees at these

laboratories, as part of a national network, are being

trained,and the laboratories are being proficiency 

tested, and supplied with control samples to perform

official tests. They will be linked through a reporting

database. Currently, 10 laboratories with which APHIS

has contracted perform CWD testing, and APHIS is

working to bring another 5 on line by January 2003.

This capacity is more than sufficient to handle the

increased surveillance testing planned this fall to

determine the geographic distribution and prevalence

of CWD in the United States.

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employer.

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture