Chronic Wasting Disease (CWD)

Chronic Wasting Disease (CWD) is a deadly illness in white-tailed deer and other members of the deer family, called cervids. 

CWD belongs to a family of diseases called transmissible spongiform encephalopathies (TSEs), or prion diseases. Similar diseases include Mad Cow Disease in cattle, CJD in humans, and scrapie in sheep. The Centers for Disease Control and Prevention (CDC) states that prion diseases are always fatal.

CWD will eventually kill all deer and other cervids it infects by causing fatal lesions in the brain. CWD can take years to infect an animal to the point of it showing symptoms: excessive salivation, drooping head/ears, tremors, emaciation, and changes in behavior such as a lack of fear of humans and a lack of coordination.

You can’t tell if an animal has CWD until symptoms show during the final stages of the disease. CWD can’t be confirmed until after the animal dies and a tissue sample can be taken for testing.

The disease has been found in Missouri and is slowly spreading. The Missouri Department of Conservation is working with conservation partners to find cases and limit its spread. CWD remains relatively rare in the state at this time.

Why CWD is a Threat

• CWD has the potential to greatly reduce deer numbers and deer hunting and watching over time for Missouri's nearly 500,000 deer hunters and almost two million wildlife watchers.
• Missouri offers some of the best deer hunting in the country, and deer hunting is an important part of many Missourians' lives and family traditions.
• Deer hunting is also an important economic driver in Missouri and gives a $1 billion annual boost to the state and local economies.

How CWD Spreads

• CWD is spread from deer to deer through direct contact and through contact with soil, food, and water that have been contaminated through feces, urine, saliva, or carcasses of infected deer.
• Potential for transmission increases when deer gather in larger, concentrated numbers.
• Young bucks can also spread CWD to new areas as they search for territories and mates.
• Moving carcasses out of the immediate area where harvested and improperly disposing of them can spread the disease.
• It can take months or years for a deer infected with CWD to show symptoms. However, an infected deer can spread the disease to other deer and contaminate the environment while appearing healthy.
• The neurological disease infects deer and other members of the deer family (called cervids) by causing degeneration of brain tissue, which slowly leads to death.
• CWD is caused by misshapen proteins called “prions.”
• The disease has no vaccine or cure.

Signs of CWD

Deer infected with CWD do not always look sick. Symptoms include:

• Excessive salivation
• Drooping head/ears
• Tremors
• Emaciation
• Change in behavior, such as a lack of fear of humans and a lack of coordination.

Differences between CWD and Hemorrhagic Disease (HD)

• CWD is new to the state with the first cases being detected in 2010 and 2011 in captive deer.
• CWD is caused by misshapen proteins called “prions” that concentrate in the brain, lymph nodes, spine, and eye tissue of infected animals and lead to the slow death of all infected animals.
• CWD is a 100-percent fatal disease that is spread from deer to deer through body fluids.
• CWD symptoms include excessive salivation, drooping head/ears, tremors, emaciation, and change in behavior such as lack of fear of humans and lack of coordination.
• CWD can take more than 18 months for an infected deer to show symptoms.
• CWD has the long-term potential to significantly reduce deer numbers in the state over time.
• HD has periodically affected deer in Missouri and other states for many decades with no long-term impacts to the overall deer population.
• HD is caused by a naturally occurring virus spread by midge flies during the summer and fall and ends when cold weather kills the flies.
• HD symptoms include fever, reduced activity, and swollen neck, tongue or eyelids. Because sick deer are feverish, they are often found near water.
• HD-infected deer typically show symptoms within days of being infected and those that die do so within weeks of being infected. 
• HD does not kill all deer it infects. Deer that survive HD develop antibodies for future immunity.
• HD can have significant short-term impacts on a deer herd, but has never been shown to have long-term impacts on deer populations.

CWD Cannot Be Eliminated Once It’s Well Established

• CWD remains relatively rare in the state at this time.
• Research from other states that have had CWD much longer than Missouri shows that the disease has been impossible to eliminate once it has become well established in an area.
• MDC will continue to focus on managing the levels of the disease where it has been found and reducing the risk of introducing the disease to new areas of the state.

CWD Has Not Always Been In Missouri

• MDC has determined CWD was recently introduced to Missouri. This is based on the low current prevalence and limited distribution of CWD in Missouri.
• Once well established, CWD increases in both prevalence and distribution over time. This means the number of cases of the disease present in a particular deer population in a particular area increases over time. It also means the disease spreads to many new areas over time.

Where CWD Came From

• Researchers do not know the origin of CWD.
• CWD was first found in captive mule deer at a research facility in Colorado in the 1960s.
• It was first found in the wild in Colorado in the 1980s and has spread to almost two dozen states.
• The first cases in Missouri were found at private big-game breeding and hunting facilities in 2010 and 2011.
• The first cases in free-ranging deer were found in 2012 near the private breeding and hunting facilities.

Deer Die Directly From CWD

Like other prion diseases -- such as CJD in people and Mad Cow Disease in cattle – scientific research shows that CWD is a fatal disease. Scientific research also shows that CWD spreads over time. Without management intervention, the percent of a deer population that is infected with CWD (also known as the prevalence rate) increases slowly over years and decades and regional population declines can occur.

Not seeing deer dead from CWD in Missouri is a good sign

We are fortunate in Missouri that we are not routinely seeing deer that are dying directly from CWD. It shows that the prevalence of the disease (the percent of a deer population that is infected) is still very low in the state. MDC wants to keep it that way! MDC’s CWD surveillance and management efforts aim to keep CWD rare in Missouri and to prevent CWD from spreading to a level where sick deer are routinely observed, and regional population declines occur.

Other states are finding deer dead from CWD

Other states that have had CWD longer than Missouri and that have more cases of CWD are routinely seeing deer die from the disease and are finding dead deer that show symptoms of CWD and test positive for the disease. These states are also seeing drops in deer numbers in areas with high levels of CWD – not from targeted culling or other CWD management practices, but from infected animals dying from the disease. Wisconsin, Arkansas, Colorado, and Wyoming, consistently document deer that are dying (or have died) because they are infected with CWD.    

Few carcasses are found

Like with many other fatal diseases, symptoms and secondary infections from CWD also cause and contribute to deer deaths that would not occur if the animals did not have the disease. Animals with CWD will eventually show drastic weight loss (wasting) and slowly starve to death. CWD also causes infected animals to lose coordination, become listless and weak, and lose fear of people and other predators -- making them easy prey targets. Animals with CWD also die from secondary infections such as pneumonia. These deer become easy food sources for predators and scavengers – decreasing the chances that testable carcasses are found.

Learn more

Here are link to indepth studies related to deer dying from CWD and CWD driving population declines:

• Lions and Prions and Deer Demise
• Endemic chronic wasting disease causes mule deer population decline in Wyoming
• Chronic Wasting Disease Drives Population Decline of White-Tailed Deer

Get More General Information

• National Wildlife Health Center
• Chronic Wasting Disease Alliance