Chronic Wasting Disease in Animals

Causes and Spread

CWD is a type of prion disease. These diseases occur when proteins normally in the body misfold, causing symptoms that eventually result in death. CWD affects many different species of animals with hooves. Other prion diseases in animals are bovine spongiform encephalopathy in cows and scrapie in sheep and goats.

Scientists think CWD spreads between animals through contact with saliva, blood, urine or feces of an animal with CWD. They suspect it can also spread indirectly through the environment, such as in soil, drinking water or food.

Once an animal gets sick, the disease moves to its brain and spine and eventually kills the animal. CWD does not appear to naturally infect cows, other livestock or pets.

The U.S. Department of Agriculture regulates deer and elk raised on farms for meat. The agency operates a voluntary national CWD herd certification program. In the program, states and herd owners agree to take actions to decrease the risk of CWD in their herds. Privately owned herds that are not in the program may be at increased risk for CWD.

Warning Signs

CWD can infect farmed and free-range deer and elk of all ages. It may take months to years before an infected animal shows symptoms. These symptoms may include:

• Drastic weight loss (called wasting)
• Stumbling or lack of coordination
• Drooling
• Listlessness (appearing "out of it")
• Excessive thirst or peeing
• Drooping ears
• Lack of fear of people

It is often difficult to diagnose an animal with CWD based on these symptoms alone. This is because many CWD symptoms also occur with other diseases and when animals aren't eating enough. CWD is always fatal to the animal.

Occurrence

As of February 2024, CWD has been reported in animals in at least 32 U.S. states, and four Canadian provinces.

CWD was identified in captive deer in Colorado in the late 1960s and wild deer in 1981. It spread more widely in Colorado and southern Wyoming in the 1990s. CWD has been found in many other areas since 2000. CDC expects the disease to continue to spread.

It is also possible CWD may also occur in other states but cases haven't been detected yet. Once CWD is established in an area, the CWD proteins (prions) say in soil and water for years.

Nationwide, the overall rate of CWD in free-ranging deer and elk is low. However, in areas where CWD has been found for years or decades, infection rates may be much higher.

Nationally, statewide rates may exceed 1 in 10 animals. In hotspots, rates of 1 in 4 have been reported. In captive deer, rates are much higher. A rate of nearly 4 in 5 was reported in at least one captive herd.

Research on CWD in animals

In external research labs, CWD has been shown to infect squirrel monkeys, and mice that carry some human genes.

A study by Canadian and German scientists is looking at whether CWD can be spread to macaques—a type of monkey. Macaques are genetically closer to people than any other animal that has been infected with CWD. A 2017 summary showed that CWD spread to monkeys that ate infected meat or brain tissue from CWD-infected deer and elk. This included meat from infected deer without CWD symptoms.

This study contradicted a previous study that showed macaques were not infected with CWD by eating infected meat. It's not clear why the results were different.

To date, there is no strong evidence that CWD infects people. However, these experiments raise the concern that CWD may pose a risk to people. They show the importance of preventing people from eating CWD-infected deer.

Scientists are conducting studies to learn more about the potential risk to people who hunt or eat deer or elk meat. In particular, scientists want to know if this group of people is diagnosed with prion diseases more often. It may take years to decades for symptoms to appear. So, it will be a long time before results are known.