Fact Sheet: Chronic Wasting Disease and Theories of Transmission



Chronic wasting disease, a fatal brain and nervous systems disease that affects certain wildlife such as elk and three relatives – mule deer, white tail deer and black tail deer – has a significant impact on the state’s economy and wildlife health, but the disease itself remains somewhat of a mystery.

Animals show signs of infection that include staggering, listlessness, abnormal behavior, loss of fear of humans, excessive drooling and drinking, frequent urination, drooping ears, a rough coat and weight loss.

The disease is caused by a transmissible agent, meaning that the disease can be transmitted from one animal to another through an "agent;" it is spread through some form that is different from a bacteria or germ through which an illness like a common cold is spread.

Scientists have not yet pinpointed what the transmissible agent is that causes CWD to spread, although researchers have studied the transmission of CWD and other diseases of the same family, called transmissible spongiform encephalopathies, for more than 50 years.

What is the disease?

–     TSEs affect other animals including cattle, mink, cats, sheep and people.

–     TSEs are not a new disease. Sheep are known to have been susceptible to scrapie, a TSE, for at least 300 years. TSE research began in Britain in 1955 due to the economic impact of scrapie in that country.

–     TSEs cause microscopic empty spaces in brain matter, giving the brain a spongy appearance.

–     TSEs appear to be transmitted from animal to animal, but scientist are not sure how. CWD is believed to be transmitted among deer and elk by direct contact with infected animals and from living in highly contaminated habitat.

–     Scientists have not discovered the exact mode of transmission, but believe that the infectious agent is passed through feces, urine or saliva.

–     Scientists do not agree on how TSEs started, including CWD. Some believe it is a naturally occurring disease among species; others believe that CWD was contracted by deer and elk who grazed near sheep with scrapie.

Two leading scientific theories about how the disease spreads and what causes the disease

Prion Theory

–     The most widely accepted theory in the scientific community.

–     A prion is a small, irregular or defective protein that is made from normal protein-making genes common to all animals. Unlike "regular" protein, this protein cannot be broken down in the body and accumulates in the brain of infected animals. The accumulated protein causes the degeneration of the brain and, eventually, death.

–     Scientists believe that prions may spread from digestive tracts to the central nervous system. Gastrointestinal fluids digest most proteins, but small amounts pass through the body with out being destroyed.

–     A TSE infection does not spread through all species the same way. In early stages of the disease, CWD has been identified in mule deer in their lymph nodes, tonsils and lymph tissue in the small intestine. In cattle, which contract the TSE called bovine spongiform encephalopathy, commonly called BSE or mad cow disease,  the infection is only detected in lymph tissue in the small intestine – and sometimes not at all – at the same stage.

–     This theory is the first scientific theory to introduce the thought that an infection can be spread through a protein, as opposed to the accepted modes of infection which include bacteria, viruses, fungi or parasites.

Spiroplasma Theory

–     Spiroplasmas are a kind of bacteria that do not have a cell wall and are believed to be among the oldest forms of life on earth.

–     Spiroplasma-like structures have been found in tissue infected with TSEs and these structures are very similar to the TSE agent: they contain internal proteins with the same characteristics as proteins in TSEs.

–     Spiroplasmas gather in the central nervous system.

–     Research shows that some animals infected with spiroplasmas develop a spongy change in brain tissue, but some tests show that these animals are not infected with spiroplasmas.

Theories about how TSEs are transferred from species to species:

–     Scientists believe that the transmission, or "jump," of a TSE from one species to another are one million times less likely than a transmission between two animals of the same species.

–     Once a transfer from one species to another occurs, the new strain can more easily jump from species to species.

–     A transfer of CWD from species to species is still being investigated. Pronghorn antelope, Rocky Mountain bighorn sheep, mountain goat, moose, and exotic antelope called blackbuck have been in contact with deer and elk infected with CWD and their habitat and have not developed the disease.

–     At this stage, CWD is considered to have no human health implications, but the consumption of infected meat from deer and elk is discouraged.

Colorado State University is a leading CWD research institution, and the disease was first observed in Fort Collins in 1967 at a government research facility.

For more information about chronic wasting disease, visit the following links to policy reports written by Colorado State University Cooperative Extension at:

Economic Implications of Chronic Wasting Disease:


Chronic Wasting Disease: Government and Private Sector Action http://dare.agsci.colostate.edu/extension/apr03-06.pdf

Chronic Wasting Disease and Theories of Transmissible Spongiform Encephalopathy Transmission: http://dare.agsci.colostate.edu/extension/apr03-05.pdf

Chronic Wasting Disease Overview: Hunter Information: http://dare.agsci.colostate.edu/extension/apr03-04.pdf


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