Prion diseases are a related group of rare, fatal brain diseases that affect animals and humans. Also known as transmissible spongiform encephalopathies (TSE), they include bovine spongiform encephalopathy ("mad cow" disease) in cattle; Creutzfeldt-Jakob disease in humans; scrapie in sheep; and chronic wasting disease in deer and elk. Read about NIAID disease-specific prion disease research.
Chronic Wasting Disease
A different transmissible spongiform encephalopathy (TSE) disease, called chronic wasting disease (CWD), has been detected in U.S. deer and elk populations in 11 states. So far, scientists have uncovered no evidence that deer or elk with CWD might transmit some form of TSE disease to people who eat or have close contact with deer or elk.
More research is necessary to determine whether CWD poses any risk to humans, particularly because it is spreading over a wider geographical area in the United States. There have been several reported cases of Creutzfeldt-Jakob disease (CJD) in people who have eaten venison, most much younger than the typical age associated with CJD. In each of these instances, careful investigations by the Centers for Disease Control and Prevention have shown no causal link between CJD and CWD in deer and elk populations. Continued surveillance is important, however, to assess any possible risk of CWD transmission to humans.
Variant Creutzfeldt-Jakob Disease
In some cases, transmissible spongiform encephalopathy (TSE) diseases appear to be infectious. For example, there is good evidence that a newly discovered TSE disease—variant Creutzfeldt-Jakob Disease (vCJD)—was first spread to humans from cattle infected with bovine spongiform encephalopathy (BSE). Identifying the transmissible agent is still an ongoing area of research.
A leading hypothesis about the origin of vCJD is that abnormal forms of prion protein ingested through diseased meat may sicken people by causing normal human prion protein to form incorrectly. This change from normal to abnormal prion protein spreads in the brain, where the misshapen protein aggregates in the spaces between brain cells and may produce disease. An alternative hypothesis is that the disease could be transmitted by a virus that subsequently triggers the process of forming abnormal prion protein.
Health officials in the United Kingdom have responded to evidence that beef infected with BSE may have spread vCJD to humans. Since the 1980s, when the BSE epidemic began in the United Kingdom, millions of cattle in Europe have been destroyed. In 1997, the U.S. Department of Agriculture (USDA) then reacted by outlawing feeding meat and bone meal products to cattle intended for U.S. consumption.
As of fall 2006, worldwide, there have been 200 reported cases of vCJD, according to the Centers for Disease Control and Prevention (CDC). All of these cases were among people who ate beef in a country with a BSE outbreak, and nearly all (164 cases) were in the United Kingdom. Three U.S. cases have been reported, though they were likely exposed to BSE agent while in the United Kingdom.
The first North American case of BSE was found in Canada in May 2003. Discovery of the first U.S. case of BSE followed in December 2003 in a Washington State cow. In June 2005, a second cow in the United States tested positive for BSE in Texas. USDA ascertained that the cow was born in 2001, 4 years after the 1997 USDA ban. A third BSE-infected cow was found in Alabama in March 2006. No cases of endemic vCJD have been associated with consumption of domestic beef in the United States.
Another human type of TSE, CJD, is similar to vCJD but progresses much more quickly and affects older people. CJD is extremely rare with an estimated 1 case per million people. Although a majority of cases occur without a known cause, the disease may be inherited. Rare transmission of CJD through infection also has been documented.