Mad cow disease is the common name for bovine spongiform encephalopathy (BSE), a type of transmissible spongiform encephalopathy (TSE) that only affects cattle. It is characterized by a long incubation period (three to eight years) and progressive nerve and brain damage and is always fatal. According to the U.S. Centers for Disease Control and Prevention (CDC), the first probable infections of BSE occurred in cattle during the 1970s.

Another type of TSE that only affects humans, called variant Creutzfeldt-Jakob disease (vCJD), has been linked to eating meat and meat byproducts from cattle infected with BSE. These two diseases appear to have the same infectious agents. The agent that causes BSE in cattle appears to remain in the animal tissue even after the animal has been slaughtered and processed as food. When people consume contaminated beef, they may become infected with vCJD. Although the infection spreads quickly and easily among cattle, it is not as easily transferred to or spread by humans. Infectious agents from one species may be less likely to create additional infectious agents in another species, a complication known as the “species barrier.” Some experts believe that the BSE infectious agents may be 10 to 100,000 times less infectious in humans as it is in cattle.  

BSE, vCJD and other forms of TSE are associated with certain types of proteins. Proteins are long strings of amino acids that are folded into particular shapes. These shapes allow the proteins to perform their functions in the body. Prions, which are also called proteinaceous infectious particles, are a certain type of harmless protein – they are not viruses, bacteria or other cellular organisms. In their normal state, prions are noninfectious.

However, prions can misfold, or fold into abnormal shapes and become infectious. When these infectious prions come into contact with other proteins, they cause those proteins to misfold and become infectious as well. Eventually, enough abnormal prions are present to produce symptoms of disease. It is believed that these abnormal prions kill nerve tissues (e.g., brain cells) with which they come into contact.

The theory that prions alone are responsible for TSE is widely accepted, but it has not been proven. One alternate theory claims that a slow-acting virus is the infectious agent responsible for TSE, and that the abnormal prions are a result of the infection, not the cause. However, no actual virus, bacteria or other cellular organism that may have caused the disease has been identified.

BSE and vCJD are most common in the United Kingdom, but have also occurred in several other European countries, Japan, Saudi Arabia, Canada and the United States.

According to the U.S. Food and Drug Administration (FDA), there have been more than 180,000 cases of BSE in the United Kingdom. Most of these cases have occurred in dairy cattle. While BSE in cattle has reached crisis levels, vCJD in humans has remained relatively rare, even in areas where BSE is common. Since vCJD was first identified in 1996, there have been fewer than 200 patients reported with the disease, from 11 different countries. According to the CDC, more than 80 percent of these cases occurred in the United Kingdom. Only two cases have occurred in the United States as of 2006.

A widespread outbreak, or epizootic, of BSE struck cattle in the United Kingdom in 1986 and reached its peak in January 1993. According to the FDA, almost 1,000 new cases per week occurred at this time. BSE epizootics have also been reported in Ireland, Switzerland, France, Liechtenstein, Luxembourg, the Netherlands, Portugal and Denmark.

In December 2003, BSE was confirmed in a cow in the state of Washington (but originally from Canada). After the cow died following complications of pregnancy, it was slaughtered for food, but when BSE was discovered in a sample of the cow’s brain tissue, the meat was recalled. The FDA and the U.S. Department of Agriculture (USDA) responded to this incident by implementing additional security measures against the disease. In June 2005, the first case of BSE in a cow born in the United States was confirmed in Texas. In March 2006, BSE was confirmed in a cow in Alabama. Investigators were not able to identify where the cow, which had no tags or distinctive marks, was born.

There has been some concern that even though the occurrence of BSE has decreased dramatically, more cases of vCJD may occur in the near future. Fears are that the condition may be more widespread than realized, with many current cases still in the incubation period.