There are several different types of transmissible spongiform encephalopathies (TSEs). Some of these affect humans and some only affect animals. One TSE in particular has received a great deal of attention in recent years. Bovine spongiform encephalopathy (BSE), more commonly known as mad cow disease, is a TSE that affects cattle. BSE has been linked to a human TSE called variant Creutzfeldt-Jakob disease (vCJD).

Scientists believe that vCJD infection occurs due to eating meat and meat byproducts from cattle infected with BSE. It appears that the agent (prion) that causes BSE in cattle remains in animal tissue after the animal has been processed as food. When people consume food contaminated with this agent, they may become infected with vCJD. However, the risk of this transmission in the United States is extremely rare.

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. Since vCJD was first identified in 1996, there have been nearly 200 patients reported with the disease, from 11 different countries. According to the Centers for Disease Control and Prevention (CDC), more than 80 percent of these cases occurred in the United Kingdom. Only two cases had occurred in the United States as of 2006.

Unlike other TSEs, vCJD in the United Kingdom tends to affect people at a younger age. According to the CDC, the average age at death for persons with this disease is about 29 years.

In addition to vCJD, other TSEs that may affect humans include:

• Creutzfeldt-Jakob disease (CJD). While there are similarities, classic CJD and variant CJD (vCJD) are distinctly different conditions. Classic CJD has been known and studied for much longer than vCJD, and it is not linked to BSE. The disease progression of CJD is typically faster than vCJD, and it tends to affect older rather than younger adults. According to the National Institute of Neurological Disorders and Stroke (NINDS), symptoms of CJD tend to first occur around age 60. After symptoms are present, about 90 percent of patients die within 12 months.
  
  CJD does not appear to be transmitted through food. According to the NINDS, it most often develops spontaneously in people with no known risk factors for the disease, a form known as sporadic CJD. In about 5 percent to 10 percent of cases, CJD is genetically inherited. In less than 1 percent of cases, it is transmitted to brain or nervous system tissue during certain types of medical procedures. The nationwide incidence of CJD is rare, affecting about 200 people per year in the United States.
  
  
• Gerstmann-Straussler-Scheinker disease (GSS) and fatal familial insomnia (FFI). These are two very rare, hereditary forms of TSE, only known to occur in a few families worldwide. Unlike other TSEs, FFI causes disturbances in the endocrine system and autonomic nervous system (which regulates body functions). This can lead to excessive sweating, high body temperatures, increased heart rate and high blood pressure.
  
  
• Kuru. This was among the first identified and best studied TSEs in humans. It occurred in an isolated tribe in Papua New Guinea called the South Fore. It appeared to be related to ritual cannibalism. Since cannibalism has been stopped among these people, kuru has practically disappeared.

BSE is the only animal form of TSE directly linked to human disease. While other animal forms of TSE have yet to cross the species barrier to humans, researchers warn that such transmission may eventually be possible. Studies on this possibility and potential methods of preventing it are ongoing.

Beside BSE, additional TSEs that may affect animals include:

• Scrapie. This oldest known TSE occurs in sheep and goats. Unlike BSE, it can be passed from a mother sheep to her lambs. It may also be acquired through contaminated feed. There have been no cases of humans acquiring any TSE from eating lamb or mutton contaminated with scrapie. However, some scientists believe that the use of scrapie-contaminated sheep and goats in supplements and feed for cattle may have caused or accelerated the development of BSE.
  
  
• Chronic wasting disease. The TSE that affects deer and elk appears to be spreading in America. The disease’s cause and how it spreads are not well-known.
  
  
• Transmissible mink encephalopathy, feline spongiform encephalopathy and ungulate spongiform encephalopathy. These TSEs affect farm mink, cats (both domestic and those in zoos) and zoo ruminants (animals that chew food they have already partially digested), respectively. While the cause of these diseases is not positively known, it is likely that they are linked to contaminated feed. Cattle products have often been used as protein supplements in animal feed, and some of these products may have been contaminated with BSE.