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ACVP BSE Fact Sheet (Mad Cow Disease)

Mad Cow Disease (BSE) Factsheet

Definition
Mad cow disease is the popular term for Bovine Spongiform Encephalopathy (BSE), one of a group of diseases in animals and humans called transmissible spongiform encephalopathies (TSE), or prion diseases. BSE is a fatal degenerative disease affecting the central nervous system of adult cattle. Pathologic changes are confined to the central nervous system and consist of sponge-like microscopic changes. The average incubation period is about five years, but it may range from two to eight years. Clinical signs include changes in temperament such as nervousness or aggression; abnormal posture; lack of coordination, decreased milk production; loss of body condition despite continued appetite and eventually, difficulty in rising. Following the onset of clinical signs, the animal's condition deteriorates - usually for two weeks to six months - until it dies or is killed.

Significance
BSE is the first TSE transferable from animal to humans by consuming food products from animals, causing a major health/food safety issue.

History
The first disease of this nature to be described was scrapie of sheep, known for over 200 years, but only later recognized as a TSE. BSE in cattle was first identified in 1986 in the United Kingdom (UK). Its source is uncertain, but there is convincing evidence that the outbreak was related to the use of meat and bone meal (MBM) in cattle feed that was contaminated with a scrapie-like agent. Regardless of the initial origin, it is clear that the epidemic was sustained and amplified by the recycling of BSE infected cattle material to other cattle from the mid 1980s onwards. In the UK, supplements in cattle (and other livestock) feed have been produced with MBM made from animal carcass material since the 1920s. Changes to these rendering procedures in the 1970s/1980s may have allowed the infectious agent to survive during the production process.

Most cases of BSE have been reported in the UK. Through April 2005, 180,780 BSE cases had been confirmed in more than 36,000 herds of cattle. At the peak of the epidemic in January 1993, nearly 1,000 suspect cases were reported each week. To date (28 June 2005), there has been two reported case of BSE in the United States. The first was an adult dairy cow imported from Canada in October, 2001. The second case was confirmed in June 2005.

Known TSEs of animals, other than BSE, include scrapie of sheep and goats, transmissible mink encephalopathy, and chronic-wasting disease (CWD) of deer and elk. Feline spongiform encephalopathy, reported in domestic cats and some wild felid species in zoos, as well as spongiform encephalopathy in several African species of ruminants in zoos, are considered to result from infection with the BSE agent.

Cause and Transmission
The exact nature of the infectious agent and its mode of transmission are unknown. The most common hypothesis about the cause points to abnormally formed proteins (prions) in MBM. Prions are highly resistant to heat, ultraviolet light, ionizing radiation and common disinfectants that normally inactivate viruses and bacteria. They do not elicit a detectable immune response in the host and cannot be seen microscopically. Cattle become infected by eating MBM contaminated with the BSE agent (prion). Unless controls are in place the disease is further spread by feeding other cattle prion-contaminated MBM produced from infected cows. There is no evidence that BSE spreads by contact between infected cattle and other cattle or other species.

Link to Disease in Humans
TSEs in humans include Creutzfeld-Jakob disease (CJD), Kuru, Gerstmann-Straussler-Scheinker Syndrome, and Fatal Familial Insomnia. The finding that the same strain of prion that causes BSE also causes a variant form of CJD (vCJD) in humans provides strong evidence that humans can contract vCJD from eating by-products from infected cattle. Central nervous system tissues (brain, spinal cord) and some portions of the intestine are the main sources of prion-contaminated material from infected cattle. Apart from BSE, no other animal TSE agent is known to cause human disease.

As of June 2005, 156 cases of definite or probable vCJD cases have been reported in the UK. To August 2004 there had also been 7 cases in France and one each in the Republic of Ireland, Italy, Canada and the United States. Three of the French cases and the Canadian and United States cases are considered to have resulted from exposure to the BSE agent whilst living in the UK during the BSE epidemic.

Prevention
There is no effective treatment for BSE. To stop its spread, many preventive measures have been established. The USDA-APHIS has restricted the importation of live ruminants and certain ruminant products from Europe. Since 1997, the FDA has banned the use of mammalian protein in the preparation of ruminant feed. All U.S. cattle showing abnormal neurological behavior are examined and tested for BSE. Additional public health protection measures implemented in 2004 include, blocking of non-ambulatory cattle from entering the food chain, use of a rapid screening diagnostic test on slaughtered cattle, removal and destruction of certain organs and restrictions on advanced meat recovery.

Drug products containing animal tissues may not be manufactured with tissue from a country with BSE. Blood centers must exclude potential donors who spent six or more months in the UK during 1980--1996. There is surveillance for human TSEs including cases of vCJD. Many scientists, including ACVP members, are studying BSE, CWD, vCJD and related neurologic diseases to better understand the pathogenesis of these enigmatic diseases and the means to control them.

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