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.
BSE is the first TSE transferable from animal to humans by consuming food products from animals, causing a major health/food safety issue.
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.
Through February 2011, BSE surveillance has identified 22 cases in North America: 3 in the U.S. and 19 in Canada (one was imported from the United Kingdom).
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.
Strains of BSE
There is increasing evidence that there are different strains of BSE: the typical BSE strain responsible for the outbreak in the United Kingdom and two atypical strains (H and L strains).
- Typical BSE strain -- The BSE strain responsible for most of the BSE cases in Canada is the same classic or typical strain linked to the outbreak in the United Kingdom. It is known to be preventable through elimination of BSE contaminated feed and has been causally linked to vCJD in humans. This typical strain has not yet been identified in any U.S.-born cattle.
- Atypical BSE strain -- In July 2007, the UK Spongiform Encephalopathy Advisory Committee (SEAC) suggested that atypical BSE may be a distinct strain of prion disease. Unlike typical BSE, cases of atypical BSE, according to SEAC, may have risen spontaneously (although transmission through feed or the environment cannot be ruled out). Recently reported French surveillance data support this theory that unlike typical BSE, atypical BSE appears to represent sporadic disease
Both of the U.S.-born BSE cases and two of the 19 Canadian-born BSE cases were 10 years of age or older. Of these older North American cases, three were linked to an atypical BSE strain known as the H-type. The strain type for the fourth older North American case, a 13-year-old BSE-infected Canadian cow, has been identified as the L-type.
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.
175 cases of vCJD were reported in the United Kingdom of Great Britain and Northern Ireland (United Kingdom), and 49 cases in other countries from October 1996 to March 2011. Following the successful containment of BSE epidemin in cattle, the number of cases of VCJD in the UK has declined since 2000.
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.