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Bovine Spongiform Encephalopathy (BSE)
What is Bovine Spongiform Encephalopathy (BSE)? BSE (bovine spongiform encephalopathy) is a progressive neurological disorder of cattle, and has been called “mad cow disease.” Its symptoms are similar to “scrapie,” a brain disease that occurs in sheep. Cattle affected by BSE experience progressive degeneration of the nervous system. Affected animals may display changes in temperament, such as:
Affected cattle die and currently preventive treatment, including a vaccine, does not exist. The incubation period (the time from when an animal becomes infected until it first shows disease signs) is from 2 to 8 years. Following the onset of clinical signs, the animal’s condition deteriorates until it either dies or is destroyed. This process usually takes from 2 weeks to 6 months. Most cases
in Great Britain have occurred in dairy cows
between 3 and 6 years of age.
As of November 2003, more than 183,000 cases of
BSE were confirmed in the United Kingdom in more
than 35,000 herds. The BSE epidemic in the United
Kingdom peaked in January 1993 at almost 1,000 What causes BSE? BSE and scrapie both result from infection with a
very unusual infectious agent. However, the exact
causative agent of the disease has not yet been Three main theories on the nature of the agent have been proposed:
Currently, the most accepted theory is that the agent is a modified form of a normal cell protein known as a prions. A prion is not a bacterium, parasite, or virus, and thus treatments usually used for treating or prevention bacterial infections (e.g. antibiotics) or viral infections are not effective against prions. Currently, it is accepted that the BSE agent:
The outbreak of BSE in the United Kingdom may have resulted from the feeding of scrapie-containing sheep meat-and-bone meal prepared from cattle to young calves. Does BSE or a similar disease occur in humans? BSE belongs to a group of progressive degenerative
neurological diseases known as transmissible
spongiform encephalopathies (TSEs) diseases are The TSE diseases include:
There are six TSE diseases that affect people:
The human diseases are very rare; for example, classical CJD has been well studied and occurs sporadically worldwide at a rate of about one case per one million people. Where is the BSE agent found? In cattle naturally infected with BSE, the BSE agent has been found in:
Additional experimental studies suggest that the BSE agent may also be present in:
How was BSE spread or transmitted? It is thought that BSE was spread via meat-and-bone
meal fed to cattle. The practice of using this material
as a source of protein in cattle feed has been In the late 1970s there was a change in the production (rendering) process used to make this meat and bone meal. One hypothesis is that this change permitted the infectious agent of scrapie (a transmissible spongiform encephalopathy, or TSE, of sheep) to survive the rendering process, and get transmitted to other animals, such as cows, that are fed meat-and-bone meal nutritional supplements. Does BSE occur in the United States? On Dec. 23, 2003, the U.S. Department of Agriculture (USDA) announced a presumptive diagnosis of BSE in an adult Holstein cow from Washington state. USDA has launched an epidemiologic investigation to determine the source of the disease. For up-to-date information access the USDA website at www.usda.gov. Is BSE a foodborne hazard in the United States? Strong evidence indicates that BSE has been transmitted to humans primarily in the United Kingdom, causing a variant form of Creutzfeldt-Jakob disease (vCJD). In the United Kingdom, where over 1 million cattle may have been infected with BSE, a substantial species barrier appears to protect humans from widespread illness. As of Dec. 1, 2003, a total of 153 vCJD cases had been reported worldwide; of these, 143 cases had occurred in the United Kingdom. The risk to human health from BSE in the United States is extremely low. What countries have reported BSE? The vast majority of cases of BSE (more than 99% as of 1999) have been reported from the United Kingdom during an epidemic. However, endemic cases have also been reported in other European countries including:
The numbers of reported cases by country are available on the web site of the Office International des Epizooties (www.oie.int). These numbers should be interpreted with caution, however, because the intensity and methods of surveillance probably vary over time and by country. How is the BSE agent detected? Mouse inoculation studies take a long time (up to 700 days) to detect the agent, and a negative result (that is, lack of brain tissue changes in the injected mice) may only mean that there was too little of the infectious agent to cause symptoms, not that the material was really free of the infectious agent altogether. It is also possible to detect the presence of the abnormal prion protein in tissue (such as brain) using special staining procedures although these methods do not allow an accurate assessment of infectivity of the infected material. What has the British government done in response to the BSE epidemic? In response to the BSE epidemic, the British Government instituted a series of measures to minimize the risk of disease transmission among both animals and humans. These included:
Following institution of these measures, Great Britain has seen a decrease in the number of cattle with BSE from a peak incidence of 36,680 confirmed cases in 1992 to 2,254 confirmed in 1999 (information on the BSE epidemic in Great Britain is available at www.defra.gov.uk). Where can I get more information? Access the Centers for Disease Control and Prevention website at www.cdc.gov, the USDA website at www.usda.gov or contact the Southern Nevada Health District, Office of Epidemiology at (702) 759-1300. |
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