Discovery

Reports of suspected clinical cases of BSE date back to 1985. However, the disease was not officially confirmed until November of 1986. ^{1, 3} Most of the BSE cases have been reported in cattle born in the United Kingdom. Ireland, France, Portugal, Belgium, the Netherlands, Luxembourg, Liechtenstein, Denmark, Switzerland, Germany, Spain, Italy, Greece, CzechRepublic, Slovakia, Slovenia, Austria, Finland, Japan, Israel, United States and Canada have also reported cases of BSE in indigenous cattle, but the number of cases in these countries is significantly lower than in theU.K.²¹

Most indigenous cases that have been detected in other countries appear to be the result of importation of contaminated feed.

Incidence

As of June 15, 2005 the total number of confirmed BSE cases in U.K. cattle was 184,266. ²¹ The epidemic peaked in 1992-93 with almost 1,000 cases per week.

On June 1, 2004, the USDA began an enhanced BSE surveillance program to determine prevalence of BSE in the US. The plan provided for testing as many cattle as possible, somewhere in excess of 200,000, animals at high risk for the disease, as well as 20,000 healthy, aged animals.  On June 24, 2005, USDA announced the first indigenous case of BSE in the United States.

The USDA, the Food and Drug Administration (FDA) and many arms of the livestock industry have taken a number of measures over the years to prevent the spread of BSE. See the BSE Timeline included in this document for additional details.

Transmission

The epidemiological data demonstrate that BSE in the U.K. is an extended common source epidemic involving feed containing TSE-contaminated meat and bone meal (MBM) as a protein source. There are different scientific hypotheses concerning the origins of BSE. The origin of the BSE causative agent is suspected to be from either Scrapie-affected sheep or cattle with a previously unidentified TSE. ^{3, 10} The 1997 FDA ban on feeding ruminant-derived supplements to cattle was designed to prevent amplification of BSE in cattle in this country.

Changes in rendering and feeding practices in the late 1970s and early 1980s may have played a part in the appearance of BSE and the subsequent amplification of the agent in the cattle population. However, no renderingprocess achieves complete inactivation of the BSE agent, so changes in rendering can not explain the epidemic entirely.

Cases that have been detected in other countries appear to be a result of importation of live cattle that have been slaughtered, rendered, and fed back to ruminants; or, more significantly, contaminated feed from the U.K. ¹⁵ There is no evidence that BSE spreads horizontally (i.e.,by contact between unrelated adult cattle and from cattle to other species).Limited evidence suggests that maternal transmission may occur at an extremely low level but that it would not perpetuate the epidemic under current British farming conditions. Research continues in this area.

Documented studies report that the BSE agent, to date, only has been found in brain, spinal cord and retina (eye) tissue of naturally infected cattle. Evaluation of experimentally inoculated cattle has found BSE infectivity in additional nervous and other tissues, specifically the dorsal root ganglia (nervous tissues connected to the spinal cord) and trigeminal ganglia (nervous tissue connected to the brain), as well as distal ileum (tissues in the intestines) and bone marrow. This research involved a series of experiments in which calves were either intracranially injected with or fed relatively large amounts of heavily infected brain from clinical BSE cases. Examination of the animals fed BSE-infected brain showed infectivity in the distal ileum six to eight months after exposure and in other central nervous tissues 30 months or more after exposures. Reports on the research state that muscle meat and other tissues were tested for infectivity at every stage of these experiments, and no infectivity was found.

Sheep have been experimentally infected with BSE by oral inoculation. Thus, the importation of sheep exposed to potentially BSE-contaminated protein concentrates in Europe are banned in the United States.Although other species have been experimentally infected with BSE, cattle currently are the only known food animal to be naturally infected with BSE.

Clinical Signs

Cattle with BSE display gradual changes in several aspects of their behavior, including:

• temperament changes such as increased nervousness or disorientation
• abnormal posture
• coordination problems
• difficulty in rising or walking
• decreased milk production
• severe muscular twitching
• loss of body weight despite a continued appetite

It is important to note that earlier in the clinical course of the disease, symptoms may be undetectable.In addition, not all animals show all signs of the disease.

The incubation period for BSE is usually four to five years. Following the onset of clinical signs, the animal’s condition rapidly deteriorates until it dies (usually within six months) or is destroyed. The disease is fatal, and there is no treatment or vaccine to prevent BSE.

Diagnosis/Diagnostic Tests

Diagnostic testing for BSE relies on direct examination of brain tissue for presence of abnormalities or assay for the presence of the resistant PrP proteins. The tests used to detect PrPRes are referred to as immunohistochemistry, immunoblot or ELISA (see Glossary) ^{3,4, 35, 36}

There currently are no proven practical tests available to detect BSE in live cattle. Several laboratories are in the process of developing new tests with practical applications, but these methods are still being evaluated.

For a complete list of BSE prevention measures, see BSE Timeline.

For additional information on worldwide incidence of BSE, link to the Office of International Epizootics/World Organization for Animal Health at http://www.oie.int/eng/info/en_esbru.htm