American Veterinary Medical Association - BSE

Background

Bovine spongiform encephalopathy (BSE), sometimes called "Mad Cow Disease" is a slowly progressing, degenerative disease affecting the central nervous system of cattle. The disease belongs to a group of related diseases known as transmissible spongiform encephalopathies (TSEs). The TSEs known to occur in animals in the United States include scrapie, which affects sheep and goats; chronic wasting disease of mule deer and elk; and transmissible mink encephalopathy. TSEs in humans includes Creutzfeldt-Jakob Disease (CJD). CJD occurs sporadically worldwide at a rate of 1 case per 1 million people per year. In 1996 in Great Britain, another rare human neurological disease, Variant Creutzfeldt-Jakob Disease (vCJD) was first described and an association with BSE was recognized.

BSE was first recognized in Great Britain (England. Wales, Scotland) in 1986. Since then the disease has been confirmed in native cattle in other European countries, and in animals imported from Great Britain to several other countries, including Canada with a case in 1993. There have not been any cases of BSE found in any U.S.A. cattle.

Causative Agent

The nature of the BSE agent is still a matter of debate. According to the prion theory, the agent is composed largely, if not entirely, of a self-replicating protein, referred to as a prion. Another theory argues that the agent is virus-like. Strong evidence supports the prion theory, but the ability of the BSE agent to form multiple strains is more easily explained by a virus-like agent.

The agent is highly stable, resisting ultraviolet radiation, ionizing radiation, common disinfectants, freezing, drying and heating at normal cooking temperatures, even those used for pasteurization and sterilization.

Clinical Signs

Cattle affected by BSE display no pathognomonic clinical signs, but they may exhibit some of the signs of a progressive degeneration of the nervous system such as nervousness, changes in temperment, head-shyness (easily frightened), abnormal posture, incoordination, difficulty in rising, and eventual death. Decreased milk production may also occur. The most frequently cited clinical signs are changes in mentation, sensation, and locomotion. The herdsman who has a thorough familiarity with the animal can incoordination, difficulty in rising, and eventual death. Decreased milk production may also occur. The most frequently cited clinical signs are changes in mentation, sensation, and locomotion. The herdsman who has a thorough familiarity with the animal can notice such changes. The majority of cases early in the course of the disease would not be readily evident to a casual observer or to a veterinarian that does not have extensive experience with BSE. Following onset of clinical signs, the animal’s condition deteriorates within 2 weeks to 6 months. Currently there is no treatment and no test available to detect the disease in live animals. Confirmation of BSE is made by postmortem histopathological examination of brain tissue. Immunocytochemistry or electron microscopy provide adjunctive diagnostics.

Epidemiology

Epidemiological studies conducted in Great Britain suggest that the source of BSE was cattle feed prepared from carcasses of ruminants. Speculation as to the cause of the appearance of the disease has ranged from spontaneous occurrence in cattle, the carcasses of which then entered the cattle feed chain, to entry into the cattle feed chain from the carcasses of sheep with a similar disease, scrapie, with subsequent adaptation of the agent to cattle. The emergence of the epidemic in Great Britain, under either hypothesis, is linked to enhanced survival of the agent in ruminant-derived protein subsequent to changes in the rendering process. The recycling of infected cattle parts through rendering and subsequent feeding of ruminant-derived protein amplified the epidemic.

After its emergence in the mid-1980s, the number of BSE cases in Great Britain peaked in 1992. During that year, as many as 1,000 new cases were reported each week. The number of cases has gradually decreased, and the British epidemic is expected to die out early in the twenty-first century. However in late 2000, the number of cases in other countries in Europe began to increase partially because of aggressive testing programs. Epiderniological studies in some of these countries have shown the likely cause of infection was exposure to feed containing B SE-contaminated protein products of animal origin, much of it originating from Great Britain. Implementation of an effective control program across Europe lagged behind Great Britain. BSE also has been associated with the emergence of TSEs in exotic ruminants (e.g., nyala, gemsbok), exotic cats (e.g.. cheetah, puma), and domestic cats.

Control Measures in Great Britain

The most critical control measures seem to have been (1) mandatory notification of suspect cases and adequate farmer compensation; (2) control of live animal movements; (3) efficient diagnostic capabilities; (4) prohibitions on feeding ruminant-derived rendered protein to all ruminants; and (5) stringent rules for rendering raw materials. In 1996, the original ban on ruminant-to-ruminant feeding was extended to prohibit the use of mammalian-origin meat and bonemeal for all farm animals.

To further protect human health, the British Government instituted a series of other measures to minimize the risk of disease transmission. These included removal of some "high risk" materials (such as brain, spinal cord and intestines) from cattle at slaughter, and a ban on cattle over 30 months of age from being used for food.

Preventive Measures in the U.S.A.

No BSE case has ever been detected in the United States. A series of actions have been implemented to prevent the entry of BSE and to prevent its amplification if it should occur in the United States. These measures include prohibitions or restrictions on certain animal and product importations, surveillance, education, a ruminant-feed ban, and development of an emergency response plan in the event of an introduction of BSE.

In 1989, the United States prohibited the importation of ruminants and most ruminant products from countries affected with BSE. The cattle that were imported from the United Kingdom or Ireland between 1981 and 1989 were traced and all but 32 of the 496 cattle were found. The few animals still alive are under quarantine and are monitored. All cattle of unknown status would be greater than ten years of age and would have a reduced likelihood of developing BSE at this late date. Two cows imported from Belgium in 1996 and 33 from other European counties are also under quarantine. No evidence of BSE has been found in any of these imported animals. Additionally 2 flocks of sheep imported from Europe are under quarantine. An undifferentiated TSE has been diagnosed in these sheep. Because they may have been exposed to BSE infected feed in Europe, the sheep are quarantined.

In 1997, the United States prohibited importation of live ruminants and most ruminant derived products from Europe, pending a thorough risk assessment. In 2000, a prohibition was placed on all imports of rendered animal protein products, regardless of species, from Europe. This followed a determination by the European Union that feed of non-mammalian origin was potentially cross-contaminated with the BSE agent.

In 1990, the United States began an aggressive and active surveillance program to ensure timely detection and swift response in the event an introduction were to occur. The surveillance program included locating the imported cattle from countries with detected BSE and targeted active and passive surveillance for either BSE or another TSE in cattle. Samples of brain for BSE surveillance of adult cattle are obtained from (1) field cases of cattle exhibiting signs of neurological disease; (2) cattle condemned at slaughter for neurological reasons; (3) rabies-suspect cattle submitted to public health laboratories; (4) neurological cases submitted to veterinary diagnostic laboratories and teaching hospitals; and (5) random sampling of aged cattle (predominantly dairy) that are nonambulatory at slaughter. In 1999, the USDA expanded surveillance of fallen stock and, in 2000, expanded this surveillance even further. The USDA also regionalized states to increase coverage. As of December 31. 2000, 11,954 brains had been examined. No evidence of either BSE or another TSE in cattle has been found.

In addition to this active surveillance, general surveillance takes advantage of existing data sources. These include a database with diagnoses submitted from 27 US veterinary schools, CNS antemortem condemnation data from USDA Food Safety and Inspection Service, necropsies performed at zoos on various species, and a veterinary diagnostic laboratory reporting system. Referrals of unusual cases by private practitioners to veterinary schools and diagnostic laboratories provide additional surveillance.

In 1991, the United States rendering industry began a voluntary ban to exclude high-risk tissues (sheep heads and scrapie-infected sheep) from being rendered. In 1996, cattlemen initiated a voluntary ban on ruminant-to-ruminant feeding. To further reduce the risk associated with animal feeds, the Food and Drug Administration prohibited feeding of most mammalian protein to ruminants effective in 1997. Aggressive enforcement action has been instituted.

In 1991, analyses were performed to assess the risk factors associated with BSE. The risk analysis was updated in 1993 and 1996. These risk factor analyses continue to demonstrate that the overall risk of BSE in the United States, based on what is known about TSEs, is extremely low. In 1998, the USDA entered into a cooperative agreement with Harvard University’s School of Public Health to analyze and evaluate the Department’s prevention measures. The two-year study is reviewing current scientific information, assessing the ways that BSE could potentially enter the United States, and identifying any additional measures that could be taken to protect human and animal health. A report is expected in the spring of 2001.

Selected References:

  • Transmissible Spongiform Encephalopathies, Task Force Report, Council for Agricultural Science and Technology, October 2000.
  • Bovine Spongiform Encephalopathy, World Health Organization Fact Sheet, December 2000.
  • Questions and Answers on Bovine Spongiform Encephalopathy, FDA Center for Biologics Evaluation and Research, December 2000.
  • Questions and Answers — Bovine Spongiform Encephalopathy, USDA APHIS Veterinary Services, January 2001.
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