By: Jacob Chacko, D.V.M.
Public Health Veterinarian,
United States Department Of Agriculture
Bovine Spongiform encephalopathy (BSE) is a progressive neurological disorder of cattle that results from infection by an unconventional transmissible agent. Strong scientific evidence has seen for a relationship between outbreaks in Europe of disease in cattle called bovine spongiform encephalopathy (BSE, or “mad cow disease”) and a disease in humans originally called new variant Creutzfeldt-Jakob disease (vCJD). Both disorders are fatal brain diseases with unusually long incubation periods measured in years. Although there is very strong evidence that the agent responsible for the human disease is the same agent responsible for the BSE outbreaks in cattle, the specific foods that might be associated with the transmission of this agent from cattle to humans are unknown. However, bioassays have identified the presence of the BSE agent in the brain, spinal cord, retina, dorsal root ganglia (nervous tissue located near the backbone), distal ileum, and the bone marrow of cattle (called Specific Risk Materials or SRMs) experimentally infected with this agent by the oral route.
Through the end of November 2003, more than 183,000 cases of BSE were confirmed in the United Kingdom alone in more than 35,000 herds. The outbreak may have resulted form the feeding of scrapie-containing sheep meat-and-bone meal to cattle. There is strong evidence and general agreement that the outbreak was amplified by feeding rendered bovine meat-and-bone meal to young calves.
The nature of the transmissible agent is unknown. Currently, the most accepted theory is that the agent is a modified form of a normal cell surface component known as PRION protein. The pathogenic form of the protein is both less soluble and more resistant to enzyme degradation than the normal form.
On December 23, 2003, the U.S. Department of Agriculture announced a presumptive diagnosis of bovine spongiform encephalopathy (BSE, or “mad cow” disease) in an adult Holstein cow from Washington State. Samples were taken from the cow on December 9 as a part of USDA’s BSE surveillance program. The BSE diagnosis was made on December 22 by histopathology and immunohistochemical testing at the National Veterinary Service Laboratory, Ames, Iowa. The diagnosis was confirmed by an international reference laboratory in Weybridge, England, on December 25. Preliminary trace-back based on an ear-tag identification number suggests that the BSE-infected cow was imported into United States from Canada in August 2001.
Strong evidence indicates that BSE has been transmitted to humans primarily in the United Kingdom, causing a variant form of Creutzfeldt-Jakob disease (vCJD). As of December 1, 2003, a total of 153 vCJD cases had been reported worldwide; of these, 143 cases had occurred in United Kingdom. The Centers for Disease Control and Prevention (CDC) monitors the trends and current incidence of CJD in the United States.
An experimental study reported in June 1996 showed that three cynomologus macaque monkeys inoculated with brain tissue obtained from cattle with BSE had clinical and neuropathological features strikingly similar to those of variant CJD (Nature 1996; 381:743-4). An experimental study involving inoculation of a panel of inbred mice with the agents causing BSE and variant CJD substantially increased the strength of the scientific evidence for a causal association between variant CJD and BSE (Nature 1997; 389:498-501). In this study, groups of inbred mice and a group of cross-bred mice inoculated with brain homogenates from variant CJD cases were reported to have had latency periods and lesion profiles consistent with the BSE pattern.
Preventive Measures: Public health control measures, such as enhanced BSE surveillance, the culling of sick animals, and bans of specified risk materials (SRM), have been instituted in countries of Europe and United States to prevent potentially BSE-infected tissues from entering the human food chain. The most stringent of these control measures, including an “Over Thirty Months Scheme” that excludes all animals older than 30 months from the human food and animal feed chain, have been applied in the United Kingdom and appear to be highly effective. In June 2000, the European Union Commission on Food Safety and Animal Welfare strengthened the European Union’s set of control measures in relation to BSE by adopting a decision requiring all member states to remove SRMs from the animal feed and human food chains. Milk and milk products from cows are not believed to pose any risk for transmitting the BSE agent.
Reference:
- Journal of the American Medical Association, November 8, 2000 (Volume: 284, No.18, pp.2322-3).
- Clinics of Laboratory Medicine, December 2002 (Volume 22, pp. 849-62).
- Nature 1996; 381: 743-4.
- Nature 1997; 389: 498-501.
- Proc National Acad Sci. 1999;96 : 15137-42