Mad cow disease investigation
Mad cow disease, or bovine spongiform encephalopathy (BSE), is a fatal neurological disease affecting cattle, first identified in Great Britain in 1986. The disease peaked in prevalence in 1992, impacting around 200,000 cattle, and has been linked to a variant form of Creutzfeldt-Jakob disease (vCJD) in humans, first diagnosed in 1995. This transmission occurred through the consumption of contaminated beef, resulting in approximately 233 diagnosed vCJD cases by mid-2024. The investigation into BSE is complicated by the disease's long incubation period, the lack of effective detection methods, and the unique nature of its infectious agent—a prion, which is a misfolded protein that does not elicit an immune response and does not contain genetic material. The initial outbreak was exacerbated by contaminated cattle feed, leading to strict regulations on animal feed practices to curb the spread of BSE. Global cases of BSE have since been reported in 29 countries, prompting ongoing surveillance programs aimed at preventing further human transmission and ensuring food safety. Understanding the origins and implications of BSE continues to be vital for public health and agricultural practices.
Mad cow disease investigation
DATE: Began in November 1986
THE EVENT: The lethal neurological disease bovine spongiform encephalopathy, commonly called mad cow disease, was first recognized in 1986 in Great Britain; the incidence of the disease peaked in 1992, and about 200,000 cattle were affected. The disease was transmitted to people who consumed contaminated beef, leading to a variant form of Creutzfeldt-Jakob disease that was first identified in 1995; the incidence of this disease peaked in 2000, and it has killed approximately two hundred people.
SIGNIFICANCE: The investigation of mad cow disease and its control were hampered by the unique nature of the infectious agent, the long incubation time involved in the disease process, the lack of a rapid detection system for infected animals and people, and the inadequacy of tests for contaminated feed and food.
Bovine spongiform encephalopathy (BSE) is one of several spongiform encephalopathies that may be heritable or transmissible. The infectious agent that causes BSE in cattle and the related variant form of Creutzfeldt-Jakob disease (vCJD) in people is novel in that it does not contain any nucleic acid (DNA or RNA) and it does not elicit an immune response, making it difficult to detect other than at autopsy, when spongelike lesions are seen in the brain. The infectious agent appears to be a protein called a prion. Heritable spongiform encephalopathies are caused by mutations in the prion gene, producing abnormal prions that adopt an unusual conformation and clump together over time to cause the brain pathology and neurological symptoms characteristic of the diseases. The diseases can be transmitted to susceptible animals or persons through the insertion of fragments or extracts from diseased tissue into the brain or bloodstream or, much less efficiently, by ingestion.
Spongiform Encephalopathies and Human Risk
Scrapie, found in sheep, is the earliest known spongiform encephalopathy. First noted during the 1730s in Britain, it is present in most countries of the world, except for Australia and New Zealand. It is passed from animal to animal, probably through contact with saliva, urine, or feces. The infectious agents, abnormal prions, decay slowly in the environment and may take years to dissipate from a contaminated pasture. Scrapie has not been known to infect humans, either through husbandry or through consumption of meat or milk.
Chronic wasting disease is a transmissible spongiform encephalopathy of deer, elk, and moose. It was first observed in 1967 in a captive mule deer in Colorado and has been identified in thirteen US states and two Canadian provinces. No transmission to humans has been documented.
Since appearing in 1986, BSE has been diagnosed in twenty-nine countries. It occurs in cattle between two and eight years of age and is fatal. In the course of the disease, the animals lose coordination and show extreme sensitivity to sound, light, and touch. Although BSE may be transmitted from mother to calf, the major cause of the BSE epidemic in Britain was cattle feed containing contaminated ruminant-derived protein. Sick or dead animals, as well as parts of the carcasses of slaughtered animals not sold for human consumption, are routinely converted to a protein-rich supplement.
As cattle feed became increasingly contaminated with BSE, the epidemic exploded. The incidence of the disease peaked in 1992; cases of BSE decreased after the feeding of ruminant-derived protein to cattle was banned. In Europe, more than four million at-risk cattle were destroyed. By May 2007, 188,535 cattle had been identified with the disease; this is a substantial underestimate, however, because cattle disposed of before age two would not show outward signs of the disease, and until 1990 few of these animals would have been tested.
Since the British outbreak, most developed countries have established BSE surveillance programs, and fewer than 100 new cases are reported per year. By mid-2024, more than 185,000 cases of BSE had been confirmed worldwide since 1986.
When the British BSE outbreak occurred, concerns arose in regard to the implications of BSE for human health, despite the fact that scrapie was long known not to be a risk to humans. A surveillance unit was established in 1990, and 10 cases of vCJD were reported in 1996. In Britain, the incidence of vCJD peaked in 2000. By mid-2024, 233 people had been diagnosed with it; this is an underestimate, given that only in Britain must any possible vCJD case be reported to a surveillance unit.
Consumption of contaminated beef was likely the source of vCJD; cooking does not inactivate the infectious agent. The ability of BSE to cross species barriers is one of its most insidious features. It has also been transmitted to zoo animals and domestic cats inadvertently fed contaminated meat or feed.
Perspective and Prospects
The BSE epidemic arose in Britain because protein supplements fed to cattle were derived from cattle that contained the abnormal prions. In the United States, the protein supplements fed to cattle come primarily from soybean meal. The origin of the infectious agent is likely to have been a spontaneous mutation of bovine prions that led to an abnormal conformation. The abnormal prions were amplified through the feeding of ruminant-derived protein back to cattle. It would be possible to create a BSE epidemic by incorporating the infectious agent, which is difficult to detect and to destroy, in an area’s food, water, or blood supplies. Manufacturing the agent would be a daunting task, however, and malicious use of the agent would not offer instant gratification in view of the long incubation period (years to decades) necessary for the disease to manifest itself.
Continued monitoring of BSE and vCJD is warranted. In addition, surveillance of the food supply should persist to prevent consumption of meat from BSE cattle. Bans on feeding ruminant-derived protein to cattle must be maintained and enforced until methods can be developed to ensure the destruction of the abnormal prions they may contain.
Bibliography
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Harris, D., ed. Mad Cow Disease and Related Spongiform Encephalopathies. New York: Springer, 2004.
Morgan, Kendall K. "Mad Cow Disease and vCJD: Should I Worry?" WebMD, 6 Feb. 2024, www.webmd.com/brain/mad-cow-disease-basics. Accessed 15 Aug. 2024.
Packer, Richard. The Politics of BSE. New York: Palgrave Macmillan, 2006.
Prusiner, Stanley B. “Detecting Mad Cow Disease.” Scientific American 291 (July, 2004): 86-93.
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Schwartz, Maxime. How the Cows Turned Mad: Unlocking the Mysteries of Mad Cow Disease. Berkeley: University of California Press, 2004.