Mad cow disease

DEFINITION: Cattle disease affecting the central nervous system

The sheep disease scrapie is generally believed to have jumped the species barrier to become mad cow disease. It breached the species barrier again to infect humans who ate beef. The introduction of the pathogen into the human food chain has been attributed to the use of scrapie-infected meat and bonemeal in cattle feed.

Mad cow disease, formally known as bovine spongiform encephalopathy (BSE), is believed to be related to variant Creutzfeldt-Jakob disease (vCJD), a fatal human illness. It is marked by the deterioration of brain tissue and progressive degeneration of the central nervous system, which causes symptoms such as impaired physical coordination, staggering, unusual aggression, and other abnormal behavior. Since it was first identified in Great Britain in 1986, BSE has infected cattle throughout the European Union. Scattered cases have also been found in the United States, Canada, the Falkland Islands, Israel, Oman, and Japan.

BSE is a transmissible spongiform encephalopathy (TSE), a class of diseases so named because of the brain damage that characterizes them—the tissue is left riddled with small holes, like a sponge. Other species affected by TSEs include deer, elk, and antelope (chronic wasting disease), sheep and goats (scrapie), mink (transmissible mink encephalopathy), cats (feline spongiform encephalopathy, or FSE), and humans (kuru, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and Creutzfeldt-Jakob disease, or CJD). All TSEs have long incubation periods that range from months to years. Diagnosis is confirmed only through autopsy and examination of the brain tissue. All known TSEs are incurable, untreatable, and fatal.

BSE is believed to have arisen because of an present in cattle feed during the 1970s and 1980s. Meat and bonemeal, a protein concentrate produced by rendering plants (facilities that process animal carcasses and slaughterhouse wastes into commercial products), was added to cattle feed as a nutritional supplement with increasing frequency during the 1980s. Among the rendered wastes from which the supplement was made were the carcasses of sheep that had died from scrapie. Through infected protein supplement, scrapie is thought to have crossed the species barrier from sheep into cattle and become a new TSE. An increase in Great Britain’s sheep during the late 1970s and early 1980s elevated the number of scrapie cases, and thus more infected material was rendered. Coincidentally, at about this time, Britain’s rendering industry made changes to the rendering process—eliminating the solvent extraction of fats and a subsequent steam-stripping treatment—that may have allowed more infectious agent to pass into the finished product.

The British BSE Epidemic

Not long after it was first identified in 1986, BSE reached epidemic proportions in the United Kingdom. The epidemic peaked in 1992, with 37,280 confirmed cases. Mandatory reporting of BSE cases and destruction of symptomatic animals began in 1988, as did a ban on the feeding of ruminants (cattle, sheep, goats, and deer) with ruminant-derived protein. In 1989, the British government banned the human consumption of cattle brains and other “specified offals” believed to be possible carriers of infection. In 1990, a British domestic cat was diagnosed as the first victim of FSE, causing public concern over the possible spread of BSE beyond livestock and a temporary drop in the nation’s beef consumption. That year, the British government established a system for reporting and tracking CJD to monitor possible BSE effects on human beings.

In 1993, the British National CJD Surveillance Unit began to receive reports of unusual CJD cases. While CJD typically affects one person in one million between the ages of fifty-five and eighty, CJD was appearing in younger patients, with uncharacteristic patterns of brain damage and with atypical frequency. By early 1996, eight cases of variant CJD (vCJD) in people under forty-two years of age had been identified. Global attention turned to Britain’s mad cow crisis in March 1996, when British health secretary Stephen Dorrell announced that ten cases of vCJD had been confirmed and that they were most likely the result of to BSE-contaminated beef before the 1989 specified offal ban. The announcement triggered a nationwide panic in Britain, caused a sharp drop in British beef sales, and exacerbated political and economic friction between Britain and the rest of the European Union (EU).

Researchers have since found more conclusive evidence linking BSE and vCJD, but they have not determined the exact cause of these and other TSEs. One of the more widely accepted theories is that the illnesses are induced by tiny proteinaceous infectious particles, or prions. Prions resemble an abnormally folded version of protein, a harmless, naturally occurring protein found in the brains of all mammals. Prions do not break down in mammalian digestive systems and provoke no immune response. Lacking genetic material (unlike more familiar infectious agents, such as bacteria, viruses, and fungi), prions are believed to spread by invading cells and converting normal prion protein into the aberrant form. As the prions accumulate, they damage brain cells and ultimately kill the organism.

Another theory is that BSE originated as a cattle TSE called bovine amyloidotic spongiform encephalopathy (BASE). Researchers found that the prions associated with BASE, first identified in 2003 during BSE testing in Italy, could convert to BSE prions in lab tests. Yet another hypothesis is that BSE’s ability to manifest as multiple strains suggests instead a viruslike with nucleic acids that can transmit genetic information. Whatever the agent that causes BSE, it is highly resistant to ultraviolet light, and temperature extremes, ionizing radiation, and many chemical disinfectants.

Responses

British efforts to contain a livestock epidemic that was only partially understood while protecting human health and allaying public fears about beef consumption created new problems. In a government-instituted culling program, some 4.4 million animals considered at were slaughtered. Neither the designated rendering plants that processed the remains nor the designated incinerators that burned the rendered product were able to keep up with the slaughter rate. The resulting backlog of hundreds of thousands of tons of beef and rendered products were warehoused pending disposal. Incineration plans proved unpopular with Britons, many of whom were unconvinced that burning cull material was safe. Public concern also arose over the possible contamination risk posed by cattle carcasses landfilled and liquid rendering wastes discharged to the during the early years of the epidemic.

Another matter of concern was the possible person-to-person transmission of the disease through medical procedures such as blood transfusions, human growth hormone treatments, and cornea transplants. The first patient believed to have contracted vCJD from infected blood died in 2003, about six years after receiving the suspect transfusion. The donor had given blood in 1996 before precautions to protect blood supplies were implemented; he died in 1999. In 2016, blood screening technology was developed that can detect vCJD with great accuracy before symptoms emerge. This discovery was revolutionary to the vCJD epidemic.

The number of BSE cases in the United Kingdom declined sharply after 1993 and by 2008 was down to thirty-seven. From 2014 to 2021, only six cases of BSE in the United Kingdom were confirmed. Cases reported outside the United Kingdom have generally decreased since 2005. Countries that have learned from the British experience have used feed bans, programs, precautionary recalls, and other measures to contain the spread of the disease and keep BSE pathogens out of the human food chain. Perceptions that the United States does not employ sufficient precaution against BSE have led many countries to implement partial or full restrictions on the import of American beef, at least temporarily.

As of 2021, a total of 232 people had contracted and died from vCJD since the disease was first identified: 178 in the United Kingdom, 28 in France, 5 in Spain, 4 from Ireland, 4 in the United States, 3 in the Netherlands, 2 in Portugal, 3 in Italy, 2 in Canada, and 1 each from Japan, Saudi Arabia, and Taiwan. It is believed that two of the four cases each from the United States and Ireland, one of the two cases from Canada, and the single case from Japan, were likely exposed to the BSE agent while living in the United Kingdom. Three of those who had died in the United Kingdom were known to have contracted the disease through blood transfusions. The number of vCJD cases has generally declined since the year 2000, but the disease’s long incubation period makes it impossible to forecast its future trends.

Bibliography

Becker, Geoffrey S., Curtis W. Copeland, and Sarah A. Lister, eds. Mad Cow Disease (Bovine Spongiform Encephalopathy). New York: Nova Science, 2008.

"Bovine Spongiform Encephalopathy (BSE)." Centers for Disease Control and Prevention, 10 May 2024, www.cdc.gov/mad-cow/php/animal-health/index.html. Accessed 19 July 2024.

Caruso, Catherine. “New Test Spots Human Form of Mad Cow Disease with 100 Percent Accuracy.” Scientific American, 21 Dec. 2016, www.scientificamerican.com/article/new-test-spots-human-form-of-mad-cow-disease-with-100-percent-accuracy/. Accessed 3 Feb. 2023.

Lyman, Howard F., and Glen Merzer. Mad Cowboy: Plain Truth from the Cattle Rancher Who Won’t Eat Meat. 1998. Reprint. New York: Touchstone, 2001.

"Mad Cow Disease Fast Facts." CNN. Cable News Network and Turner Broadcasting System, 11 Jan. 2015. Web. 2 Feb. 2015.

Nunnally, Brian K., and Ira S. Krull, eds. Prions and Mad Cow Disease. New York: Marcel Dekker, 2004.

Prusiner, Stanley B. Prion Biology and Diseases. 2d ed. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2004.

Rampton, Sheldon, and John Stauber. Mad Cow U.S.A. Updated ed. Monroe, Maine: Common Courage Press, 2004.

Rhodes, Richard. Deadly Feasts: The “Prion” Controversy and the Public’s Health. New York: Simon & Schuster, 1997.

Ritchie, Diane L., Alexander H. Peden, and Marcelo A. Barria. "Variant CJD: Reflections a Quarter of a Century on." Pathogens, vol. 10, no. 11, 2021, doi:10.3390/pathogens10111413. Accessed 19 July 2024.

Schwartz, Maxime. How the Cows Turned Mad: Unlocking the Mysteries of Mad Cow Disease. Berkeley: University of California Press, 2004.

“Single Case of Classical BSE Confirmed on a Farm in Somerset.” GOV.UK, Sept. 2021, www.gov.uk/government/news/single-case-of-classical-bse-confirmed-on-a-farm-in-somerset. Accessed 3 Feb. 2023.

Stone, Andrea. "Fourth Death from Mad Cow Disease Confirmed in US." National Geographic.National Geographic Society, 4 June 2014. Web. 2 Feb. 2015.

"vCJD (Variant Creutzfeldt-Jakob Disease)." CDC. Centers for Disease Control and Prevention, 3 June 2014. Web. 2 Feb. 2015.

Yam, Philip. The Pathological Protein: Mad Cow, Chronic Wasting, and Other Deadly Prion Diseases. New York: Copernicus Books, 2003.