Bovine Spongiform Encephalopathy
By: Josephine Hall – Large Animal B
On March 13, 2006, the United States Department of Agriculture announced that a cow in Alabama had tested positive for Bovine Spongiform Encephalopathy (BSE), commonly known as Mad Cow Disease. This is the third reported case in the United States since BSE emerged, in 1985, as a new disease in the United Kingdom (U.K.) (BSE par. 1). Believed to have origins connecting it to the ovine disease scrapie, most of the scientific community believes the agent of BSE to be a prion, and it has evolved into a disease that is fatal to humans who contract it.
BSE turns placid cows into staggering, aggressive animals, hence the nickname Mad Cow Disease (Ferreiro 8). Autopsies of infected cattle indicate that this is a degenerative disease affecting the brain; holes in the gray matter give the brain a spongy appearance. Transmissible Spongiform Encephalopathy (TSE) is a name for all diseases that cause this spongy appearance, including BSE, Creutzfeldt-Jakob disease (CJD) in humans, and scrapie in sheep. Scrapie has been endemic in the U.K. for over 250 years (Ferreiro 40). Since then, it has occurred throughout Europe and much of the world, but while scrapie remained a concern for farmers and veterinarians, the general public was not troubled by it. However, with the evolution of BSE, people started to wonder if this disease could jump species to humans, and scientists began to look into how and why this TSE had suddenly started to affect cattle. The simultaneous nature of the outbreak throughout the U.K. led scientists to a common denominator: the feeding of meat-and-bone-meal (MBM) to cattle. MBM is produced by rendering, a practice that has existed for centuries. The process of rendering is the boiling and breaking down of those parts of animal carcasses that are not fit for human consumption. MBM is fed to livestock to accelerate growth and, therefore, increase production. However, this practice goes against nature by turning natural herbivores into carnivores and, even more alarming, into cannibals. There is little doubt that, in the U.K., many of the sheep carcasses recycled by rendering would have been infected with scrapie: the disease has a long incubation period, and infection is impossible to detect until clinical signs appear. So, if BSE is linked to scrapie, why did it not appear until 1985? Changes in the rendering process in the 1970’s, prompted in part by the energy crisis, led to rendering at lower temperatures for shorter durations, and the use of solvents in the process was stopped. These changes are believed to be responsible for allowing the BSE agent to transform into a pathogenic entity (Yam 114). Also, a marked increase in the U.K. sheep population, from 1980 to 1988, implies a relative proportional increase in scrapie infected carcasses in the MBM food chain. Scientists believe these changes increased cattle exposure to the scrapie agent which, in turn, led to asymptomatic infected cattle also being recycled in the rendering process. These factors, along with an 11% increase in the amount of MBM fed to animals during the 1980’s (Ferreiro 14), suggest how scrapie was able to mutate and jump species, develop into BSE, and ultimately cause a new variant CJD (vCJD) in humans.
Exactly what form the BSE agent takes has had scientists baffled for years, and even today not all of the scientific community agrees. However, years of work on the scrapie agent has led to some understanding of the cause of BSE. As scientists tried to isolate the scrapie agent, many thought it was a virus due to its small size, but if this was true, it was an unusually durable virus. In 1935, William S. Gordon developed a vaccine for the ovine disease louping-ill. In the process, he used formaldehyde which killed the live virus. Unfortunately, two years after inoculation, many of the sheep showed signs of scrapie. This outbreak was due to contamination of one of the batches of vaccine (Yam 56): the formaldehyde had failed to kill the scrapie agent. The scrapie agent also fails to create an immune response within the animals infected, something that viruses and all other known agents of disease do to some degree. Furthermore, ionizing radiation and ultraviolet light experiments led to the suggestion that this agent did not require DNA or RNA for reproduction which went against all understanding of molecular biology (Ferreiro 64). In 1967, J.S. Griffith, a mathematician from Bedford College in London, theoretically suggested the scrapie agent was a malformed version of a normal protein (Yam 59). Experiments, by American neurologist and biochemist Stanley B. Prusiner, expanded this theory, and, in 1982, he gave the agent a name: “proteinaceous infectious particles” or “prions” (Yam 63). Normal proteins can be broken apart and destroyed by the enzyme proteinase K, normal prion proteins included. While the pathogenic prion proteins differ from normal prions only in shape, they are more resistant and not readily broken down by proteinase K (Yam 68). Pathogenic prions spread simply by contact with normal prions, turning them into pathogenic entities. The lack of immune response in the body is explained by the fact that this is simply a normal protein; there is no foreign body for the immune system to react to. However, while this is the most accepted theory for what causes BSE, it should be noted that the scientific community has not ruled out other, currently undiscovered possibilities.
When BSE originally broke out in 1985, the British government was slow to respond. Some people assumed that, like scrapie, it would not pose an immediate danger to human health. Since beef plays a huge role in the British economy, there is little doubt that economics played a part in the timing of announcements during this crisis. It was not until the year following the outbreak that BSE was officially identified as a disease by the government. Also, it was not until 1988 that infected MBM was banned from ruminant feed, despite having been recognized as the source of BSE in 1987 (Ridgway 16). Although the British government was in no hurry to implicate problems with British beef, there was a lot of speculation in the media during this time. The concern of the general public was evident not only in the U.K., but also abroad: “On July 28, 1989, the European Union banned U.K. cattle born after July 18, 1988” (Yam 121). The following year, cats started to die from a TSE disease, elevating fears that BSE had crossed species. The government response was the insistence that British beef was safe to eat. However, cases of CJD in the U.K. began to increase. CJD is generally considered an old person’s disease, but now it was affecting an increasing number of young people. In 1995, following the death of 19 year old Stephen Churchill, an autopsy revealed that his brain looked more like that of a cow with BSE than a patient with traditional CJD (Ridgway 7). Further similar deaths forced the government to announce, in 1997, a “probable link” between BSE and vCJD (Ridgway 18). As of March 31, 2006, 155 people in the U.K. have died of vCJD (CJD par. 2).
The evolution of BSE from scrapie is now a generally accepted fact, and the prion remains a challenge for the scientific community. Thanks to changes in agricultural practices, cases of BSE and vCJD have decreased substantially. However, governments throughout the world should learn from the mistakes made in the U.K. Three cases in the United States could mean that America is relatively free of this disease, or it could simply indicate that testing is inadequate. This disease can only be contained if health and safety is considered ahead of industry and profit.
BSE in an Alabama Cow. 15 March 2006. Centers for Disease Control and Prevention.
21 March 2006. <www.cdc.gov/ncidod/dvrd/bse/news/alabama_cow_031506.htm> .
CJD Statistics. 31 March 2006. The National Creutzfeldt-Jakob Disease Surveillance Unit, The University of Edinburgh. 29 April 2006. <www.cjd.ed.ac.uk/figures.htm>.
Ferreiro, Carmen. Deadly Diseases and Epidemics - Mad Cow Disease (Bovine Spongiform Encephalopathy). Philadelphia: Chelsea House Publishers, 2005.
Ridgway, Tom. Epidemics: Deadly Disease Throughout History - Mad Cow Disease: Bovine Spongiform Encephalopathy. New York: The Rosen Publishing Group, Inc., 2002.
Yam, Philip. The Pathological Protein – Mad Cow, Chronic Wasting, and Other Deadly Prion Diseases. New York: Copernicus Books, 2003.
