Joy Carerra Luba Mad Cow Disease: Bovine Spongiform Encephalopathy Bovine Spongiform Encephalopathy (BSE), otherwise known as Mad Cow, is a fatal disease that affects the central nervous system of cattle. The name comes from the microscopic pores seen in the brain of an infected specimen that cause it to resemble a sponge (Epidemiological Bulletin, March 2001). This relatively new disease has had tremendous impact on the political and economic status of countries that rely heavily on the market of raising and exporting cattle. Since the discovery of BSE in 1986, much research has been done to determine the particular epidemiology and pathology of the disease. It is believed that the causative agent of Bovine Spongiform Encephalopathy (BSE) is a proteinaceous infectious particle, or prion. Prions are proteins that contain no DNA or RNA, yet are able to replicate themselves without the use of genes. The prion protein is found at the surface of cells, mainly those of the spinal cord and brain. There are two types of prion protein: normal (PrPC); abnormal or infectious (PrPSc). The normal strain of the prion is important to cell maintenance. It is soluble and protease sensitive. It is the abnormal form of the protein that causes BSE. (Epidemiological Bulletin, March 2001). The abnormal PrPSc prion proteins are insoluble and partially protease resistant so the body has difficulty expelling them (American Association for the Advancement of Science, May 9 2003). The infectious proteins replicate themselves by forcing normal, healthy prions into conforming to the shape of the abnormal ones. The normal form of the protein has a helical shape which the abnormal protein changes into a sheet-like shape Current Science, March 5 2004). It is the infectious prion s ability to transform normal, useful prions into exact duplicates that makes it difficult to estimate gestation times and track the progress of the disease. While the first case of BSE was identified in Great Britain in 1986, it is believed that the first cattle infections were as early as the 1970 s (Centers for Disease
Control, July 7 2005). It is believed that the infectious prions entered the cattle by way of being feed meat and bone meal made from sheep that were contaminated by Scrapie, a relative of BSE that infects the sheep (Centers for Disease Control, July 7 2005). The beginning of the BSE epidemic appears to be related to the changes made in the way that livestock carcasses were processed into animal feed in the late 1970s (American Scientist, Jul/Aug 2004). The process of liquefying separate batches of animal carcasses was replaced by a process in which the carcasses moved continuously through the rendering machines, which could have prevented an even exposure to heat. The practice of extracting the material with hydrocarbon solvents under steam was also eliminated (American Scientist, Jul/Aug 2004). These changes could have allowed the infectious proteins to survive in the meat and bone meal, which was subsequently fed to the herds of cattle in Great Britain. Following the ban of meat and bone dietary supplements for cattle in 1987, there was a significant decrese in the number of BSE infected cows (American Scientist, Jul/Aug 2004). BSE became of heightened interest to the medical community after the disease was discovered in some domestic cats and and wild cats, such as lions and tigers, kept in captivity. It is believed that the domestic cats ingested the infectious proteins in meat and bone meal based foods, while the wild cats contracted the disease by eating raw tissues that included those from the central nervous system of contaminated cattle (Epidemiological Bulletin, March 2001). The finding of BSE in animals other than cattle caused the scientific community to worry the BSE may be able to infect humans. A decade after the first documented cases of BSE, the scientific community saw its fears become reality. In 1996 a variant of Creutzfeldt-Jakob Disease (CJD), another relative of the BSE infection, was found in a human in the Great Britain. While CJD is most commonly a hereditary disease, the variant CJD prion most resembles the prion that causes BSE. The way the prion proteins were folded in the variant CJD infections greatly resembled those in the BSE infected cattle(american Scientist, Jul/Aug 2004). Variant CJD is also found predominately in victims under thirty years of age, while the normal CJD infection manifests itself in the fifty to seventy age range (American Scientist, Jul/Aug 2004). The time frame in which the variant CJD incubates is another link to BSE.
The ten year span between the exposure to contaminated meat products and the onset of variant CJD is consistant with the ten year incubation period of the BSE prion (Centers for Disease Control (June 29 2005). As of yet there is no sure way to determine if a person has been infected with the variant CJD. The only way to confirm the presence of the abnormal prion is through autopsy, and like BSE, the variant CJD is always fatal (Centers for Disease Control, June 29 2005). It is believed that the BSE epidemic began in the United Kingdom due to the large ratio of sheep to cattle farming. Seeing as how BSE is thought to be an altered form of the similar sheep disease Scrapie, which has been recorded to have been around since the 1800s, it is reasonable to assume that the disease would originate in a region high in the number of sheep herds (Epidemiological Bulletin March 2001). The following table illustrates the trend in the number of cattle infected with BSE from its discovery in 1986 up until the year 2000. Year Number of cases of BSE reported in the United Kingdom Great Britan Northern Ireland Isle of Man Jersey Guernsey Total United Kingdom Up to 1987 442 0 0 0 4 446 1988 2469 4 6 1 34 2514 1989 7137 29 6 4 52 7228 1990 14181 113 22 8 83 14407 1991 25032 170 67 15 75 25359 1992 36682 374 109 23 92 37280 1993 34370 459 111 35 115 35090 1994 23945 345 55 22 69 24436 1995 14302 173 33 10 44 14562 1996 8016 74 11 12 36 8149 1997 4312 23 9 5 44 4393 1998 3179 18 5 8 25 3235 1999 2274 7 3 6 11 2301 2000 1076 14 0 0 11 1101 (Epidemiological Bulletin, March 2001) The data shows that the epidemic hit the largest proportions from 1991 to 1994. It is believed that this crest in the number of infected animals was due to the use
of undiagnosed sick animals in the production of cattle feed (Epidemiological Bulletin, March 2001). The beginning in the downtrend can possibly be explained by the meat and bone meal ban in the late 1980s. The BSE prion proteins have been found in areas such as the brain, spinal cord, dorsal root ganglia and the bone marrow of cattle that were orally infected with BSE. This points out that these are the areas most susceptible to the transmission of the disease (Centers for Disease Control, June 27 2005). When experimental rodents were inoculated with the abnormal protein it first began to transform the cells of the spleen and lymph nodes and progresses to the splanchnic nerves, where the sympathetic nerves leave the abdomen. From there the pathogen travels through the spinal cord and stops in the brain. If the contaminant is taken orally, as is the case with the meat and bone meal feed, the pathogen can travel directly from the stomach and into the brain stem via the vagus nerve (American Scientist, Jul/Aug 2004). The abnormal proteins then transform the normal proteins in the brain into the pathogenic form and this process continues to the next protein, and so on, until there is a large build-up of the pathogen in the cells of these tissues (Medical Microbiology And immunology, Aug. 2003). The accumulation of the proteins in the brain kill off healthy neurons leaving the spongy holes that the disease is named for (Current Science, March 5 2004). The direct mechanical damage is hard to detect until after death because the prions emit no toxins, nor do they illicit an immune response. Since prion proteins are already present in the body, the similar pathogenic proteins are not detected by the body s immune system, making it impossible to detect antibodies that may leave some clue to the disease s presence (Epidemiolgical Bulletin, March 2001). Since the disease is only positively diagnosed after death by a biopsy of brain tissue during autopsy, there is no way to treat the disease. As the brain is the organ ultimately affected by the prion build-up symptoms include a wide range of psychiatric and sensory symptoms when it first begins to present itself. These symptoms may include ataxia in the early stages and dementia towrd the end of the phase (Centers for Disease Control, June 29 2005). Furthermore, the infected host may suffer from loss of motor control, progressive dementia, some paralysis, and wasting (Prusiner). As of yet there is no cure for BSE or any of its relatives. BSE and the human form, variant CJD,
are always fatal. In fact, the best prognosis is that the victim lives two years after the disease presents itself (5% of cases). Most (90%) live only half that long after the onset of the disease (Prusiner).