Bovine Spongiform Encephalopathy (“Mad Cow” Disease)
Bovine Spongiform Encephalopathy (“Mad Cow” Disease)
Bovine spongiform encephalopathy (BSE) is a progressive infection of the brain and nervous system found in cattle. It is often known as “mad cow” disease, because of the way affected animals stagger. There is much evidence that BSE can be transmitted from cattle to humans via the consumption of infected beef, resulting in an invariably fatal brain disorder called variant Creutzfeldt-Jakob disease (vCJD). An epidemic of BSE in the United Kingdom (U.K.) in the 1980s and 1990s has been linked to several cases of the human form of vCJD, mainly among younger people. The impact of BSE on Britain's farmers and beef industry was severe, as countries rushed to boycott imports of meat that might have come from infected cows. Although the BSE epidemic has largely died away, occasional cases still appear around the world. Meanwhile, many scientific questions on how BSE is transmitted remain unanswered.
BSE is a relatively new disease of cattle which was first identified in the United Kingdom in 1986. It proved to be one of a group of diseases called the transmissible spongiform encephalopathies (TSEs). On post-mortem examination with a light microscope, the brain tissue of an animal with a TSE shows a characteristic spongy appearance because the pathology of the disease creates holes within the brain tissue—hence the term “spongiform.”
TSEs affect other animals, including humans. For instance scrapie, a TSE found in sheep, has been known since the eighteenth century and is found at a low level in many parts of the world. The name comes from the tendency of animals with the disease to scrape their fleece against trees and bushes. TSEs have also been found in mink (transmissible mink encephalopathy) and in mule, deer, and elk (chronic wasting disease). CJD is the most significant TSE in humans; it is very rare, usually occurring at a rate of around one per million of the population. The cases that arose from exposure to BSE in the United Kingdom from the mid–1990s represent a new form of CJD.
BSE occurs in adult animals of both sexes. The incubation period—the time lag from exposure to the appearance of symptoms—of TSEs is usually measured in years. Therefore the disease is rarely seen in very young animals, even though they may be infected. Animals with BSE exhibit abnormalities of movement and posture and changes in mental state which an experienced vet or farmer would be able to detect. The disease lasts for several weeks and is invariably both progressive and fatal.
TSEs can be transmitted from one animal to another. However, there is a species barrier, which means that transmission within species is more likely than transmission between species. For instance, there are no known instances of scrapie being transmitted to humans.
It is widely (but not universally) accepted that BSE arose in cattle from exposure to feed derived from sheep infected with scrapie. Adding protein from the carcasses of ruminants (sheep and cows) to animal feed is a long established practice. The U.K. BSE Inquiry, which was set up to look at the underlying causes of the BSE epidemic, concluded that changes in the way the feed was processed probably allowed infectious material to survive and infect the cattle consuming it. From the time the BSE epidemic first took hold there were fears that the disease might be transmitted to humans through exposure to meat and meat products (such as hamburgers) from infected animals. These fears were realized with the announcement of the first case of variant CJD in 1996.
However, it has been hard to prove for certain that exposure to BSE causes variant CJD. This is because the infective agent in TSEs is an unusual entity known as a prion. Research on infected tissue has shown that prions are not destroyed by either heat (which would destroy bacteria) or ultra-violet light (which would destroy viruses). Prions are an abnormal form of a protein that is found normally in the brain. When it infects the brain, the prion corrupts the normal prion protein molecules. These newly formed abnormal prion protein molecules go on to corrupt further normal prion molecules, beginning a cascade effect. The accumulation of more and more abnormal prion molecules triggers the brain damage that produces the symptoms of TSEs.
WORDS TO KNOW
ENCEPHALOPATHY: Any abnormality in the structure or function of the brain.
PRIONS: Prions are proteins that are infectious. Indeed, the name prion is derived from “proteinaceous infectious particles.” The discovery of prions and confirmation of their infectious nature overturned a central dogma that infections were caused by intact organisms, particularly microorganisms such as bacteria, fungi, parasites, or viruses. Since prions lack genetic material, the prevailing attitude was that a protein could not cause disease.
TRANSMISSION: Microorganisms that cause disease in humans and other species are known as pathogens. The transmission of pathogens to a human or other host can occur in a number of ways, depending upon the microorganism.
ZOONOSES: Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.
By September 1, 2006, approximately 183,139 cases of BSE had been confirmed in the United Kingdom, according to the Department for Environment, Food and Rural Affairs. The epidemic peaked in 1992, with 36,680 confirmed cases in that year. In 2006, there were only 15 cases.
Although BSE has reached epidemic levels only in the United Kingdom, it has affected other countries too. The World Organization for Animal Health collects data on BSE. While there have been no cases, to date, in Australia, New Zealand, Africa and much of Asia, there have been several in European countries such as France, Germany, Ireland, and Portugal. As of August 23, 2006, there had been twelve confirmed cases of BSE in North America, nine in Canada, and three in the United States.
There is no treatment for BSE, but much has been done to prevent its spread—both to other cattle in a herd and to humans. The government of the United Kingdom has introduced a number of measures to keep BSE under control. In July 1988, it imposed a ban on feeding cattle with potentially infected material. This measure kept animals that were not already infected from becoming infected and has been adopted in many countries, including those who are currently BSE-free. This measure alone made a major contribution to halting the growth of the U.K. BSE epidemic. However, because BSE has a long incubation time, there was a lag between introducing this ban and a fall in the number of cases. This is why the number of cases continued to rise from 1988, despite the ban. In 1997, the U.K. also began a selective cull—slaughtering those animals that were at risk of contracting BSE. This further reduces the risk of the spread of infection.
IN CONTEXT: EFFECTIVE RULES AND REGULATIONS
Variant CJD (vCJD) is the human form of bovine spongiform encephalopathy (BSE) disease that emerged in Britain in the mid–1990s. The vCJD outbreak in Britain echoed the emergence in the 1950s of a strange and invariably fatal condition called kuru (meaning “trembling with fear”) among the Fore people of New Guinea. After years of living among the group, American doctor Carlton Gajdusek (1923–)—who went on to win the Nobel Prize for Medicine or Physiology in 1976—came to the conclusion that the disease was transmitted in the ritualistic eating of the brains of the deceased, a Fore funeral custom. He suspected that one of these brains, at least, must have belonged to someone with sporadic or familial CJD. There were some striking parallels between the emergence of vCJD and its links with the earlier epidemic bovine spongiform encephalopathy (BSE or mad cow disease), one of the transmissible spongiform encephalopathies (TSEs) found in cattle. The latter prompted a public enquiry to investigate the cause of the outbreak.
The picture that emerged from the inquiry was, briefly, that vCJD is, indeed, the human form of BSE (mad cow disease). The Inquiry concluded that infected material—either from sheep infected with scrapie (a sheep TSE) or from BSE-infected cattle— was incorporated into cattle feed. Further, it was found that changes in the processing of carcasses used for animal feed were the likely cause of this contamination. Fortunately, the epidemic, though tragic for the victims and their families, was limited by steps such as the wholesale slaughtering of infected cattle and a ban on imports of British beef.
The inquiry led to a variety of developments. For example, in an attempt to restore public confidence, a Food Standards Agency was set up in the United Kingdom to advise on food safety issues. Regulatory authorities are moving towards eliminating animal products from the manufacture of medicines and other items destined for human consumption. The BSE inquiry also led to changes in the supply of blood and blood products, in an attempt to screen out donors that are, unknowingly, carrying vCJD.
The spread of BSE to humans has been limited by restricting imports of meat and meat products that might be infected. In 1996, cattle over 30 months old were no longer allowed to enter the food chain— instead, they were incinerated after slaughter. This ban has now been lifted and replaced by BSE testing—only meat that tests negative can enter the food supply.
The BSE epidemic hit British farmers and the United Kingdom meat industry hard. In 1996, the government of the U.K. admitted a link between BSE and variant CJD, and shortly afterward, France and many other European countries announced a ban on imports of British beef and related products. South Africa, Singapore, and South Korea soon joined in. The Meat and Livestock Commission stated that the bans had caused half of the U.K.'s slaughterhouse workers to lose their jobs. The import bans were gradually lifted over the next few years, as the BSE epidemic began to die down. But it has taken many years for British beef sales to begin to recover, both at home and abroad.
In 1998, a public inquiry into BSE and variant CJD began. This concluded with lessons to be learned to stop such a catastrophe from happening again. People in Britain and elsewhere are now more aware of safety issues around food. They wish to know where their food comes from and what is in it. In 2000, the government of the United Kingdom set up the Food Standards Agency, a department that looks after public health and consumer interests with respect to food. This was a response to public distrust generated by the way the government was seen to have handled the BSE crisis. Formerly, food and agriculture had been the responsibility of the same department, which many felt marginalized the interests of the consumer.
BSE has substantial economic impact. During the 1990s BSE outbreak in the United Kingdom, hundreds of animals were destroyed. Quarantined farms and slaughterhouses lost business. New regulations governing cattle feed and BSE testing programs proved expensive to implement. However, in most nations where BSE is detected, the most significant economic impact is the loss of revenue from the export of beef products. Beginning in 2001, several nations restricted the import of American beef products, concerned that the United States beef industry lacked sufficient testing and identification methods for BSE. In 2003, when the U.S. Department of Agriculture announced that BSE had been discovered in one cow in Washington state, approximately 60 nations temporarily banned the import of U.S. beef. The infected cow was later traced to a herd in Canada, but the discovery of BSE in the North American herd resulted in approximately $4.7 billion in beef industry losses that year.
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