Bovine spongiform encephalopathy (BSE or more commonly mad cow disease) is a fatal, neurodegenerative disease of cattle, which is transmissible to humans. Misshaped prion proteins cause the degeneration and spread the disease between individuals. Very rarely BSE may arise spontaneously, but more often it spreads in epidemic fashion. Spontaneous disease arises in animals that carry a rare mutant prion allele, which expresses prions that contort by themselves into the disease-causing shape. Transmission of BSE occurs when healthy animals consume tainted tissues from others with the disease. Practices recently banned in many countries allowed this to occur.
Epidemics in cattle are believed to have originated in sheep, in which the related prion disease scrapie is common. The tissues that contain most of the pathogenic molecules are those of the brain and the nervous system, although contagious amounts appear sometimes to be present in the blood. In the brain, these proteins form plaques, which lead to the appearance of holes in the brain, degeneration of mental abilities and death.
Following an epidemic of BSE in Britain, 152 people (as of 2003) acquired and died of a more or less identical disease. For many of them, direct evidence exists that they had consumed tainted beef, and so this is assumed to be how all the individuals contracted it. Disease incidence also appears to correlate with slaughtering practices that led to the contamination of hamburger and other beef with nervous system tissue. The human disease was designated variant Creutzfeldt-Jakob disease (or vCJD), after an extremely rare genetic prion disease whose symptoms it closely resembles.
Although the diseased herds were culled long ago, more people are diagnosed with vCJD each year. This implies that the disease has a long latency, as a result of which, public health experts do not know yet how many ultimately will contract it. The ease with which the disease can be contracted from beef therefore is not yet known.
Rodents injected with brain tissue from diseased cows begin to succumb to a similar neurological disorder in one or two years. With current tests, it is not possible to detect abnormal prions in the brains of all of these animals.
The epidemic in British cattle
Of the 152 human cases so far, 143 occurred in the United Kingdom, 6 in France, and one in Italy. Three cases of vCJD occurred in people who had lived in or visited Britain -one each in Ireland, Canada and the United States. There is also some concern about those who work with (and therefore inhale) cattle meat and bone meal, such as horticulturalists who use it as fertilizer.
The BSE epidemic in the United Kingdom was discovered in 1986. BSE is thought to have spread by the practice of feeding cattle meat and bone meal, a high-protein substance obtained from the remnants of butchered animals. This practice allowed the accumulation of prions over many generations. The use of meat and bone meal as a protein supplement in cattle feed was widespread in Europe prior to about 1987. Soybean meal is the primary plant-based protein supplement fed to cattle. However, soybeans do not grow well in Europe, so cattle raisers throughout Europe turned to the less expensive animal byproduct feeds as an alternative.
BSE in North America
At least three BSE-infected cattle have been identified in North America. The first was in 1993, involving an animal born in Britain. The second was reported in Canada on May 20, 2003. It occurred in a single older cow that may have contracted the disease from contaminated feed in earlier years. The animal had been destroyed and declared unfit for consumption prior to being diagnosed. The United States issued a temporary ban on all Canadian beef.
On December 23, 2003, the first case of BSE in the United States was found in a single Holstein cow in Washington State. Agriculture Secretary Ann Veneman called the discovery "a clear indication that our surveillance and detection program is working." However, the United States tested only 20,526 cows in 2003 out of the roughly 35 million slaughtered. Current tests reveal the presence of misshaped prions when they are abundant, but it is not known how far the disease must progress in an individual to transmit it to others. Therefore, it is possible that even among those cattle that are tested and classified as negative, a proportion nevertheless may be contagious. As a result, U.S. authorities have very little idea of how many American beef cattle might have the disease.
The government plans to double the number of cattle tested in 2004, and has banned the use of "downer cows" for human consumption. While the Washington cow that tested positive for BSE was indeed unable to stand, veterinarians say the condition was unrelated to BSE. Therefore it is not clear by how much the ban is liable to reduce the number of infected cattle consumed. Only 200,000 cows slaughtered in 2003 were downers.
The meat of the BSE-positive cow went to market, but some of it was successfully recalled before it was sold to consumers. U.S. authorities called for a switch to the testing procedure that is used in the United Kingdom, which yields its results in one day. Until the switch, U.S. surveillance relied on a test that gave results only after two weeks, after which time the meat from an animal usually has all been sold.
Shortly after the U.S. discovery of BSE, Japan and South Korea instituted a temporary ban on the import of U.S. beef, until more information about the US BSE outbreak becomes available. Since Japan and South Korea are the first- and third-largest importers of US beef, respectively, the economic impact of their bans is significant both for American cattle ranchers and for Japanese and Korean beef consumers. [1]
No case of variant Creutzfeldt-Jakob disease has occurred in North America so far, except among those who have travelled to Europe.
Soybean meal is cheap and plentiful in the United States. As a result, the use of animal byproduct feeds was never as common as it was in Europe. However, U.S. regulations only partially prohibit the use of animal byproducts in feed. Regulations prohibit the feeding of cows, sheep and goat byproducts back to themselves. But the byproducts of ruminants can still be legally fed to pets and to other food animals such as pigs and chickens. It is also legal for these food animals to eat byproducts of their own species, which ruminants may legally be fed as well. [1]
In February 2001, the USGAO reported that the FDA, which is responsible for regulating feed, had not adequately policed the various bans. [1] Compliance with the regulations was shown to be extremely poor before the discovery of the Washington cow, but industry representatives report that compliance is now 100%. Even so, critics call the partial prohibitions insufficient.
Husbandry practices in the United States relating to BSE
Research on treatment
On September 26, 2003, it was reported that an experimental treatment given to a Northern Irish teenager, Jonathan Simms, halted the progress of brain damage caused by variant Creutzfeldt-Jakob Disease (vCJD). The drug, called pentosan polysulphate and commonly used to treat cystitis, was injected into the patient's brain. The patient's weight and heart rate returned to normal levels after receiving the treatment. Still, there is no cure for vCJD, a fatal disease. [1]Further Research
