Mad cow disease

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Bovine spongiform encephalopathy (BSE or commonly mad cow disease) is a fatal, neurodegenerative disease of cattle. The disease is believed to be 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. Organometalic chemicals may be a factor in the spontaneous occurrence of the disease.

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 a disease with similar neurological symptoms. For many of them, direct evidence exists that they had consumed tainted beef, and this is assumed to be the mechanism by which all affected individuals contracted it. Disease incidence also appears to correlate with slaughtering practices that led to the mixture of nervous system tissue with of hamburger and other beef. The human disease was designated variant Creutzfeldt-Jakob disease (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.

On February 17 2004 a research team headed by Salvatore Monaco reported in the Italian Proceedings of the National Academy of Sciences that a second strain of mad-cow disease had been detected. It is not known if this second strain is transmissible to humans.

The epidemic in British cattle
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. A change to the rendering process in the early 1980's appears to have resulted in a large increase of the infectious agents in the cattle feed. A contributing factor seems to have been lax British laws that did not require a high temperature steriliziation of the protein meal. While other european countries like Germany required the said animal byproducts to undergo a high temperature steam boiling process, this requirement had been eased in Britain as a measure to keep prices competitive.

Of the 152 cases of vCJD in humans 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.

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 reportedly unable to stand, veterinarians say the condition was unrelated to BSE. Furthermore, there is some dispute as to whether the cow was a downer or not. 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. 

No case of variant Creutzfeldt-Jakob disease has occurred in North America so far, except among those who have travelled to Europe.

Husbandry practices in the United States relating to BSE
Soybean meal is cheap and plentiful in the United States. As a result, the use of animal byproduct feeds was never 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 mammalian byproducts to ruminants such as cows and goats. But the byproducts of ruminants can still be legally fed to pets or other livestock and poultry such as pigs and chickens. And it is legal for ruminants to be feed byproducts from some of these animals. 

In February 2001, the USGAO reported that the FDA, which is responsible for regulating feed, had not adequately policed the various bans. 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.

BSE statistics by country
The following table summarizes reported cases of BSE and of vCJD by country. BSE is the disease in cattle, whilst vCJD is the disease in people.

The figures given above for BSE are certainly too low, and most likely by a considerable amount. The tests used for detecting BSE vary considerably as do the regulations in various jurisdictions for when, and which cows, must be tested. For instance, in the EU cows tested are older (30 months+), while many cattle are slaughtered earlier than that. Tests are also difficult as the altered prion protein has very small levels in blood or urine, and no other signal has been found. Newer tests are faster, more sensitive, and cheaper, so it is possible that future figures may be more comprehensive. Even so, currently the only reliable test is examination of tissues during an autopsy.

It is noticeable that there are no cases reported in Australia and New Zealand where cattle are mainly fed outside on grass pasture and, mainly in Australia, non-grass feeding is done only as a final finishing process before the animals are processed for meat.

As for vCJD in humans, autopsy tests are not always done and so those figures too are likely to be too low, but probably by a lesser fraction. In the UK anyone with possible vCJD symptoms must be reported to the UK Creutzfeldt-Jakob Disease Surveillance Unit and so it is unlikely that any cases would be missed.

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 Creutzfeldt-Jakob disease (CJD). 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. Several other treatments have been proposed, including a vaccine, but none has reached human clinical trials, nor is any in even experimental use. There is currently no cure or even much palliative treatment for vCJD, a fatal disease.