Bovine spongiform encephalopathy
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.
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.
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