04/01/1996 08:00:00

Question of the month: What exactly is mad cow disease?

Question of the Month Answered by: Michael Harrington, Member of the Beckman Institute at Caltech in biology

Formally called bovine spongiform encephalopathy—or BSE for short—mad cow disease is the common name of a fatal illness that many cattle in the United Kingdom have. Bovine means related to cattle; encephalopathy has Greek roots and means brain disease; and spongiform means literally "in the form of a sponge." Put it all together, and BSE is a disease of cattle in which the brain ends up looking like a sponge, full of holes.

The term mad cow disease comes from the strange behavior and odd staggering gait of affected cattle. The sick animal loses its sense of balance and lurches or stumbles around in circles, making it appear crazy.

BSE is closely related to a centuries-old disease in sheep called scrapie, and to some human diseases, notably an illness called Creutzfeldt-Jakob disease, or CJD for short. Scientists believe the disease is spread when one animal eats infected tissue from another animal. In fact, scientists believe the current epidemic among British cattle was caused by using the processed offal of scrapie-infected sheep as a supplement in cattle feed. While the disease can occasionally move between species, the source for CJD in humans is not known, except for a small number of families in which it is genetic.

Public-health experts in Britain have been aware of BSE since the mid-1980s, and have repeatedly assured the public there was no chance that humans could be infected by eating beef. Then in March, after looking at 10 new cases of Creutzfeldt-Jakob disease in which the patients were much younger than usual, they announced that it might be possible for humans to be infected by eating beef after all.

While scientists know that the disease can be spread between animals when animals eat infected tissue, scientists know few of the details. The infectious agent is not a bacteria or a virus, but a poorly understood type of protein called a prion. Prions were discovered in the 1980s, and no one knows yet how they result in disease.

More puzzling still, cooking contaminated beef doesn't inactivate the infectious agent, as it does with bacteria and parasites. Prions heated to a temperature that will destroy most proteins (130 degrees Celsius, or 266 degrees Fahrenheit), retain their ability to transmit the disease.

Although the British government stressed that there is only very weak and indirect evidence for a link between the illness in cattle and in humans, British beef sales plummeted. As more information has become available and beef prices have dropped over the last several weeks, Britons have resumed eating beef, but at slightly lower levels than before.

American agriculture and health experts believe this country is safe from BSE, because British beef imports to the United States have been banned since 1989 to protect U.S. cattle from infection. Also, American farmers don't use sheep products as a supplement in cattle feed.