“Mad cow” disease is one of a small family of diseases that are like no others.
The vast majority of infectious illnesses are caused by a virus or bacteria -- living organisms, or in the case of viruses, quasi-living organisms -- with their own genetic blueprint that allows them to invade the body and reproduce. Mad cow disease apparently is not.
Instead, many scientists now believe, mad cow disease -- more accurately known as bovine spongiform encephalopathy, or BSE -- is apparently caused by a protein gone bad. The prevailing view is that BSE and related diseases are caused by a normal brain protein that somehow gets twisted into a toxic form called a prion.
These prions damage brain cells, producing characteristic holes that make the brain look like a sponge. More important, when they come into contact with the healthy form of the protein, they somehow induce it to convert into a prion as well. And when they are transferred from one animal to another via blood or through ingestion of infected proteins, they transmit the disease.
This highly unlikely scenario was first proposed in the early 1970s by neurologist Stanley Prusiner of UC San Francisco, only to be met with great derision. But a number of crucial laboratory experiments subsequently seemed to verify the basic tenets of the theory, and Prusiner was awarded the Nobel Prize for medicine in 1997 for his discovery.
Nonetheless, many researchers still resist the idea, arguing that there must be some genetic material associated with the disease transmission and that scientists have simply not yet discovered it.
The disease sometimes occurs spontaneously in humans in a form known as Creutzfeldt-Jakob disease. There are about 20 cases of the disease in California each year, most of them genetic in origin, and there seems to be no evidence that it is transmitted from person to person under normal circumstances.
It can be transmitted under more unlikely circumstances, however. In the early part of the 20th century, researchers identified a disease called kuru among the South Fore people of New Guinea. Decades of research eventually showed that it is caused by a prion and that it is transmitted through the cannibalistic mortuary practices of the victims. As part of their burial practices, the South Fore eat the brains of the dead, and the infectious agent is transmitted during the process.
The current interest in mad cow disease began in 1985, when farmhands noticed that a black-and-white Holstein cow in Britain began to stagger, head-butt other cattle and shy away from humans. It was subsequently found to have BSE, but the more colorful name stuck.
Researchers ultimately discovered that the disease was caused by supplementing the dairy herd’s feed with ground-up sheep infected with a related disease called scrapie.
In the 1980s and 1990s, Britain destroyed 3.7 million cattle in an effort to eradicate the disease, but the effort was not entirely successful and BSE ultimately spread to herds throughout Europe. The U.S. Department of Agriculture says there has never been a case in the United States, however.
At first, scientists did not believe that BSE affected humans. But in 1996, they discovered the first human case, which is now called variant Creutzfeldt-Jakob disease. The disease has subsequently killed or sickened 129 people: 119 in Britain, six in France and one each in Italy, Ireland, Canada and the United States. The U.S. victim was infected abroad.
There is no treatment for the disease. Prusiner’s team reported two years ago that the antimalarial drug quinacrine worked in mice, but subsequent studies in humans found no benefit. Workers are searching for drugs against the disease and for a vaccine.
Although there is no strong evidence that variant Creutzfeldt-Jakob disease is transmitted via blood, people who have spent any length of time in Britain are not allowed to donate blood in this country.
A related disease called chronic wasting disease has recently been moving through herds of deer and elk in the U.S. and Canadian west, spread via saliva and feces, and hunters have been warned against eating meat from the animals.
Elk with the disease are known to live in the remote part of Alberta where the cow with BSE was discovered, but there is no evidence that the elk are the source of the infection.