A frenzy of worry over “mad cow” disease has again erupted in England. Last week, chilling scenarios of infection from chicken salad splashed across the tabloids. The research that set off this media frenzy was a report that mice, thought to be resistant to mad cow disease, in fact are not. Infected mice do, it turns out, develop the brain lesions that are the hallmark of the disease, but mice only live a few years — too brief a time, it seems, for infected individuals to get sick. As mice are not a major part of the typical British diet, this report did not seem to me particularly alarming.
The uproar did set me to thinking that it has been over a year since I wrote about mad cow disease in this column, on July 1,1999. It seems a good time to take another look at what’s going on.
“Mad cow disease” is a fatal and communicable brain disease of cows that has a very long incubation period. Decades after infection, the brains of infected cattle develop numerous small cavities as nerve cells die. The holes produce a marked spongy appearance that gives the disease its scientific name, bovine spongiform encephalopathy (BSE). Central nervous system function is progressively degraded, until death eventually occurs. There is no cure.
Mad cow disease is remarkable in that it is not transmitted by a virus or other microbe. Instead, a protein spreads it from one individual to another!
Proteins are made as long strings, like spaghetti, each then folding itself up into a compact shape. The shape it assumes determines how the protein functions in an organism.
BSE is thought to have started when a key brain protein called a prion made a mistake and folded itself up incorrectly. Tragically, this protein, when it encounters others of the same sort, is able to induce them to refold into this same mistaken shape. A wave of misfolding relentlessly expands through the brain like a chain reaction. And, like a spreading rumor, any brain receiving a copy of the misfolded protein experiences the process anew.
Therein lies the problem. Prions from the brain of an infected cow can cause the disease in other cows. Because cows normally eat grass rather than each other, you would not expect prion infection to be a problem for cows. However, until recent years it was the practice in Britain to supplement cattle feed with extra protein, and guess where the added protein came from! Often, from the “rendered” bodies of cows that had died in the field. It would be difficult to invent a more perfect way to spread the BSE disease.
Something was bound to happen, and in the mid-1980s it did. There was a major outbreak of mad cow disease in Britain, with many thousands of cattle affected each year for more than a decade.
The British public were very worried that BSE might be transmitted to humans. Some 750,000 infected British cows entered the human food chain as hamburgers, sausages and other meat products before the infected herds were slaughtered and the outbreak brought to an end. Even baby food was protein-supplemented. There is a rare human disorder, typically of the elderly, called Creutzfeldt-Jacob disease (CJD) with an array of symptoms and brain lesions similar to BSE. Could BSE cause a variant of CJD in humans?
In late 1995 the first cases of variant CJD (vCJD) were reported. Two teenagers were diagnosed with Creutzfeldt-Jacob disease, followed soon by a 29-year-old. They did not live long. Analysis of their brain tissue revealed dense deposits of prion proteins called “plaques,” similar to those seen in BSE-infected cows but very different from what is usually seen in CJD. Researchers concluded that infection from BSE “was the most plausible interpretation” of these case of vCJD.
By April 1996, seven more cases of vCJD had been identified. Over the following years, more and more cases of vCJD have surfaced, almost entirely in younger people — the average age of vCJD victims is over 30 years younger than for CJD cases.
Last month the British government’s CJD surveillance unit reported in the journal The Lancet that the incidence of vCJD is starting to rise sharply. So far this year 14 Britons have died of vCJD. That is as many as died all last year, and five others are known to be dying from the disease.
This is very sad news, as it tells us the British are probably in the early stage of an epidemic. While only 80 cases have been reported so far, many more vCJD cases are expected to appear as the prions from all that BSE-infected beef work their way through the British population. Researchers can only guess how many people have been infected. Mathematical models predict 136,000. Other researchers suggest that about 10,000 cases would be a more realistic estimate. Either number is a tragedy.
There has never been a case of vCJD reported in the United States. However, it is not difficult to understand why this summer the US Dept. of Agriculture ordered the killing in Vermont of nearly 400 imported Belgian sheep suspected of being infected with a BSE-like disorder. Carrying out tests to determine what form of the disease infects these sheep would take years. Death on suspicion seems a cruel policy. I support it, however, to the consternation of some of my animal-loving friends. I judge the costs of starting our country down the British path to be too great to justify the risk of waiting to see. vCJD is cruel too, and will not let us take back any mistake.
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