Can It Happen Here? The Puzzle of Mad Cow Disease

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January 02, 2008 | 958 views

It's an unappetizing accompaniment to those summer-evening barbecues: the nagging concern that mad cow disease, which raised fears over the safety of British beef in 1996, may lurk here as well. Behind the concerns are occasional reports of similar symptoms in other countries, and not just in cattle.

A new book about the outbreak and similar diseases, "Deadly Feasts," by Richard Rhodes, makes for unsettling reading.

There's no reason to think that the British mad cow epidemic, or the fatal human disease that has apparently been linked to it, has crossed the Atlantic. But given what's known about how mad cow disease arose in Britain, there's reason to believe that we could breed our own native version.

Should U.S. consumers worry about the safety of the meat they're eating? There's clearly no reason to panic. But unfortunately, no one knows precisely how widespread such diseases are, or how hazardous they might be to people, which makes it difficult for consumers and policy-makers to decide what to do.

An Odd Group of Disorders

The several fatal brain diseases now under close scrutiny are called TSEs, for transmissible spongiform encephalopathies. The name is based on the major effect: As the disease progresses, the brain typically becomes riddled with spongelike holes. Diverse evidence suggests that the surprisingly similar diseases sometimes seen in people (Creutzfeldt-Jakob disease, or CJD), sheep (scrapie), cows (mad cow disease) and other animals are all caused by a previously unrecognized type of infectious agent, a mutant cell protein that can apparently induce normal cell proteins to mimic its shape.

Recent evidence from Britain suggests that the disease can jump from species to species when a diseased animal is eaten. Mad cow disease has caused the deaths of 160,000 cattle in Britain in the past decade, and more are still coming down with it. The best guess is that the disease was transmitted in cattle feed. The feed at first contained remains from scrapie-infected sheep carcasses, and then from infected cattle as well.

In 1996, the cows' disease was suspected of somehow infecting a number of people in Britain who contracted CJD at an unusually young age (there are 16 such cases so far). The concern is great enough that Britain has banned the feeding of mammal protein to all food animals. Other countries have banned the import of British beef.

Adding to the concerns: Whatever agent causes this class of disease, it's incredibly durable. It can survive very high temperatures (cooking does not destroy it), common disinfectants, even 10 years of soaking in formaldehyde at a research laboratory.

What makes the investigation difficult, and the threat potentially worse: The infection can go undetected in animals when they're slaughtered. And in people, it may take years for symptoms to appear. The uncertainties are so great that one risk analysis projects the number of human deaths in Britain from infected beef over the next 20 years at anywhere from 100 to 80,000.

What About This Country?

If some version of mad cow disease exists in the U.S., it seems rare. In the past eight years, the USDA has examined the brains of 5,000 slaughtered cattle and found no clear-cut evidence of the disease. But the tests are not definitive and other evidence indicates that we may have the necessary ingredients for trouble ahead.

The same feeding practices once used in Britain have been commonplace here for the past couple of decades, and TSEs already are present to some degree in some animals in the U.S. Some sheep have scrapie, some wild deer and elk have a similar disease, and recently a suspicious case was discovered when researchers examined 20-year-old movies and brain samples of pigs with similar symptoms.

An odd but possibly important clue comes from several incidents in which groups of ranch-raised mink died of TSEs (after they were fed meat from cows that had been deemed unfit for humans to eat).

There's no direct evidence that people in this country have been infected with any sort of TSE through food. But two small studies of Americans with CJD (considered a rare disease) demand follow- up. One of them, in 1973, found that CJD patients were more likely than other people to have eaten brains, particularly hog brains. The other, in 1985, found that they were more likely than other people to have eaten certain meats, including lamb and several types of pork. The authors concluded that the results could mean that a scrapie-like disease might exist in swine and might be infecting people.

What To Do

To judge from the available evidence, even if TSEs were somewhere in the food supply, the odds that the meat you eat is affected are extremely small. If you eat meat and are determined to keep your risk as close to zero as possible, here's what the evidence suggests about various kinds of meat:

Brains. The presumed infectious agent is found on nerve cells and so is most concentrated in the brains of affected animals. In a British experiment, a gram of infected brain, when fed to a calf, was enough to induce infection. The British have banned the sale of brains from British cows, sheep and goats to consumers.

Other meat cuts. The infectious agent has been found in nerve tissue in the spine and elsewhere, and in organs like the spleen. You can't avoid nerves entirely, since they run throughout the body. But whole cuts of meat have less nerve tissue than some other meat products. Hot dogs and sausage can be made with organ meat and with mechanically deboned meat, which can end up containing parts of the spinal cord.

Hamburger isn't supposed to contain spinal tissue, but recent USDA tests at meat-processing plants discovered that some hamburger does. If you want to take the absolutely safest course, buy a cut of meat and ask the butcher to grind it for you.

Fish, poultry. There's no reason to think that poultry or fish have TSEs. While scientists have succeeded in inducing such diseases in cattle, pigs and sheep, all efforts to infect poultry have failed. In addition, evidence so far suggests that milk and dairy products pose no risk of transmitting TSEs.

Consumer Reports Recommendations

The most effective way to be certain that the meat we're eating stays safe is to prohibit the feeding of animals that might be infected to animals that people might eat. In our view, the FDA should stop practices that could spread TSEs in U.S. food animals. It could do that by banning the feeding of any mammal remains to food animals, as the British government has now done. And the sosoner the better. Even after a comprehensive ban, it will take several years before all the meat in the supermarket comes from animals that have never consumed animal protein.