How Can Mad Cow Disease be Transmitted to Humans?
Dr. Jean Handy, Associate Professor of Microbiology and Immunology and of Pathology and Laboratory Medicine, UNC School of Medicine. Associate Director, Clinical Microbiology/Immunology Laboratories, UNC Hospitals, explains...
My question was, "How can humans get mad cow disease?" I'll try to answer it. There is a group of diseases called "spongiform encephalopathies" because they create lesions in the brain that look like a sponge under the microscope. These diseases take a long time to develop (usually years) and are acquired by eating nerve tissue from an animal already infected. The infectious agent is called a "prion," and it is even smaller and simpler than a virus because it seems to have only protein, and no DNA or RNA. How this protein manages to replicate itself is a matter for intensive research right now. It seems to involve the way the protein folds itself, and it may serve as a pattern to refold a normal protein found in nerve cells.
An example of spongiform encephalopathy is "scrapie" in sheep (which makes them develop neurological symptoms and also itch, so they "scrape" themselves). An example in humans is kuru, a fatal brain disease which formerly occurred among the Fore tribe in New Guinea, when they used to follow a practice of ritual cannibalism (eating a portion of the brains of their dead for religious reasons). The disease has disappeared among them since they discontinued this practice. Bovine spongiform encephalopathy, or "mad cow disease," is a similar disease which started to appear among cows in Great Britain who had been fed processed protein from the slaughterhouse as a feed supplement instead of soy protein. Protein from this source is no longer fed in Britain, and has never been used in the United States. Last year it was reported that there were 14 cases of "variant Creutzfeldt-Jacob disease" in Great Britain, and it has been suggested that these cases were acquired by eating the meat from cows with "mad cow disease." Creutzfeldt-Jacob disease (CJD) is a spongiform encephalopathy of humans which usually occurs among older adults and sometimes clusters in families. The "variant" CJD was unusual because the patients were much younger. The possibility of transmitting "mad cow disease" to humans has led to the destruction of all cows in Britain old enough to have eaten the questionable feed, and also to widespread worry and speculation.