•  Brain & Nervous System Disorders •  Infections & Infectious Diseases

Mad cow disease, or bovine spongiform encephalopathy (BSE), is a degenerative central nervous system disease that affects the brains of cattle, killing brain cells and creating spongy holes in the tissue. Cows afflicted with the disease stumble around as if they've lost their balance. They act demented (or "mad," as the British say) and quickly die of the disease.

BSE belongs to a family of debilitating brain diseases that can affect other animals bred for human consumption. And when humans eat boned meat and organs from these infected animals, they risk becoming infected as well.

Fortunately, mad cow disease is very rare in the United States. According to the Centers for Disease Control and Prevention (CDC), as of 2008, there have only been three reported cases in people in the United States, and there is strong evidence that all three were exposed to BSE outside the United States.

Most of the reported cases around the world are from Great Britain, where an outbreak of BSE occurred between 1980 and 1996. Between 1994 and 1996, a dozen people between the ages of 19 and 39 came down with what seemed to be Creutzfeldt-Jakob Disease (CJD), another brain disorder. Scientists later found that the victims' brains looked more like the brain of a cow infected with BSE than that of a person with classic CJD. Scientists eventually called the disease "new variant Creutzfeldt-Jakob disease" (vCJD).

Scientists are still debating the cause of the disease. Some researchers initially suspected a virus, but most now believe that this family of brain diseases is caused by so-called "renegade" proteins called prions. These abnormal prions enter brain cells and turn healthy proteins into a lethal form that damages brain cells and causes them to die.

It's likely that BSE was spread among cattle through the food that cows eat. Until 1997, it was common to supplement cattle feed with meat and bone meal made from animal carcasses, including sheep and other cows. Some experts speculate that this practice enabled a form of the disease found in sheep to make the jump to cows, causing BSE.

Once the prions from an infected cow get into the digestive system of a healthy cow, they can spread to the nervous system and cause BSE. Researchers believe that these proteins, when consumed by other animal species, can jump the species barrier. Likewise, scientists believe that prions can cause vCJD in people who eat animal organs infected with BSE.

Not very likely, but government officials haven't ruled out the possibility. In 2001 researchers at Harvard University's Center for Risk Analysis at the School for Public Health conducted a study to determine the likelihood of a mad cow outbreak in the United States. Although some experts contend it is only a matter of time before the disease appears in the United States, the Harvard study concluded that "BSE is extremely unlikely to become established in the U.S."

Still, federal officials are wary, given recent threats. There have been three cases of BSE identified in cows in the United States (in 2003, 2004, and 2006). The 2003 case involved a cow born in Canada, where it presumably caught the disease before being imported into the United States. The 2004 case was the first endemic case of BSE in the US, and officials have not been able to determine the origin of the cow diagnosed in 2006.

To prevent the risk of contamination, the USDA in 1997 banned the import of meat and meat products from Britain, as well as cattle feed made from cows and other livestock. But the discovery of mad cow disease in the United States in 2003 triggered a chain of new reforms geared to make beef even safer. In December 2003, the U.S. Department of Agriculture banned cows that can't walk (also known as "downer cows") for human consumption. The agency also prohibited the sale of brains and spinal cord tissue -- the organs most likely to be contaminated -- from cattle 30 months old or older.

In March 2004, the USDA also ramped up its surveillance program. Between 2004 and 2006, over 750,000 cows were sampled. Since then, the agency has determined that the prevalence of BSE in cows in the United States is extremely low (less than one cow per million adult cattle). As a result, its surveillance program now samples about 40,000 cows per year.

Although the CDC estimates the risk of acquiring vCJD through eating beef to be very low -- at the most, about one case per 10 billion servings -- taking some precautions can help to ease your fears. Experts at the Center for Science in the Public Interest (CSPI) suggest choosing cuts of meat and products that are less likely to be infected with the disease. Products with no known risk include cuts of boneless beef, beef from grass-fed cattle, and all dairy products.

Cuts of beef containing a bone, such as prime rib, porterhouse and T-bone steaks, and rib roasts carry what the CSPI describes as a "minuscule" risk. Potentially riskier products include beef sausages, hot dogs, pizza toppings, and hamburger meats in which different cuts of beef are mixed together. The most risky products are those that contain brains and the spinal cord.

Although swearing off beef is unnecessary, you might want to take a few precautionary steps anyway.

-- Chris Woolston, MS, is a health and medical writer with a master's degree in biology. His reporting for CHI on occupational health earned him an award from the Northern California Society of Professional Journalists. Jennifer Robb is a senior editor at CHI.

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First published March 24, 2004
Last updated October 14, 2008
Copyright © 2004 Consumer Health Interactive

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