Yet, no trace of the animal disease, Bovine Spongiform Encepathalopathy (BSE) -- also known as mad cow disease -- has been detected in the United States. Neither has its human counterpart, Creutzfeldt Jakob disease (CJD), associated with consuming meat and meat products. Based on the findings of a recent Harvard study, this is likely to remain the case for the foreseeable future.

The study confirms that banning meat and bone meal in animal feed -- a measure imposed by the cattle industry years ago to prevent the spread of mad cow disease into the United States -- is the prime defense against the disease. For this reason, the study concludes the chances of mad cow ever posing a threat to U.S. cattle – and ultimately humans in the form of CJD -- are remote.

The study is based on reviews of American and European prevention programs, data on the spread of the disease throughout Europe, and surveys of meat-processing practices in the United States. Its findings support what many experts have suspected for a long time: outbreaks of mad cow disease in Europe stemmed from the once common European practice of feeding cattle with offal – in this case, sheep renderings containing brain and spinal tissue. More recent outbreaks in Britain have been attributed to the use of bone meal in feed.

Neither of these feeding practices has ever been used in the United States.

Though they are not yet completely certain how the disease occurs, scientists believe the culprit behind both diseases is an aberrant protein known as a prion, which European cattle presumably ingested from sheep offal and later from bone meal.

"We’re not quite sure what these proteins really are and even how they reach the brain," says Dr. Jean Weese, Alabama Cooperative Extension food scientist. "All we know is that they are not living things, like bacteria, although they concentrate in the brain like a toxin."

Equally baffling to scientists is the fact that these prions are virtually impervious to heat.

Testing, for example, shows that prions remain intact at temperatures as high as 2000 degrees Celsius, Weese says.

Even so, Weese says, people shouldn’t worry about mad cow disease or it’s human equivalent, CJD.

Even in Europe, where outbreaks have occurred, one is far more likely to be struck by lightening than to contract the disease, Weese says. In fact, only about 100 people have died from CJD since the outbreaks first occurred in Europe more than 4 years ago.

Studies have shown death from exposure to prions associated with CJD is far less likely to occur than from food pathogens such as listeria, salmonella and E.coli 0157:H7. Even deaths from food pathogens are very few, Weese says.

For example, listeriosis, associated with contaminated meats, dairy products and raw vegetables, and salmonella, most often linked with raw eggs, cause slightly more than 500 deaths a year in the United States – out of a population of almost 300 million.

The risk of mad cow disease is likely to be even more remote in the future: Auburn University researchers are developing tests that will detect any animal tissue in livestock feed that has been linked with mad cow disease.