U.S. Cattle Resistant to Mad Cow Outbreak Unlikely Even if Infected Animals Arrive, Study Says

By David Brown Washington Post Staff Writer Saturday, December 1, 2001; Page A11

The nation's cattle herds and food supply are "highly resistant" to the introduction of mad cow disease, and if a few infected animals were to arrive it is unlikely they would spread the disease, according to a long-awaited report.

The existing ban on putting rendered cattle tissue into cattle feed -- which is the main way mad cow disease spreads -- is enough to reduce the mad cow risk here to near zero, according to the report prepared by the Harvard Center for Risk Analysis, a branch of the Harvard School of Public Health. Several other regulatory changes may be able to reduce it further.

"We have a system in place that acts to check the disease and, if it does get in, to gradually eliminate it," said George M. Gray, a toxicologist at the Harvard center who directed the study, which the Agriculture Department commissioned.

Mad cow disease, known formally as bovine spongiform encephalopathy (BSE), is an incurable brain disease in cattle discovered in England in 1986. On rare occasions, it can apparently cause a form of fatal dementia in human beings, called variant Creutzfeldt-Jakob disease (vCJD). Neither disease has been found in the United States.

Despite the findings, Agriculture Secretary Ann M. Veneman, speaking at news conference, announced several actions, aimed at further protecting against the disease. The government will double the number of BSE tests on cattle carcasses next year, and it proposed a ban on certain stunning devices used in slaughterhouses that have been shown to occasionally drive brain matter into other organs.

"We cannot let down our guard or reduce our vigilance," she said.

She also said the department will consider a prohibition on rendering for animal feed the carcasses of animals that die spontaneously on farms. Currently, such material can be used for chicken or hog feed, but not cattle feed. In the report, the Harvard analysts noted these animals might be more likely to have BSE than slaughtered animals simply because their deaths were unexpected. Diverting them completely from the rendering industry is a way of further lowering the chance the disease is transmitted, should it happen to exist undetected now.

BSE elicits fear, in part, because it is caused by agents called "prions," which are neither viruses nor bacteria and whose biology is shrouded in mystery. Prions survive heat and chemical disinfection (to some degree at least), and are not killed by antimicrobial drugs. They do their damage by reshaping a normal protein found in the nervous system into a form that leads eventually to a porous ("spongiform") appearance of the brain.

Classical CJD is itself very rare, and is found almost exclusively in the elderly. The variant form has struck people in their thirties. Human beings presumably acquire it by eating beef containing small amounts of infected cattle brain or spinal cord. One of the early cases of vCJD was found in a person who had not eaten meat for years, however, and the exact mechanism of transmission in all cases is murky.

There have been more than 178,000 cases of BSE in Britain since 1986. The link between the disease and vCJD was discovered in the early 1990s, and it led to massive culling of British cattle herds and, in some quarters, panic about the safety of beef. In all, about 100 cases of vCJD have been found, most of them fatal within a year or two of symptom onset.

New cases of BSE continue to appear in small numbers around the world, despite widespread bans on the import and export of bone meal and rendered cattle tissue from affected countries. Japan recently reported its second case, Italy its 39th. Some observers believe it is only a matter of time before an American case is found. The new report, however, argues there is a good chance the disease is not here and will never arrive.

The analysis took three years to perform and consists mainly of a mathematical model predicting the amount of contaminated material -- principally cattle brain and nervous tissue -- that might circulate in the United States under various scenarios. It did not involve testing cows or sampling meat.

The unit of contamination the analysts use is the "ID-50" -- the amount of infectious material that has a 50 percent chance of infecting a cow that eats it. Biologists assume that one ID-50 is not enough to infect a person, as millions of such doses circulated in Britain and only 100 people were infected. The report estimates that 10 BSE-infected animals would present about 35 ID-50 doses to the American population over 20 years.

The chance of that many infected animals coming in is small, however, as imports of live cattle and most cattle byproducts from Britain have been banned since 1989, and more recently from other BSE-containing countries. More important, the ban on putting cattle material in cattle feed blocks the spread of disease -- making the overall risk of its appearance very small, they conclude.

Although the Harvard Center for Risk Analysis sometimes takes money from industry groups to do its work, this study was financed entirely by the U.S. Department of Agriculture, which paid about $500,000 for it.

-- Anonymous, December 01, 2001