Mad cow cases confound experts

WASHINGTON – Two cases of mad cow disease in Texas and Alabama seem to have resulted from a mysterious strain that could appear spontaneously in cattle, researchers say.

Government officials are trying to play down differences between the two U.S. cases and the mad cow epidemic that has led to the slaughter of thousands of cattle in Britain since the 1980s.

It is precisely these differences that are complicating efforts to understand the brain-wasting disorder, known medically as bovine spongiform encephalopathy, or BSE.

The Texas and Alabama cases – confirmed last year and this one, respectively – are drawing international attention.

At a meeting in London last month, experts presented research on the U.S. cases and similar ones in Europe.

These cows appear to have had an “atypical” strain that scientists are only now starting to identify. Such cases have been described in about a dozen cows in France, Italy and other European countries, as well as in Japan.

In the two U.S. cases, researchers did not detect the telltale spongy lesions caused by prions, the misfolded proteins that deposit plaque on the brain and kill brain cells. In addition, the prions in brain tissue samples from the Texas and Alabama cows seemed to be distributed differently from what would be expected to be found in cows with the classic form.

Laboratory studies on mice in France showed that both the classic and atypical strains could be spread from one animal to another. But scientists theorize the atypical strain might have infected cattle through an unusual way.

Linda Detwiler, a former Agriculture Department veterinarian who consults for major food companies, said that other theories suggest the atypical strain might come from a mutation of mad cow disease or even from a related disease in sheep.