Tests explore next generation of defenses. (Mad Cow Future).

As Canadian health officials press on in their investigation of mad cow disease within their borders and other countries bar Canadian beef, scientists are taking disparate approaches to developing defenses against such brain diseases.

Researchers in the United Kingdom studying livestock that resist so-called transmissible spongiform encephalopathies report mixed results. Although earlier tests had found some sheep resistant to infection by natural routes, extreme challenges--injections of diseased material directly into the brain--brought on the disease in 3 out of 19 animals, says Fiona Houston of the Institute for Animal Health in Newbury in the May 29 Nature.

Looking at meat treatments, U.S. and Italian researchers are blasting hot dogs with pressure and heat to inactivate agents for a spongiform encephalopathy. The researchers report in the May 13 Proceedings of the National Academy of Sciences that they have quashed risk of infection from the food.

The diseases actually leave brains full of holes, or spongy, ultimately killing susceptible species, which include minks, house cats, elk, and people (SN: 11/30/02, p. 346). Twisted forms of brain proteins called prions spread the disease, making normal proteins misfold.

Some scientists argue that cattle can get mad cow disease spontaneously, says Lisa Ferguson, a veterinarian for U.S. Department of Agriculture's Animal and Plant Health Inspection Service. "I think that's a bit difficult to accept," she says. If it were true, she'd expect that mad cow disease would have shown up in the United States by now.

Until May 20, North Americans hadn't reported any apparently homegrown case of mad cow disease. Canadian officials then announced that tissue from an 8-year-old cow slaughtered in January had tested positive for mad cow disease. Teams have been tracing the farms where the cow lived and where its calves have gone. The number of farms under quarantine has been climbing, and several hundred cows have been slaughtered for testing. The sick cow's most recent companions, a herd on an Alberta farm, have all tested negative for mad cow disease in a rapid test.

Britain has proclaimed an effort to breed livestock to get rid of prion diseases. Scientists are focusing on a form of a sheep gene called ARR. They're working to increase the frequency of that form because it has been linked to resistance to spongy-brain diseases. Sheep carrying two copies of the gene form and that were fed tissue from infected cattle still looked healthy after almost 6 years, but if the sheep were instead inoculated with the bovine material, they revealed symptoms after 3 years. Moreover, sheep without the gene form developed symptoms as early as 16 months after inoculation.

In other experiments, Paul Brown of the National Institutes of Health in Bethesda, Md., and his colleagues exposed meat to pulses of pressure starting at 100,000 pounds per square inch and temperatures ranging from 121[degrees] to 137[degrees]C. This dramatically reduced the dangers of infection, Brown says.