Mad Cows, Sane Cats: Making Sense of the `Species Barrier'

[Andrew Gach]

When thousands of cattle started collapsing from mad cow disease more than a
decade ago in Britain, experts stated firmly that people would never get the
disease from eating beef.

A species barrier - a kind of biological Hadrian's wall - would protect
roast beef lovers from harm.

So much for scientific hubris. After young people began dying in 1996 from a
new variant of Creutzfeldt-Jakob disease, a rare malady thought to occur
only in older people, the experts were forced to admit they were wrong.

The people had died because they ate meat from infected cows. A species
barrier had crumbled.

Unlike SARS, a disease that first appeared in Chinese vendors who prepared
wild animals infected with an exotic virus, mad cow disease is caused by an
even more mysterious infectious agent, a misfolded protein called a prion.

When animals eat the remains of their own species or other species,
infectious prions sometimes pass among them.

Prions can incubate for very long periods of time before they result in
full-blown disease, with each animal or person experiencing the effects
differently. But questions about which animal can infect which other animal
in the animal world, including humans, continue to perplex researchers
worldwide.

Can sheep infect cows? Can cows infect sheep? What about goats and pigs? Or
chickens? Can deer infect cattle living in the same pasture without
cannibalism going on? Can deer infect people?

Are some animals silent carriers of the disease, giving meat lovers of the
world a false sense of security?

Answers matter because regulations in United States intended to keep mad cow
disease out of the country and to control related prion disorders like
chronic wasting disease in deer and elk are based on fully understanding the
biology of species barriers.

Ranchers, for example, are not allowed to feed their cows the rendered
remains of other cows and closely related species.

But such remains can be fed to more distant species like pigs, chicken and
fish. Wastes from those animals, once rendered, can be fed back to cows.

http://www.nytimes.com/2003/06/03/science/03SPEC.html?tntemail0