Mad cow disease may be even more infectious and persistent than experts had thought, U.S. scientists said Wednesday.

They found that prions - the tiny mutated brain proteins that cause the brain-wasting disease - can exist undetected for long periods in animals that were thought to be resistant to the disorder.The research raises the possibility that infected animals other than cattle could be linked to Creutzfeldt-Jakob disease, the human form of bovine spongiform encephalopathy or mad cow disease.

"We report results that raise concern over the possible long-term persistence of infectivity in such clinically resistant species which may have implications for the control of BSE," Richard Race and Bruce Chesebro, of the National Institute of Allergy and Infectious Diseases in Hamilton, Montana, said in a letter in the science journal Nature.

So far there is no evidence that BSE can be transmitted from resistant animals, such as chickens or hamsters, to more susceptible species, but Race and Chesebro said more research is needed.

Mice, goats, mink, pigs and some members of the cat family can all get BSE. Hamsters suffer from their own prion disease, called scrapie, to which mice are completely immune.

Race and Chesebro injected the infectious agent for hamster scrapie into the brains of mice. Although the mice did not develop signs of the disease, the scientists found that a year after the mice were infected, their contaminated brain and spleen tissue could cause the disease when it was re-injected into hamsters.

"Although we have not tested whether similar results would be obtained after oral ingestion, this unexpected and prolonged survival of a foreign scrapie agent raises the possibility that BSE infectivity might persist in various `resistant' species exposed to BSE-contaminated feeds," they added.

The scientists voiced particular concern about poultry raised for human consumption, and said feed containing animal carcasses should not be fed to any animals.

"Additional experiments should be carried out to detect possible BSE infectivity in clinically normal BSE-exposed animal species," they added.