LONDON (Reuters) - Mad cow disease may be even more infectious and persistent than experts had thought, U.S. scientists said on 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 (CJD), the human form of bovine spongiform encephalopathy (BSE) 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. Scientists believe the BSE epidemic that broke out in British herds in 1986 was caused by cattle feed containing carcasses of sheep that died of scrapie, a prion brain disease similar to BSE. Britain banned such feed for cattle in 1988, but it was still fed to pigs and chickens until it was outlawed for all animals in 1995.

The European Union slapped an export ban on British beef in March 1996 after London admitted a possible link between BSE and a new strain of CJD. Britain also slaughtered millions of cattle to eliminate the epidemic and reduce the risk of any possible transmission to humans. So far 24 people have died from a new strain of CJD which scientists believe is caused by eating contaminated beef. Unlike the previous strain of CJD, a one in a million disease of mainly elderly people, the new variant hits people under 40. ``What comes out from this (research) is that we don't know where humans got new variant CJD from, and it's very difficult to do studies to find it out,'' Dr Steve Dealler, a British microbiologist who has studied prion diseases, said in a telephone interview.

``It shows that the exposure that the human population, particularly in the UK, would have had to BSE is not just from cows but from all sorts of directions we haven't thought of.'' Dealler said that pigs and chickens, and not just infected cattle, might have caused the disease in humans. A spokesman for Britain's agriculture ministry said it will examine the U.S. study and is always open to new research from the scientific community. Last month it said that new data suggested the BSE epidemic would fall to an insignificant level by 2001.