Mad cow disease might linger longer.

A rare but deadly human illness spread by cannibalism has an incubation period in some individuals of about 4 decades, researchers in New Guinea have discovered. The finding implies that a related human illness caused by eating beef from cattle with mad cow disease could also lie dormant for many years.

Scientists have identified a handful of diseases caused by misfolded proteins called prions, including mad cow disease, scrapie in sheep, and chronic wasting disease in deer and elk. In people, eating contaminated beef can cause Creutzfeldt-Jakob disease. Like the other prion diseases, Creutzfeldt-Jakob disease kills brain cells and is fatal.

The only other known prion disease in people is kuru, found in members of New Guinea tribes who at one time practiced ritual cannibalism that included eating human brains.

Cannibalism was banned in the late 1950s in New Guinea, and no one born after 1959 has contracted kuru, say authorities there.

Nevertheless, a study by John Collinge of University College London and his colleagues has found that some older people have come down with the disease in just the past decade. Between 1996 and 2004, the researchers identified 11 people living in the affected areas who had developed kuru. All were born before 1950. The scientists calculate that the minimum incubation time for the disease in these cases was between 34 and 41 years. The study appears in the June 24 Lancet.

On the basis of these findings, the authors say that an incubation period of 30 years or more for Creutzfeldt-Jakob disease caused by tainted beef is "possible, if not probable."--N.S.