An unusual case of variant CJD, the human form of mad cow disease, has revived fears that some people might still be infected and yet to develop symptoms.

Some cases of infection might have very long incubation periods; the genetic make-up of a 30-year-old man who died from vCJD suggests making several unaware that they were suffering. Doctors from the MRC Prion Unit and National Prion Clinic at the UCL Institute of Neurology and National Hospital for Neurology and Neurosurgery in London also accepted this in a journal.

Those who were unaware might have acquired the disease from eating beef as young children. The beef eaten might be with the cattle disease bovine spongiform encephalopathy (BSE).

In the UK including four suspected victims who are still alive the total number of vCJD cases now stands at 170.

The recent victim, who died in January after suffering symptoms of memory loss, unsteadiness and visual hallucinations and personality change, had the heterozygous MV form as the same gene can also take the form VV or MV. Scientists investigating the case led by Professor John Collinge, from the Medical Research Council Prion Unit in London, pointed out that related brain diseases in people with the heterozygous MV prion gene had incubation periods that were long. They included kuru, thought to be linked to cannibalism in Papua New Guinea, and CJD triggered by treatment with growth hormones.

The researchers said in the Lancet medical journal that some MV patients with kuru had the disease for 50 years or more. The scientists also uspect in people infected with vCJD the MV gene might cause similar incubation periods.