More people may be incubating variant CJD, the human version of so-called “mad cow disease”, than was previously thought, according to scientists who today report an unusual case of the disease. All those tested worldwide since 1994 when the first cases were identified have been MM homozygous.

However, a 30-year-old man who died of vCJD in January this year was found to have a different genetic makeup from the rest of the 200 or so people diagnosed around the world, and identified as MV heterozygous.

Six months before the man was diagnosed with the disease, he had been admitted to hospital suffering from personality changes, unsteadiness in walking that became progressively worse, and intellectual decline. He told doctors he had severe leg pain and memory problems. Two months later, he developed visual hallucinations. The symptoms got progressively worse and an MRI scan confirmed vCJD .

Doctors from the MRC Prion Unit and National Prion Clinic at the UCL Institute of Neurology, and the National Hospital for Neurology and Neurosurgery in London, report the unusual case in today’s Lancet medical journal.

The observation could be of concern. In some other human prion diseases, such as kuru – thought to be linked to cannibalism in Papua New Guinea – people who are MV heterozygous have incubated the disease for longer than those who are MM homozygous before symptoms have shown. Some MV heterozygous patients are reported to have incubated kuru for over 50 years.

It is possible, doctors say, that vCJD takes longer to develop in people who are MV heterozygous than in MM homozygous people.