The human form of BSE is unlikely to kill more than 200 people in Britain – less than twice the number currently diagnosed – scientists say today in their most optimistic assessment yet of the future course of the variant Creutzfeldt-Jakob disease (vCJD).

They believe the incidence may now have peaked but the latest predictions are couched in caveats because of the many uncertainties that still surround the brain disease, which has so far infected 111 people in the UK. The time between being exposed to the BSE agent and the onset of the first symptoms of vCJD is critical in trying to gauge how many people are likely to be currently infected but who have yet to fall ill.

Researchers led by Alain-Jacques Valleron, professor of epidemiology at the French national research institute Inserm in Paris, have based their optimism on the average incubation period of the disease being 17 years – far lower than the 60 years assumed by more pessimistic computer models.

"Knowing the incubation period is the main and most important parameter in terms of trying to predict the future course of the epidemic," Professor Valleron said.

The latest assessment is that the total number of cases is likely to be 205 but that this could rise to 403 in the worst-case scenario. However, if the current assumptions on genetic resistance to vCJD are wrong, this figure could rise further although it should still not be more than 1,000 cases in total.

Epidemiologists have not until now estimated the average incubation period of vCJD and the precise figure – 16.7 years – is based on 97 patients in the study who are believed to have contracted the disease after eating contaminated beef in the 1980s.

Between 900,000 and 1.13 million cattle are thought to have been infected by BSE and between 460,000 and 482,000 are estimated to have been slaughtered for human consumption before the first anti-BSE measures were introduced in 1989.

Professor Valleron and colleagues, who included Robert Will, head of the National CJD Surveillance Unit in Edinburgh, say in a study published in the journal Science that the numbers of people exposed to the BSE agent could therefore be very high.

"One could pessimistically assume that virtually everybody in the population has been in contact with food, or bovine products, originating from BSE-infected animals," they say.

What determines the eventual number of infected individuals depends on several factors, such as the amount of BSE needed to trigger an infection, the natural barrier between cattle and humans and variations in the genetic resistance of the human population to the BSE agent.

Professor Valleron said that the one striking characteristic of the vCJD epidemic was the young age of many of those who have been affected. The average age was 28 and only six of the 90 patients in the study who had died were older than 50.

The researchers said that fact gave the most important clue to the length of the incubation period because the age of a patient at diagnosis was in effect a sum of the age at infection plus the incubation time. There were only two possible explanations for the young age profile of the vCJD epidemic: either the incubation period was shorter in younger people or younger people were more susceptible to the disease, the researchers said.

Professor Valleron believes it is the latter, with children under 15 at the time when the BSE epidemic was at its height being at most risk of contracting the infection through eating contaminated beef.

* The Japanese government is to slaughter and incinerate thousands of cattle thought to have been fed with ground animal parts, after the discovery of a second confirmed case of "mad cow" disease.

Agriculture ministry officials could begin the national measures as early as next week.

After the first case was confirmed in September, health authorities banned the import and use of feed made from meat-and-bone meal and began mandatory inspections of beef before marketing.