Anthony David, a professor of cognitive neuropsychiatry who coordinates Britain's Gulf War Illnesses Research Unit at King's College in London, said the best evidence suggested the 1991 "syndrome" was sparked by psychological stress experienced by combat veterans.

The likely causes include their dread of biological and chemical weapons, and their sense of ill health was often compounded by the difficult return to civilian life after they left the military, David said.

Exhaustive research has failed to turn up any significant evidence for a physical cause for their symptoms, he said, adding however that the phenomenon should be treated as a genuine health issue.

"There's no smoking gun, so to speak," David told a conference in London on bioterrorism. "This is a very curious condition. It relates rather to occupational additional ill-health."

David said history books showed that major conflicts were followed by a "syndrome" among returning troops -- and the current war in the Gulf would probably be no different.

But he theorised that this time around, the symptoms were likely to be of "more traditional post-traumatic stress disorder" typically experienced by soldiers who have been in close, intense and sometimes hand-to-hand combat.

The 1991 conflict saw a long, prolonged buildup that was followed by a lengthy air campaign and then a lightning thrust by allied armour to oust Iraqi troops out of Kuwait.

So far, this war has seen a far longer ground campaign, sometimes involving ambushes, house-to-house searches and face-to-face fighting.

Post-traumatic stress disorder is a well-known psychological illness experienced not just by troops but also by civilians who have been hostages or victims of violence.

It includes anxiety, avoidance of triggers that remind the sufferer of the trauma, interrupted sleep, flashbacks and startled reactions.

Gulf War Syndrome is a term popularly applied to a vast range of symptoms, from memory loss, chronic fatigue and dizziness to swollen joints, depression and lack of concentration.

About 100,000 US troops as well as thousands of British, Canadian and French troops who took part in the 1990-1991 operation against Iraq have reported one or more of these problems.

Veterans' organisations have variously ascribed the causes to toxic fumes from burning oil wells; dust inhaled from depleted uranium rounds; preventative medicine against chemical attack; exposure to pesticides and to rubber chemical warfare suits; and to the cocktail of vaccinations given to troops to combat infectious diseases.

In an overview he gave to the conference, David said the only statistical evidence that could support any of these claims was that a group of British servicemen who had been given a large batch of jabs -- seven vaccinations or more -- were twice as likely to report symptoms.

But lab tests failed to provide physiological or immunological evidence to support this.

Other research found that Gulf War veterans were far likelier to report physical ill-health but when they were objectively assessed, using neuromuscular tests, they were little different from veterans of other conflicts, he added.

Veterans were often comforted in their belief that they were sick by their contacts with other veterans, by surfing Internet web sites, lack of confidence in medical protection programmes and suspicions that the military were trying to cover things up, he said.

Even if the health problems are likely to have had a psychological cause, that did not make them any less real, David said.

"This is a bio-psychosocial phenomenon. The result is a real increase in physical and psychological symptoms, which in some people are persistent and disabling and cannot be dismissed... we should treat fear as a pathogen."

SPACE.WIRE