LONDON (Reuters) - Scientists exploring what may trigger a complex disorder known as chronic fatigue syndrome (CFS) have found clues in the way some people’s immune systems respond more actively to a health attack.

A severe illness characterized by long-term physical and mental fatigue, CFS is thought to affect up to 17 million people worldwide and around 250,000 people in Britain.

Sufferers are often bed-bound and unable to carry out basic daily activities like washing and feeding themselves.

The researchers used a drug known as interferon alpha to create a model of the syndrome and found that patients whose immune response to treatment was hyperactive or exaggerated were more likely to then develop severe fatigue.

“For the first time, we have shown that people who are prone to develop a CFS-like illness have an overactive immune system, both before and during a challenge to the immune system,” said Alice Russell of King’s College London’s Institute of Psychiatry, Psychology & Neuroscience (IoPPN), who led the work.

The condition, as well as research into it, is highly contentious, in part because its possible causes and range of debilitating symptoms are poorly understood.

Interferon alpha is used as a treatment for hepatitis C infection, and activates the immune system in the same way as a powerful infection. Many patients who receive interferon alpha experience extreme fatigue during treatment, and some continue to feel chronic fatigue for many months after the drug course is completed.

Russell’s team used this knowledge and measured fatigue and immune system markers in 55 patients before, during and after treatment with interferon alpha.

They found that the 18 of those 55 who went on to develop a CFS-like illness had a hyperactive immune system before treatment, and an highly overactive response during treatment.

“(This suggests) people who have an exaggerated immune response to a trigger may be more at risk of developing CFS,” Russell told reporters at a briefing about the findings.

IoPPN professor Carmine Pariante stressed that while the study’s main finding is a useful addition to scant scientific knowledge about CFS - also known as myalgic encephalopathy (ME) - it offers few clues on how to treat, cure or prevent it.

“It’s a light in the fog,” he told reporters.

“But a better understanding of the biology underlying the development of CFS is needed to help patients.”