Having shingles can increase the risk of a stroke by almost a third, research has shown.

The risk is significantly greater when the infection, caused by the chickenpox virus, involves the eyes. Shingles, or herpes zoster, is a painful skin rash that can occur years after a bout of chickenpox when the dormant virus reawakens.

Scientists studied 7,760 adults who had been treated for shingles between 1997 and 2001, and more than 23,000 matched "control" individuals with no history of the infection. Over the course of a year, 133 of the shingles patients and 306 of the controls had a stroke.

Suffering from shingles raised the risk of a stroke by 31 per cent. If the infection involved the skin around the eye, or the eye itself, the risk was more than four times greater. The study also found that shingles led to an almost three-fold increase in the risk of haemorrhagic, or "bleeding" stokes.

A haemorrhagic stroke is caused by a burst blood vessel in the brain. Shingles raised the risk of more common ischaemic strokes, caused by a blocked arteries, by 31 per cent. The findings were published yesterday in the American Heart Association journal Stroke. Other research has suggested that as the viruses replicate, they cause blood vessel walls to be damaged and inflamed. Eventually the blood vessels may become narrow and blocked or rupture to produce a haemorrhage.

Study leader Dr Jiunn-Horng Kang, from Taipei Medical University Hospital in Taiwan, said: "Herpes zoster infection is very easy to diagnose and antiviral medication can be used to treat the infection in the early stages.

"While the mechanism by which shingles increases stroke risk remains unclear, the possibility of developing a stroke after a shingles attack should not be overlooked."