Encephalitis is a rare, but often deadly, inflammation of the brain, which often starts as a flu like illness or headache with symptoms rapidly getting worse over a short period of time. A person may experience seizures (fits), changes in mental state, such as confusion or drowsiness, a loss of consciousness, or lapse in to a coma.

The most common known causes of encephalitis are infections by a virus. But, in most cases, a cause is never found, which makes it difficult to devise new treatments or develop interventions to prevent serious complications.

Around 700 people in England develop acute encephalitis every year and about 7% of these cases are fatal. In the largest study of its kind in England, led by scientists at the HPA, over 200 patients with encephalitis were studied. Researchers found a cause for 63% of cases, the most frequent being the herpes simplex virus, which usually causes cold sores with no serious complications. But they also found that almost 10% of all cases were caused by antibodies made by the body's immune system against certain brain proteins and not caused directly by infection. Whilst it had previously been known that this could be a cause, the frequency with which it occurred had not previously been established.

The importance of these results is that it shows drugs to "dampen down" the immune system are more likely to be required to treat encephalitis than previously thought, as brain damage is thought to occur through immune attack of the brain.

Dr David Brown, director of the HPA's virus reference department, said: "The aim of this study was to learn more about the many causes of encephalitis and we are delighted that our findings have significantly contributed to our knowledge of the disease.

"We found that almost 10% of all cases in the group of patients we studied were related to antibodies found within the brain. These patients were also shown to have the worst outcomes. But the good news is that this form of encephalitis is potentially treatable, especially when therapy is started early, so it is hoped our findings will help clinicians in diagnosing and treating patients as early as possible."

Dr Brown continued: "This important study has led to the production of a national clinical case definition for encephalitis and also better methods for identifying the cause. It has also pulled together multi-disciplinary experts from around the country with an interest and expertise in encephalitis, creating a unique knowledge network here in the UK, which could also benefit patients worldwide. We have also received lots of interest from colleagues from all around the world."