Encephalitis due to herpes simplex virus

**Definition:** Herpetic encephalitis is a cerebral infection caused by herpes simplex virus type 1 (HSV1). It presents as acute necrosing temporal encephalitis. Onset is rapid (less than 48 hours) with a fever of 40 °C, headaches, and behavioural, language and memory problems. These initial manifestations are followed by numbness and coma, which may be accompanied by convulsions and paralysis. This disease, which affects only a small minority of HSV1-infected individuals, results from a primary immune deficiency.

**Long definition:** Herpetic encephalopathy is a cerebral infection caused by herpes simplex virus type 1 (HSV1). The annual incidence varies between 1 in 250,000 and 1 in 500,000. Onset may occur at any age, but is more common in children under the age of 3 (primo-infection) or in adults over the age of 50 years (usually as a recurrent infection). It presents as acute necrosing temporal encephalitis. Onset is rapid (less than 48 hours) with a fever of 40 °C, headaches, and behavioural, language and memory problems. These initial manifestations are followed by numbness and coma, which may be accompanied by convulsions and paralysis. This disease, which affects only a small minority of HSV1-infected individuals, could result from a genetic predisposition. In fact, mutations have been identified in a few patients in four different genes involved in innate immunity against HSV-1 in the central nervous system (genes coding for UNC-93B and Toll-like receptor 3, but also genes coding for transcription factors STAT-1 and NEMO). Emergency treatment should involve intravenous administration of acyclovir, as soon as the diagnosis is suspected. The disease course is severe, with a mortality rate of 20% and severe sequelae among surviving patients.