Acute viral encephalitis.

Acute viral encephalitis is a rare but devastating complication of common virus infections. Primary viral encephalitis is due to brain invasion by the virus and subsequent replication in the central nervous system, whereas postinfectious encephalitis is thought to be immune-mediated. The most frequent causes of primary encephalitis worldwide are rabies and the arboviruses, which comprise different families of viruses distributed according to geography and seasonality. Herpes simplex virus (HSV) is the commonest cause of encephalitis in Western countries. Herpes simplex encephalitis (HSE) occurs at all ages during infancy and childhood with a peak incidence during the first year of life. Its pathogenesis remains unsolved, although defects in the innate immune response have been observed in a few cases. Direct and sensitive virological diagnosis is available, but may be difficult at early stages of the disease. Specific antiviral therapy has proven to improve the prognosis of HSE dramatically, if administered early. Despite use of antiviral drugs, sequelae remain high and relapses may occur, presumably secondary to HSV latency in the brain. In addition, deleterious host inflammatory responses might account for failure of therapy. Immunomodulatory treatments have been proposed but require a better understanding of the respective roles of the multiple pathways of the immune response.