Neurotropic alphaviruses

Alphaviruses are geographically restricted enveloped, single-strand, message-sense RNA viruses. Alphaviruses are maintained in natural cycles between vertebrate and invertebrate hosts and are transmitted to vertebrates through the bite of an infected mosquito. Eastern equine encephalitis, western equine encephalitis, and Venezuelan equine encephalitis viruses are the neurotropic alphaviruses of greatest importance as causes of human encephalomyelitis and were initially recognized for their ability to cause disease in horses. Alphaviruses primarily associated with rash and arthritis can also cause neurologic disease. Infection with Chikungunya virus has caused encephalitis in humans and Semliki Forest virus and Sindbis virus provide important mouse models for alphavirus encephalomyelitis. Brain and spinal cord neurons are the primary target cells and infection induces a mononuclear inflammatory response. Clearance of infectious virus is rapid and noncytolytic through the actions of antibody and interferon-γ, but viral RNA and proteins are cleared slowly. Diagnosis is usually made by detection of viral RNA or antiviral IgM. No licensed vaccines or treatments are available.