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Viruses from many virus families can cause infection of neurons and viral encephalitis. Most commonly diagnosed are the herpes virus infections: herpes simplex, cytomegalo and varicella-zoster viruses. They are most commonly diagnosed because there are available treatments and PCR tests for diagnosis. The most common causes of viral encephalitis are enteroviruses including Coxsackie, ECHO and enterovirus 71. These viruses are related to poliovirus. Infections occur mainly in the summer and fall and most individuals recover. Treatment is not available and diagnosis is most often accomplished by culturing stool. The third major virus groups causing encephalitis are the arthropod-borne viruses. Transmission can be by ticks or mosquitoes, but most often mosquitoes, and occurs in the summer and fall. Important diseases in this category include West Nile encephalitis, Japanese encephalitis and Eastern equine encephalitis.
Our laboratory has been interested in the determinants of outcome from virus infection of neurons using a mouse model system. Host determinants include age, genetic background and the immune response. Viral determinants include the ability of the virus to enter the CNS, to efficiently infect neurons and to damage neurons. As neurons mature, they develop an intrinsic resistance to virus infection and virus-induced cell death, so disease is often more severe in the young individual. Recovery from infection requires immune mechanisms that do not damage the infected neuron. This often includes local production of antiviral antibody and cytokines that can inhibit virus replication. However, all viral nucleic acid may not be eliminated by these noncytolytic mechanisms of clearance and prolonged suppression of virus replication may be required to prevent reactivation.