West Nile virus infections.

Since its introduction to the United States in 1999, West Nile virus (WNV) has become endemic in North America and has emerged as the most common cause of epidemic meningoencephalitis in North America and the leading cause of arboviral encephalitis in the United States. West Nile virus is maintained in nature by cycling between mosquito vectors and bird hosts; humans are incidental hosts. Transmission to humans occurs predominantly after a bite from an infected mosquito but has also occurred via transfusion of blood products, via organ transplantation from infected donors, transplacentally, and percutaneously through occupational exposure. Approximately one of 150 patients develops central nervous system manifestations, including meningitis, encephalitis, and acute flaccid paralysis/poliomyelitis. Risk factors for neuroinvasive disease include older age and immunosuppression. Imaging findings are nonspecific, and cerebrospinal fluid findings include pleocytosis, elevated protein, and normal to decreased glucose. The diagnosis is made in most patients on serological examination. Reverse transcription polymerase chain reaction tests are useful to screen blood products and for surveillance of birds and mosquitoes. The pathological findings are typical of a viral meningoencephalitis and include microglial nodules, perivascular chronic inflammation, and variable neuronal loss with necrosis or neuronophagia. Treatment is largely supportive, and control of the mosquito vectors may reduce the incidence of human infections.