This virus was first identified in 1937 in the West Nile District in Uganda. It arrived in the US during the summer of 2000. California had two imported human cases in 2002 and several cases in the Los Angeles area in 2003. Dead birds, infected with West Nile Virus, were found in the Los Angeles area in 2004.
This viral infection is spread by mosquitoes primarily to birds, but horses and humans can also be infected. Mosquitoes become infected carriers when they feed on infected birds. A very small number of people have been infected through blood transfusions, organ transplants and breast milk.
Most people who are bitten by a mosquito with West Nile Virus will not get sick. Approximately 20% will get a mild illness including fever, muscle aches, headache, nausea, and vomiting. Approximately 1 in 150 infections will result in severe neurological disease. Encephalitis is more commonly reported than meningitis.
The incubation period for this virus is thought to range from 2 to 14 days, but most typically from 2 to 6 days. Mild cases usually produce symptoms for 3 to 6 days. The most significant risk for developing severe neurological disease is advanced age.
Although West Nile Virus can be transmitted year round in warmer climates, it is most commonly seen in the summer and autumn. Prevention includes using DEET mosquito repellent, the wearing of long sleeve clothing, and avoiding outdoor activity at dusk and dawn. Make sure window screens are in good repair, and drain sources of standing water where mosquitoes may breed and lay their eggs.
Diagnostic testing for West Nile virus can be obtained through local or state health departments for patients with encephalitis or meningitis. The most efficient diagnostic method is detection of IgM antibody to WNV in serum or cerebral spinal fluid collected within 8 days of illness onset using the IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA).
Treatment is supportive often involving hospitalization, IV fluids, respiratory support and prevention of secondary infections.
West Nile virus infection is reportable to both local and state health departments. WNV encephalitis is on the list of designated nationally notifiable arboviral encephalitides.