West Nile Encephalitis 

West Nile encephalitis (WNE) is distinguished from other arthropod-borne causes of viral encephalitis (eg, western equine encephalitis, eastern equine encephalitis [EEE], Japanese encephalitis, Venezuelan encephalitis) based on its geographic distribution, clinical features, and laboratory findings. (See Etiology, Differentials, and Workup.)

WNE is endemic in the Middle East, Africa, and Asia. In North America, WNE first occurred in the northeast United States along the eastern seaboard and now extends nationwide. WNE may have existed in antiquity in the Middle East. (Some have suggested that Alexander the Great may have died from WNE.)(See Epidemiology.)^{[1, 2]}

In 1999, a late summer outbreak of West Nile encephalitis (WNE), not found previously in the United States, was implicated in several deaths in New York. By late summer 2002, West Nile virus has been identified throughout the eastern and southeastern United States. Following bird migration, the virus is presently extending westward, and by April 2003, virus activity had been detected in 46 states and the District of Columbia.

Throughout the world, outbreaks of WNE have been associated with severe neurologic disease,^[3] although in general, only 1 in 150 affected patients develops symptomatic WNE. (See History.)

For more information, see the CDC fact sheet on West Nile virus, links to state and local government web sites on West Nile virus, and the Environmental Protection Agency (EPA) article on mosquito control.

For clinical information on the Internet, see West Nile Virus: A Primer for the Clinician from the August 6, 2002, issue of the Annals of Internal Medicine, which is available online in Adobe PDF format.^[4] The Canadian equivalent, West Nile Virus: Primer for Family Physicians, was published June 10, 2005 in Canadian Family Physician.^[3]

Prognosis is excellent in all patients except those who are elderly or debilitated.^[5]

Severe cases may result in death or neurologic sequelae; however, recovery is usually complete. Most children with WNE in Asia and Africa have a benign course and only rarely die of the disease. In the United States, most fatal cases have occurred in elderly patients.

Mosquito bites, which are particularly likely during feeding times (dawn and dusk) in the summer months, transmit West Nile virus. Prolonged contact or multiple mosquito bites enhance the risk.

Seventeen species of wild birds transmit West Nile encephalitis (WNE) to humans via the Culex, Aedes, and Anopheles mosquitoes. (See the image below.) WNE first causes symptomatic or asymptomatic illness in wild migratory birds that act as viral replication factories. Wild birds infected with WNE contain high titers of the virus and remain viremic for 1-2 weeks, making them ideal hosts to perpetuate the disease. Mosquitoes transmit WNE from birds to humans. Horses, dogs, and other small animals may harbor WNE after being bitten; however, they are inefficient transmitters because viral titers are relatively low, and WNE viremia is short-lived in these animals.

WNE, like other arthropod-borne viral encephalitides, traverses the blood-brain barrier and infects the brain parenchyma, clinically manifesting as viral encephalitis. WNE may also affect the leptomeninges, resulting in a clinical presentation of aseptic meningitis (viral meningitis). Patients with WNE may present with features of encephalitis and aseptic meningitis (meningoencephalitis).

West Nile virus may also be transmitted in organ transplants and has been found in breast milk.^[6]

In North America, as previously stated, WNE first occurred in the northeast United States along the eastern seaboard and now extends nationwide.

Approximately 50% of children in Egypt have West Nile virus seropositivity. West Nile encephalitis (WNE) is the most common cause of viral aseptic meningitis or encephalitis in patients presenting to emergency departments in Cairo. WNE is common in the Middle East, Asia, and Africa.

WNE usually occurs in the summer, when mosquitoes, wild migratory birds, and humans are in close proximity outdoors. Most US cases occur in elderly patients. Worldwide, most cases occur in young children or young adults; however, elderly patients are affected more severely.

Avoid exposure, particularly from dawn to dusk, in areas where mosquitoes and West Nile encephalitis (WNE) are present. Mosquito netting and mosquito repellents may also be used.

Avoid wearing bright colors and highly aromatic perfumes, deodorants, and hair products that attract mosquitoes. Avoid handling dead or diseased wild birds without proper aseptic precautions.

For patient education information, see the Brain and Nervous System Center and the Infections Center, as well as West Nile Virus and Encephalitis.