West Nile encephalitis is an infectious disease that causes encephalitis, or inflammation of the brain. The etiology stems from the West Nile virus (WNV), a flavivirus commonly found in humans, birds, and other vertebrates in Africa, West Asia, and the Middle East. This disease is part of a family of vector-borne diseases that include malaria, yellow fever, and Lyme disease.

The virus was first documented in the Western Hemisphere in August 1999, when a virus found in numerous dead birds in the New York, New Jersey, and Connecticut region was identified by genetic sequencing as the West Nile virus. Scientists in the United States first discovered the rare strain in and around the Bronx zoological park and believe imported birds may have carried the disease, which was spread by mosquitoes feeding on the infected birds.

In temperate areas of the world, West Nile encephalitis cases occur mainly in late summer or early fall. In climates where temperatures are milder, West Nile encephalitis can occur year-round.

As of mid-November 1999, health officials at the Centers for Disease Control and Prevention (CDC) reported 56 cases of WNV infection (31 confirmed and 25 probable), including seven deaths.

The risk of contracting West Nile encephalitis is greater for residents of areas where active cases have been identified. Individuals older than age 50 and those with compromised immune systems are most at risk. At this time, there’s no documented evidence that a pregnant woman’s fetus is at risk for infection with WNV. The mortality rate for West Nile encephaltis ranges from 3% to 15%; the mortality rate is higher in the elderly population.