West Nile Virus Infection

West Nile virus (WNV) is a flavivirus commonly found in humans, birds, and other vertebrates in Africa, West Asia, and the Middle East. The disease is part of a family of vector-borne diseases that also includes malaria, yellow fever, and Lyme disease.

WNV occurs mainly in late summer and early fall. In climates where temperatures are milder, West Nile encephalitis (which stems from WNV) can occur year-round.

The risk of contracting WNV is greater for residents of areas where active cases have been identified. Individuals older than age 50 and those with compromised immune systems are at greatest risk. A number of studies have described the transmission of WNV in utero with subsequent fetal damage. There have also been documented cases of WNV transmission to infants through breast milk. The mortality rate for WNV in the general population ranges from 3% to 15%; it is higher in the elderly.

Causes

Birds serve as the reservoir for WNV; mosquitoes are the vector. The virus is transmitted to birds and humans through the bite of an infected mosquito (primarily Culex). Mosquitoes acquire WNV by feeding on birds infected with the virus. The mosquitoes may then transmit the virus to humans and animals when taking a blood meal.

Ticks infected with WNV have been found only in Africa and Asia. The role of ticks in the transmission and maintenance of the virus remains uncertain; to date, ticks have not been considered a vector for transmission in the United States.

The Centers for Disease Control and Prevention has reported that there is no evidence that a person can contract the virus by handling live or dead infected birds. However, people should be instructed to use gloves or double plastic bags to place the carcass of any dead bird or animal in a garbage can; the finding should be reported to the local health department.

Complications

Complications of WNV include progression to tremors, occasional convulsions, paralysis, coma, and (rarely) death.

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