West Nile Fever

What is West Nile Fever?

West Nile fever is an infectious disease caused by West Nile fever virus. It occurs most commonly in the Middle East but may now be found worldwide, including North America. Although it is generally a mild disease (approximately 8 out of 10 infected persons have mild or no symptoms), it may cause encephalitis (brain inflammation), especially among the elderly.

How do you get West Nile Fever?

West Nile virus is acquired through the bite of an infected mosquito. The mosquitoes become infected when feeding on infected birds. Very rarely, West Nile virus has been spread through blood transfusions, organ transplants, breastfeeding, and during pregnancy from mother to baby. Pregnant women should seek medical advice if they develop any symptoms of West Nile virus. West Nile virus is not transmitted through casual person-to-person contact.

Susceptibility and Resistance

Susceptibility is universal. Anyone can acquire West Nile virus. Individuals who contract West Nile virus become immune to further infections, so those who are most susceptible in endemic areas are mainly foreigners and young children.

Incubation Period

The incubation period for West Nile Fever is 2-14 days.

What are the Symptoms?

Asymptomatic cases of West Nile fever are very common. Approximately 80 percent of people who are infected with the virus do not show any symptoms at all. Up to 20 percent of those infected develop a mild illness involving fever, headache, muscle and joint aches, nausea, vomiting, and sometimes swollen lymph nodes or a skin rash appearing on the stomach, chest, and back. These symptoms can last from only a few days to several weeks.

Approximately one in 150 people who become infected with West Nile virus develop a very severe illness involving inflammation of the brain (encephalitis). Symptoms include high fever, headache, neck stiffness, stupor, disorientation, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. Coma can occur. These symptoms can last several weeks, and the neurological effects can be permanent. Medical attention should be sought immediately if any of these more severe symptoms occur.

Preventative Measures

The best preventive measure against West Nile virus is to avoid mosquito bites through the use of insect repellents, mosquito bed nets, and protective clothing.


There is no specific treatment for viral infections of West Nile virus other than treating the symptoms. Milder disease improves on its own through rest and symptomatic treatment (e.g., analgesics for muscle and joint pain). Severe West Nile disease usually require hospitalization and supportive care such as intravenous fluids and nursing care.

Where Does It Commonly Occur?

West Nile virus is widespread in Africa, North America, the Middle East, India, Southeast Asia, Europe, the Caribbean, Central and South America, and Australia (where it is known as Kunjin virus). Outbreaks have been seen in Canada, the Czech Republic, Egypt, France, Italy, India, Israel, Romania, Russia, and the USA.