West Nile Virus (WNV) is a mosquito-borne virus that has been found in parts of Asia, eastern Europe, Africa, and the Middle East. The virus was first detected in the United States (U.S.) in 1999 in New York City and has been making its way across the United States since.

The majority of people and animals that are infected with the virus have a mild illness or no symptoms. In rare cases, the virus can cause a more serious condition called encephalitis, an inflammation of the brain. The elderly are at a higher risk for disease caused by WNV. At the end of 2001, a total of 28 states had reported WNV activity. The virus has moved extensively further westward during 2002, bringing the total to 44 states.  California has reported one human case (Los Angeles) and the state of Washington recently listed a WNV positive raven.

How Do People and Animals Get West Nile Virus?

WNV is transmitted to people and animals by infected mosquitoes. Only certain species of mosquitoes carry the virus and very few mosquitoes are actually infective. A mosquito first acquires the infection by feeding on a bird with virus in its blood. The virus lives in the mosquito and is transmitted to a new host in the mosquito’s saliva when the insect bites a person or animal.

Humans and horses are accidental hosts for WNV. Human-to-human or horse-to-human transmission does not occur. Virus transmission is most frequent from May to October when mosquitoes are most abundant (see transmission cycle below).

What are the Symptoms of West Nile Virus in People?

Most people who are infected with WNV have no symptoms. However, of those who become ill, symptoms can include fever, headache, nausea, body aches, mild skin rash, or swollen lymph nodes. In a few cases, the disease will progress to encephalitis (inflammation of the brain). The time between the mosquito bite and the onset of illness, known as the incubation period, ranges from 5-15 days in humans. It is estimated the 1 in 150 people who are infected with WNV will require hospitalization. The elderly are particularly susceptible to clinical illness caused by WNV. There is no specific treatment for infection with WNV. Currently no vaccine for humans is available and supportive care is important.

To minimize exposure to mosquitoes and the microbes they may carry:

• Avoid outside activity at dawn and dusk during the mosquito season (May to October). This is particularly important for the elderly and small children.
• Wear protective clothing (long pants and long sleeves) and apply insect repellent (DEET) when outside.
• Make sure that doors and windows have tight-fitting screens. Repair and replace screens that have tears or holes in them.
• Drain all standing water on private property and stock permanent ponds with fish that eat mosquito larvae.
• Make sure roof gutters drain properly. Clean clogged gutters in the spring and fall.
• Sweep puddles into landscaped areas and empty containers that may collect water at least once a week.