Epidemiology Unit

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West Nile Virus


What is West Nile virus?

West Nile virus (WNV) is a mosquito-borne flavivirus that is closely related to yellow fever, dengue, Japanese encephalitis and Saint Louis encephalitis viruses. WNV is widely distributed in Africa, West Asia, and the Middle East. In the Western Hemisphere, WNV was first detected in 1999 during an outbreak of encephalitis in New York City and has since spread across the United States.


How is West Nile virus transmitted? 

Most often, West Nile virus (WNV) is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds (reservoir for WNV). Infected mosquitoes can then spread the virus to humans and other animals when they bite. WNV is not spread from person to person. In a very small number of cases, WNV has been transmitted through blood transfusions, organ transplants, breastfeeding, or during pregnancy from mother to baby.

People may develop symptoms between 3 and 14 days after they are bitten by infected mosquitoes. The incubation period may be longer in persons with weakened immune systems.


What are the symptoms of West Nile virus?

About 80% of those infected with West Nile virus (WNV) are asymptomatic.

Up to 20% of those infected with the virus may develop a fever with other symptoms such as headache, body aches, weakness, fatigue, skin rash on trunk of the body, swollen lymph glands, or eye pain. These symptoms generally last for a few days, although weakness and fatigue may last for a few weeks.

Of those infected with the virus, 1 in 150 or <1% may develop a severe illness called West Nile neuroinvasive disease (WNND) because it affects a person's nervous system. Specific types of WNND may include encephalitis, aseptic meningitis, acute flaccid paralysis, atypical Guillain-Barré Syndrome, or transverse myelitis. Clinical features of WNND may include high fever, headache, stiff neck, ataxia (failure of muscular coordination), muscle weakness, numbness, flaccid paralysis, tremors, convulsions, disorientation, altered mental status, or coma. These symptoms may last from several weeks to months, and neurological effects may be permanent. About 10% of patients with WNND die.

There is no specific treatment for WNV disease other than supportive care.


How can you protect yourself?

Currently, there are no vaccines to prevent West Nile virus (WNV) infection in people. However, people can reduce their risk of infection by using insect repellent and wearing long-sleeved shirts and long pants to prevent mosquito bites. Steps can also be taken to control mosquitoes both indoors and outdoors, such as keeping screens on windows and doors in good repair and draining all standing water sources that can be mosquito-breeding areas around the home, including buckets, old car tires, rain gutters, birdbaths, and pet bowls.


Local Data

For more information on West Nile virus in San Diego County, please see the following: 

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For more information, contact the Epidemiology Unit at (619) 692-8499 or send us an e-mail.