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Department of Health Services

Public Health Division

West Nile Virus

West Nile virus (WNV) is an infection of birds that is transmitted by mosquitoes. Occasionally, humans and other animals are infected when bitten by an infected mosquito. WNV tends to be seasonal, beginning in the early summer and continuing through fall.

Most people infected with WNV will have no symptoms. About 1 in 5 people who are infected will develop a fever and other symptoms such a headache, body aches, joint pains, vomiting, diarrhea, or rash. Less than 1 percent of people who are infected will develop a serious neurologic illness, such as encephalitis or meningitis (inflammation of the brain or surrounding tissues).

People 50 years of age and older have a higher chance of getting sick and are more likely to develop complications. Studies also indicate that those with diabetes and/or hypertension are at greatest risk for serious illness.

There are no medications to treat or vaccines to prevent WNV infection. The best way to prevent exposure to mosquito bites and therefore WNV disease is to practice the Four D's: DEET, DAWN and DUSK, and DRAIN.

  • DEET - Apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535 according to label instructions. Repellents keep the mosquitoes from biting you. DEET can be used safely on infants and children two months of age and older.
  • DAWN and DUSK - Mosquitoes bite in the early morning and evening so it is important to wear proper clothing and repellent if outside during these times. Make sure that your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens with tears or holes.
  • DRAIN - Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property, by emptying flower pots, old car tires, buckets, and other containers. If you know of a swimming pool that is not being properly maintained, please contact your local mosquito and vector control agency.

Basics and Prevention

WNV in California