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Frequently Asked Questions

Information about how West Nile Virus and how to prevent it from spreading.
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What is West Nile Virus?

West Nile Virus (WNV) is a disease spread by mosquitoes, and caused by a virus. WNV first appeared in the United States in 1999, when at least 62 people in the New York City area became ill and seven people died.

How do people get West Nile Virus?

West Nile Virus is transmitted to people by the bite of an infected mosquito. The mosquito gets infected with the virus after feeding on a bird or horse infected with the West Nile Virus. Not all mosquitoes transmit WNV.

There is currently no evidence that WNV is transmitted from person to person.

What animals can get West Nile Virus?

West Nile Virus most often affects birds, but occasionally can cause the disease in other animals, particularly horses. Most horses bitten by mosquitoes will not become ill with WNV. WNV does not usually cause illness in dogs and cats.

There is a vaccine available for horses against West Nile Virus.

There is no evidence of animal to human transmission.

What are the symptoms of West Nile Virus?

Most WNV infections produce no symptoms in people or symptoms are mild to moderate. Symptoms may include: fever, headache, body aches, skin rash, and swollen lymph glands. More severe cases can cause headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. In very rare situations, - WNV can cause death.

More information about the symptoms from Centers for Disease Control

Who is at risk of getting West Nile Virus?

Everyone is at risk of getting WNV, particularly:

  • Persons who are outdoors between dark and dusk
  • People who spend time in heavily wooded areas during the day
  • People over age 50 have the highest risk of getting severely ill
  • Infants might also be at increased risk

How is West Nile Virus treated?

There is no specific treatment for West Nile Virus, but the symptoms can be treated. In severe cases, hospitalization might be required.

There is no vaccine for humans against West Nile Virus.

What can be done to protect against West Nile Virus?

  • Eliminate standing water from collecting on your property. Mosquitoes can breed in any puddle or standing water that lasts for more than four days.
  • Dispose of, turn upside down, or regularly empty any metal cans, plastic containers, ceramic pots, and other water holding containers on your property. Pay special attention to discarded tires, a common place for mosquitoes to breed.
  • Do not allow water to stagnate in birdbaths. Change it every three to four days.
  • Clean clogged roof gutters of leaves and debris that prevent drainage of rainwater.
  • Aerate ornamental ponds or stock them with fish. In Contra Costa County free mosquitofish can be obtained by calling Contra Costa Mosquito & Vector control at 925-685-9301.
  • Limit the amount of time spent outdoors at dawn and dusk, or at other times when mosquitoes are active. Particularly during mosquito season, May to October.
  • Wear long-sleeved shirts and long pants when outdoors.
  • Use insect repellent containing one of these ingredients: DEET, Picaridin, Oil of Lemon Eucalyptus, or IR3535 - Use according to manufacturer's label.
  • Fix any holes in screens and make sure they are tightly attached to the doors and windows.

What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile disease?

Usually 2 to 15 days.

Where can I get tested for West Nile Virus?

People who have no signs or symptoms of West Nile Virus are not advised to get tested. People with symptoms such as fever, headache, chills, swollen lymph nodes, rash and fatigue should contact their health care provider who will determine whether testing is needed. People who do not have health coverage can contact Contra Costa Health Services at 1-800-771-4270 to make an appointment and enroll in health coverage or visit one of the community clinics in their area.


For more information about mosquito control, call:
Contra Costa Mosquito & Vector Control at 925-771-6196


Content provided by the Public Health Division of Contra Costa Health Services.

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