West Nile Virus
West Nile virus (WNV) entered California in 2003 and Alameda County in 2004. Since its introduction, activity throughout the state has been detected every year and is considered endemic. West Nile virus is a potentially serious disease and proper precautions should be taken to prevent infection.
Learn about West Nile virus activity in Alameda County.
What is West Nile Virus?
West Nile virus (WNV) is a mosquito-borne disease that has been found in parts of Asia, Eastern Europe, Africa and the Middle East. The virus was first detected in the United States in 1999 in New York City and has since spread throughout the continental U.S. Most people who become infected with the virus have only a mild illness or no symptoms, but in rare cases, they can become seriously ill. The types of mosquitoes that can transmit WNV are specific local mosquito species in Alameda County. Our goal is to decrease the numbers of mosquitoes in the county thereby minimizing the possibility of WNV transmission through an Integrated Vector Management (IVM) approach.
What are the symptoms?
80 percent of people who are infected with West Nile virus will show no symptoms and the disease often passes unnoticed.
About 20 percent of people will have mild symptoms that develop between 3 to 14 days after they are bitten by an infected mosquito. The symptoms, some of which resemble a cold, can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. The symptoms can last anywhere from a few days to several weeks.
In a few cases (1 in every 150 cases), the symptoms will be severe. These can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.
How is it spread?
In most cases, West Nile virus is spread by an infected mosquito to a bird. The bird in turn gets bitten by an uninfected mosquito, which then carries the virus to other birds. As you can see in the illustration below, those infected mosquitoes will occasionally bite other hosts such as horses and people, and transmit the virus to them.
West Nile virus cannot spread from person to person.
What is the treatment?
There is no specific treatment for WNV infection. In cases with milder symptoms, they pass on their own. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.
Is there a vaccine for humans?
There is currently no approved vaccine for West Nile virus (WNV) in humans.
Who is at risk?
Those most at risk of developing severe symptoms are people aged 50 and over, as well as those who have compromised immune systems. It should also be noted that if you spend a lot of time outside, your risk factor goes up sharply.
Does West Nile virus affect dogs and cats?
Dogs and cats rarely show illness if infected with West Nile virus. The National Animal Poison Control Center warns that insect repellent should not be used on pets. Cats are highly sensitive to the chemicals, and both dogs and cats lick themselves and could ingest toxins from the repellents. So far, cats and dogs have been fairly resistant to the virus.
What you can do…
There are many things that can be done to make your home and property less welcome to mosquitoes. The following are suggestions to prevent mosquitoes from breeding near your home and decrease your chances of being bitten:
- Eliminate all standing water on your property, and encourage your neighbors to do the same
- Report dead birds – call 877-WNV-BIRD (877-968-2473)
- Stock your fish pond or other water feature with mosquitofish, which are provided at no cost
- Screen your windows and doors, and make sure they fit tightly. Repair or replace screens with tears or holes.
- Minimize outside activity at dawn and dusk
- When outside, wear long sleeve shirts, long pants, and wear repellent with EPA-registered active ingredients (please follow label directions carefully)
- DEET
- IR3535
- Oil of Lemon Eucalyptus
- Picaridin
- Keep outside lighting to a minimum near entry doors (some mosquitoes are attracted to lights)
Learn more about West Nile virus
California Department of Public Health | West Nile virus
