West Nile Virus (WNV) is a mosquito-borne infection that can cause encephalitis (inflammation of brain tissue) and/or meningitis (swelling of the tissue that encloses the brain and spinal cord).  WNV is a potentially serious illness.  Experts believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.

This fact sheet contains important information that can help you recognize and prevent West Nile Virus. 

• Serious Symptoms in a Few People.  About one in 150 people infected with WNV will develop severe illness.  The severe symptoms can include high fever, headache, neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.  These symptoms may last several weeks, and neurological effects may be permanent.

• Milder Symptoms in Some People.  Up to 20 percent of the people who become infected will display symptoms which can include fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back.  Symptoms can last for as short as a few days, though even healthy people have been sick for several weeks.

• No Symptoms in Most People.  Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms at all, but there is no way to know in advance if you will develop an illness or not.

• Infected Mosquitoes.  Most often, WNV is spread by the bite of an infected mosquito.  Mosquitoes become infected when they feed on infected birds.  Infected mosquitoes can then spread WNV to humans and other animals when they bite.

• Transfusions, Transplants, and Mother-to-Child.  In a very small number of cases, WNV also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.

• Not through touching.  WNV is not spread through casual contact such as touching or kissing a person with the virus.

People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito. 

Not by casual contact.  There is evidence that West Nile Virus can be acquired via a blood transfusion or organ transplant from an infected donor.  Screening tests are now available to analyze blood and organ donation to reduce the risks of contracting the virus through blood and organ donations.

• People over 50 at higher risk to get severe illness.  People over the age of 50 are more likely to develop serious symptoms of WNV if they do get sick and should take special care to avoid mosquito bites.

• Being outside means you're at risk.  The more time you're outdoors, the more time you could be bitten by an infected mosquito.  Pay attention to avoiding mosquito bites if you spend a lot of time outside, either working or playing.

• Risk through medical procedures is very low.  All donated blood is checked for WNV before being used.  The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it.  If you have concerns, talk to your doctor.

• Pregnancy and nursing do not increase risk of becoming infected with WNV.  The risk that WNV may present to a fetus or an infant infected through breast milk is still being evaluated.  Talk with your care provider if you have concerns.

There is no specific treatment for WNV infection.  In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own, although illness may last weeks to months even in healthy persons.  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.

In extremely rare cases, yes.  In 2007, 124 people died of West Nile Virus in the US, representing about 3.4% of diagnosed cases in this country.  As of August 19, 2008, 236 cases of WNV have been reported nationwide, of which two people have died.  Elderly people and those with severe, life-threatening illnesses are especially at risk of becoming very ill.

Milder WNV illness improves on its own, and people do not necessarily need to seek medical attention for this infection though they may choose to do so.  If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately.  Severe WNV illness usually requires hospitalization.  Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.

The easiest and best way to avoid WNV is to prevent mosquito bites.

• When you are outdoors, use insect repellents containing an EPA-registered insect repellent.  The most effective repellents contain DEET (N,N-diethyl-meta-toluamide).  Products containing DEET are safe when used according to label directions. 

• Many mosquitoes are most active at dusk and dawn.  Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours. 

• Make sure you have good screens on your windows and doors to keep mosquitoes out. 

• Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels.  Change the water in pet dishes and replace the water in bird baths weekly.  Drill holes in tire swings so water drains out.  Keep children's wading pools empty and on their sides when they are not being used. 

DEET should not be used on infants less than 2 months of age. For children between the ages of 2 months and 2 years, DEET should be used with extreme caution for brief amounts of time and only at low concentrations of 10% or less. Avoid spraying any repellent on the hands and faces of infants.

There are some animals that can get WNV.

• To date, there is no evidence that dogs and cats become ill from the West Nile Virus.

• Pet birds may become sick with West Nile Virus.

• Some horses may be susceptible to the virus.  Symptoms in horses may include stumbling, weak or paralyzed limbs, circling, inability to stand, tremors and death.

• A West Nile Virus vaccine is available for horses; horse owners should consult with their veterinarians about obtaining the vaccine.

• There is no vaccine for any other type of animal or for humans.

If you find a dead bird:  Don't handle the body with your bare hands.  Contact your local health department for instructions on reporting and disposing of the body.  They may tell you to dispose of the bird after they log your report.  You can also call the California Department of Health Services (DHS) toll-free hotline at (877) 968-2473.  DHS will ask for your zip code.

• If birds in that zip code area have not yet been identified with West Nile Virus, DHS will contact the local designated agency to pick up the dead bird and send it to the state lab for testing.

• If bird in that zip code area have been identified with West Nile Virus, DHS will inform you that they are no longer testing in that zip code area.  In that case, call the Pasadena Humane Society at (626) 792-7151 to arrange to have the dead bird picked up and disposed of.

There is no evidence that West Nile virus can be acquired from handling dead birds, but we recommend you do not handle the bird, especially with your bare hands.  Use gloves, a shovel, or a plastic bag to place the bird in a trash bag for disposal or call the Pasadena Humane Society for pickup.

Yes. The Pasadena Public Health Department's Environmental Health Division uses special traps to collect mosquitoes in the evening.  Mosquitoes that are collected are identified by qualified staff the next morning and samples are sent to the state laboratory for WNV testing. 

Q. How are mosquitoes controlled in Pasadena and California cities?

More than 70 agencies in California, including Pasadena Public Health Department, conduct regular mosquito surveillance and control.  Personnel from these agencies are certified and trained to conduct safe and effective mosquito control practices.  

Q. What is being done in Pasadena to respond to WNV and to protect the community?

Pasadena Public Health Department staff has been working to reduce the number of mosquitoes in Pasadena.  Environmental Health Division staff routinely responds to residents' concerns, patrol areas known to have problems with standing water and treat the areas with pesticides as needed.  Pasadena Public Health Department now has a West Nile Virus Hotline.  Call (626) 744-6004 to report a mosquito problem in your neighborhood, ask a question about WNV, find out the dates of any scheduled community presentations, or request additional information about WNV.  

Q. How can I get more information about WNV?

Call the Pasadena Public Health Department’s West Nile Virus Hotline at (626) 744-6004. Or, visit the following official websites: California Department of Health Services www.westnile.ca.gov; Centers for Disease Control and Communicable Disease www.cdc.gov/ncidod/dvbid/westnile/index.htm; and Los Angeles County Department of Health Services www.ladhs.org.

Q. How many people have West Nile Virus illness in California?

Below is a chart on reported incidence of human West Nile Virus illness in California (as of 10/10/08)

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