Home
Public Information and Resources
  Latest Activity 
  West Nile Basics 
  Prevention 
  Repellent Info 
  Mosquito Control 
  Find Your Local Mosquito Control Agency 
Professional Resources
  Maps and Data 
  Educational Materials 
  Press/Media 
  Information for Local Agencies 
  Clinician Info 
  Veterinarian and Horse Owner Information 
 

West Nile Basics

What is West Nile Virus?
West Nile virus (WNV) is a mosquito-borne disease that is common in Africa, west Asia, the Middle East, and more recently, North America. Human infection with WNV may result in 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.

When was WNV first found in the United
States?
West Nile virus was first detected in the United States in New York in 1999. Since then, WNV has spread to 48 states, and to Canada and Mexico. Last year there were 2,448 human cases of WNV detected in the United States, including 84 deaths. This is much lower than in 2003 when there were more than 10,000 human cases of WNV detected, including 262 deaths.

When was WNV first found in California?
WNV first appeared in California in 2002 with the identification of one human case. In 2003, three human cases occurred in California and WNV activity was detected in six southern California counties. By 2004, WNV activity was observed in all 58 counties in California and 830 human infections were identified. Click Here for a summary of West Nile Virus in California in 2004.

How is WNV detected and monitored in California?
California is well prepared to detect, monitor, and respond to WNV through ongoing collaboration between over 100 public agencies. The California surveillance system includes human and horse case detection and testing of mosquitoes, sentinel chicken flocks, and dead birds for WNV.

How is WNV transmitted?

  • Infected Mosquitoes. Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers ("vectors") that 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. 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. Transmission during pregnancy from mother to baby or transmission to an infant via breastfeeding is extremely rare.
  • Not through touching. WNV is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.

How soon do infected people get sick?

People typically develop symptoms from 3 to14 days after they are bitten by an infected mosquito.

What are the symptoms of WNV?

WNV affects the central nervous system. However, symptoms vary:

  • Serious Symptoms in a Few People. Less than one percent (about one in 150 people) of individuals infected with WNV will develop severe illness. The severe symptoms 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.
  • Milder Symptoms in Some People. Up to 20 percent (about one in five) of the people who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although some 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 have any symptoms at all.

How is WNV infection treated?

There is no specific treatment for WNV infection. In cases with milder symptoms, people experience fever and aches that pass on their own. In more severe cases, people may need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing, and nursing care.

What should I do if I think I have WNV?

Milder WNV illness improves without treatment, 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. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.

If you have had WNV, are you immune to further infections?

It is thought that once a person has recovered from WNV, they are immune for life to future infections with WNV. This immunity may decrease over time or with health conditions that compromise the immune system.

Who is at greatest risk of getting severely ill from WNV?

Most people do not get ill from WNV infection. However, people over the age of 50 should take special care to avoid mosquito bites since serious symptoms of WNV are more likely to develop in this age group. In addition, people who have compromised immune systems are at increased risk.

Being outside, especially at dawn or at dusk, means you're at risk of being bitten by an infected mosquito. Pay attention to avoid mosquito bites if you spend a lot of time outside, either working or playing.

Can animals get sick from WNV?

Some species of wild birds, particularly crows and jays, are very susceptible to WNV and can die from the infection.

Horses are very susceptible to WNV and approximately one-third of horses that become ill die or are euthanized. An effective vaccine is available for horses and horse-owners should consult with a veterinarian about WNV vaccine and other vaccines against mosquito-borne viruses.

Dogs and cats rarely become ill when infected with WNV.

What can a person do to prevent getting sick from WNV?

The best way to avoid becoming sick from WNV is to prevent mosquito bites.

  • When outdoors, use insect repellents containing DEET (N, N-diethyl-meta-toluamide). Follow the directions on the package.
  • In additional to DEET based products, insect repellents containing Picaridin or oil of lemon eucalyptus have recently been recommended by the federal Centers for Disease Control and Prevention. More information on mosquito repellents may be found at: http://www.cdc.gov/ncidod/dvbid/westnile/RepellentUpdates.htm .
  • Mosquitoes that carry WNV are most active at dawn and dusk, especially during the two hours after sunset. Be sure to use insect repellent and wear long sleeves and pants if engaging in outdoor activities at these times.
  • Make sure that doors and windows have tight fitting screens. Repair or replace screens that have tears or holes.
  • Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets, barrels and other containers.
  • 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 aren't being used.

Can the public assist in detecting WNV by reporting dead birds?

Yes, the public is encouraged to report dead birds because it helps the state monitor WNV activity. Birds play an important role in maintaining and spreading this virus. Mosquitoes acquire the virus from infected birds, and then transmit the virus to people. Evidence of the virus in dead birds is often the first indication that WNV has been introduced into a new region, or that transmission risk is high. Public reports of dead birds are provided to local mosquito control agencies who use this information to target WNV surveillance and control efforts. Some dead birds are tested for WNV. Dead birds can be reported via the website http://vector.ucdavis.edu/cfm/deadbird2.cfm or by calling the hotline: 877-WNV-BIRD

What are state and local agencies doing to reduce the risk of WNV transmission?
State and local agencies conduct the following activities or provide the following services in conjunction with the statewide WNV prevention, surveillance and control program:

•  Ongoing surveillance for mosquito breeding sources .

•  Ongoing, targeted mosquito prevention and control.

•  A toll-free information and dead bird reporting hotline: 1-877-WNV-BIRD.

•  Targeted public education, emphasizing the importance of personal protective measures.

•  Rapid and comprehensive communication with the medical community and veterinarians.

•  Rapid response testing by the WNV laboratory network for timely and accurate human case determinations.

•  An information Website for clinicians, the public, local agencies and others: http:// www.westnile.ca.gov/clinician.htm.

Quick Links