West Nile Virus Fact Sheet

What is West Nile virus?

West Nile virus (WNV) is a mosquito-borne disease that is common in Africa, West Asia and the Middle East.

When was West Nile virus first found in the United States?

WNV was first detected in the United States in New York in 1999. Since then, WNV has rapidly spread to 48 states.

How is West Nile virus spread?

People usually get West Nile virus from the bite of an infected mosquito. Also, there is evidence that WNV can be acquired via a blood transfusion or organ transplant from an infected donor.

Dogs and cats become infected when bitten by an infected mosquito. There is also evidence that cats can become infected with the virus after eating experimentally infected mice.

Can West Nile virus cause illness in dogs or cats? Experimentally infected dogs showed no symptoms after infection with WNV. Some infected cats exhibited mild, nonspecific symptoms during the first week after infection, for the most part only showing a slight fever and slight lethargy.

It is unlikely that most pet owners would notice any unusual symptoms or behavior in cats or dogs that become infected with WNV.

How can my veterinarian treat my cat or dog if they become infected with WNV? There is no specific treatment for WNV infection. Full recovery from the infection is likely. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.

Does my dog/cat becoming infected pose a risk to the health of my family or other animals? There is no documented evidence of dog or cat-to-person transmission of West Nile virus. The evidence suggests that dogs do not develop enough virus in their bloodstream to infect more mosquitoes. Cats develop slightly higher levels of virus in their bloodstream, but it is unclear if this would be enough to infect mosquitoes. It is very unlikely that cats would be important in furthering the spread of the virus.If your animal becomes infected with WNV, this suggests that there are infected mosquitoes in your area. You should take measures to prevent mosquitoes from biting you (use repellent and wear protective clothing.)

Can I become infected with WNV if a dog with the virus bites me? Preliminary studies have not been able to detect virus in the saliva of infected dogs. This suggests that dog bites pose a low risk, if any, of transmission of WNV from dogs to other animals or people.

Is there a vaccine for cats or dogs? 

Should a dog or cat infected with West Nile virus be destroyed?

No. There is no reason to destroy an animal just because it has been infected with West Nile virus. Full recovery from the infection is likely. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.

Can I use insect repellent on my pets?

DEET-based repellents, which are recommended for humans, are not approved for veterinary use (largely because animals tend to ingest them when licking). Talk with your veterinarian for advice about the appropriate product for use on your pet.

What are the symptoms and treatment for WNV in humans?

  • Most people who are bitten by a mosquito with WNV will not get sick. People who do become ill may experience mild flu-like symptoms like fever, headache and body ache. It is estimated that less than 1 percent of the people who are infected with WNV become severely ill and require hospitalization. The elderly are particularly susceptible to illness caused by WNV.
  • Currently, there is no specific treatment for WNV infection. Since it is a virus, it does not respond to antibiotics. In severe cases, hospitalization and supportive care is important.
  • If you have symptoms of West Nile virus, call your doctor.

How can people protect themselves?

  • Avoid activity outside when mosquitoes are most active, especially at dawn and dusk. When outdoors, wear long pants, long sleeve shirts and other protective clothing.
  • Apply insect repellant according to label instructions.
  • Make sure that doors and windows have tight fitting screens. Repair or replace screens that have tears or holes.
  • Eliminate all sources of standing water that can support mosquito breeding.

What is the state doing to control the spread of WNV?

California has a long history of conducting surveillance for mosquito-borne viruses and has taken active steps to ensure early detection of WNV. Due to ongoing collaboration between over 70 local mosquito and vector control agencies and state public agencies, California is well prepared to detect and monitor WNV. These agencies use a variety of scientific techniques and products to control mosquitoes in their earliest stages and play a key role in reducing the risk of WNV. Also, California has launched a statewide public education effort about personal protection measures and reporting dead birds.

Why is the public urged to report dead birds?

The public is encouraged to assist in the efforts to detect and monitor WNV by calling the WNV hotline if they find a crow, raven, magpie, jay or hawk that has been dead for about a day or less. 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.

What should I do if I find a dead bird?

Birds that have been dead for less than 48 hours should be reported to the CDHS’ toll-free hotline: 1-877-WNV-BIRD. Birds can also be reported by visiting the WNV website at In southern California, all birds, other than pigeons and chickens, can be reported to the hotline. In other regions of the state, the public can report crows, ravens, magpies, jays, sparrows and finches.

Individuals should take note of the bird’s location and its condition before calling for further instructions, including assistance with identifying the type of bird found. The bird should show no signs of decomposition or maggot infestation. While there is no evidence that people can get the virus from handling live or dead infected birds, individuals should not attempt to catch or handle them.

Information provided by the California Department of Public Health and the Centers for Disease Control and Prevention.

© 2021 California Veterinary Medical Association

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