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West Nile Virus

West Nile virus is a “bird virus” that is primarily transmitted between birds by mosquitoes. It is also transmitted to humans through the bite of a mosquito infected with West Nile virus. People who get West Nile virus cannot transmit it to other people.


The virus is a mosquito-borne disease originally found in Africa. In 1999, it was detected in the eastern United States; since then, the virus has spread throughout the United States and is well established in most states. In 2003, the virus was first confirmed in California.

Today, West Nile virus is considered endemic, which means the virus will remain in the environment. 

Track West Nile virus Activity for 2022

What are the symptoms of West Nile Virus?

  • One in five persons infected with West Nile virus will exhibit symptoms. Symptoms usually occur between five and 15 days and can include fever, headache, body aches, nausea, or a skin rash. These symptoms can last for several weeks to months. One in 150 people infected with the virus will become severely ill. Severe symptoms include high fever, muscle weakness, neck stiffness, coma, paralysis, and possibly death.

How do people and animals get West Nile virus?

West Nile virus is transmitted to people and animals (birds and horses) by infected mosquitoes. There are more than a dozen species of mosquitoes in Los Angeles, but only a few species of mosquitoes carry the virus, and very few mosquitoes are actually infected.

In rare instances, West Nile virus can be transmitted through blood transfusion, organ transplants, and from mother to fetus and breastfeeding infants. Pregnant and nursing mothers are encouraged to avoid mosquitoes. Consult a physician for additional information.

Is there a vaccine for humans?

No, currently, there is no WNV vaccination for humans.

What Can I Do?

Since mosquitoes breed in standing water, you can help stop the mosquito life cycle and West Nile virus transmission by eliminating standing water on your property. Eliminate standing water by dumping or draining water in neglected swimming pools, ponds, rain barrels, discarded tires, buckets, or any container capable of holding water for more than a few days.

Following these simple actions can significantly contribute to the fight against West Nile virus.Remember, if they can’t breed, they can’t bite.

  • Avoid activity at dawn and dusk when mosquitoes are most active
  • Apply mosquito repellent containing the active ingredient DEET when outside
  • Wear protective clothing (loose, light-colored, long sleeve shirts and pants)

Not All Repellents are the Same. Find the best one for you!

Which animals get West Nile virus?

An infected mosquito can bite any animal, but not all animals will become infected. The disease most often affects birds (crows, ravens, magpies, and jays) but occasionally causes disease in other animals as well.

If you find a dead bird, particularly a crow or other corvid (i.e. raven, jay, or magpie), please call the California Department of Public Health hotline at 1-877-WNV-BIRD or submit an online report. Not all birds will qualify for testing, but they will record all reports of dead birds.

To move or discard a carcass, wear rubber or latex gloves. If gloves are not available, use a plastic bag turned inside out over your hand and invert the bag to surround the bird. Seal and discard the bag in the trash if the bird will not be picked up for testing.

Horses are also susceptible to West Nile virus. The disease does not seem to be specific to a particular breed or age of the horse. Many horses infected with West Nile virus will not develop any illness. Of those that do become ill, approximately 30% die or must be destroyed. If your horse becomes infected, it is not contagious to you or to other horses. Clinical signs can include a lack of coordination, stumbling, circling, falling, weakness or partial paralysis of limbs, muscle twitching (especially around the nose and lips), an inability to rise, and grinding teeth. There are two different vaccinations available for horses. The timing of these vaccines is critical because they must be administered prior to disease exposure. Contact a veterinarian for advice.

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.

I'd like more information. Where else can I look?

The California West Nile Virus website is a great resource for tracking WNV activity in the state (Click HERE).