West Nile Virus

WNV Header

West Nile Virus (WNV) is a mosquito-borne virus most commonly active from June through September. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and horses. In a very small number of cases, WNV has been spread through blood transfusions, organ transplants, and from mother to baby during pregnancy or delivery, and possibly through breastfeeding.

The main species of mosquitoes that spread WNV are in the genus Culex. Culex mosquitoes are widespread in California and will feed on birds, humans and other animals. Culex mosquitoes tend to bite in the morning and evening and are not known to spread Zika, dengue, or chikungunya viruses.

Butte County WNV Dashboard

View information about human infections of WNV activity in Butte County.

Mosquito & Vector Control in Butte County

BCMVCD LogoThe Butte County Mosquito and Vector Control District (District) controls mosquitoes, yellow jackets, and other vectors in Butte County and also in Hamilton City of Glenn County. One of the main goals of the District is to suppress mosquito-borne illnesses such as West Nile virus, Malaria, Zika, Dengue, Chikungunya, Yellow fever, and others. Another goal of the District is to reduce the annoyance level of mosquitoes using environmentally compatible control practices and public education.

Some of the services that the District offers are free mosquitofish, mosquito and yellowjacket annoyance service calls, tick identification, and public education. For more information, to report a dead bird, or to place a request for service please visit the BCMVCD website or call 533-6038 or 342-7350.

Prevent Mosquito Bites

Prevent WNV

The most effective way to avoid WNV is to prevent mosquito bites:

  • Use insect repellents when you go outdoors. Repellents containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide longer-lasting protection. Learn more about mosquito repellent.
  • Wear long sleeves and pants from dusk through dawn when many mosquitoes are most active.
  • Install or repair screens on windows and doors. If you have it, use your air conditioning.
  • Help reduce the number of mosquitoes around your home. Empty standing water from containers such as flowerpots, gutters, buckets, pool covers, pet water dishes, discarded tires, and birdbaths.

WNV Symptoms

People typically develop symptoms 3 to 14 days after they are bitten by an infected mosquito. The incubation period could be longer in people with certain medical conditions that affect the immune system.

There is no available WNV vaccine to prevent people from becoming ill from WNV.

No Symptoms

Approximately 80% of people (about 4 out of 5) who are infected with WNV do not develop any symptoms.

Mild Symptoms

Up to 20% of the people (about 1 in 5) who become infected will display symptoms. Acute symptoms generally last for just a few days, although fatigue and weakness can last for weeks or months. Symptoms may include:

  • Fever
  • Headache
  • Body aches
  • Nausea or vomiting
  • Swollen lymph glands or a skin rash on the chest, stomach and back

Serious Symptoms

Less than 1%% of individuals (about 1 in 150 people) infected with WNV will develop severe neurological illnesses, such as encephalitis or meningitis (inflammation of the brain or surrounding tissue). Serious symptoms may last several weeks, and neurological effects may be permanent. WNV infection can be fatal. Severe symptoms can include:

  • High fever
  • Headache
  • Neck stiffness
  • Stupor, disorientation
  • Coma
  • Tremors or convulsions
  • Muscle weakness
  • Vision loss
  • Numbness
  • Paralysis

WNV & Animals

Birds, Squirrels, Cats & Dogs

An infected mosquito can bite any animal, but not all animals will become sick. The disease most often affects birds but may occasionally cause disease in other animals. Wild birds serve as the main host of WNV for mosquitoes. When some species of birds become infected, they produce high quantities of the virus, which can then be passed on to other mosquitoes that bite them. The virus is maintained in this bird-mosquito cycle.

Tree squirrels with WNV can develop neurological symptoms such as uncoordinated movement, paralysis, shaking, or circling and may die.

Dogs and cats can be exposed to WNV in the same way as humans. However, these animals are very resistant to WNV and rarely become ill. Concerned pet owners should consult with a veterinarian.


Horses can become ill from WNV through the bite of an infected mosquito. Clinical signs of WNV illness in horses may include stumbling, circling, hind leg weakness, inability to stand, drooping lips and lip-smacking, hypersensitivity to touch or sound, muscle tremors, and death. The American Association of Equine Practitioners (AAEP) recommends the incorporation of a WNV vaccine as an annual core vaccine in equine vaccination protocols. Horse owners should consult with a veterinarian about the WNV vaccine and other vaccines against mosquito-borne viruses, such as western equine encephalitis.

For more information about WNV and horses, visit the California Department of Food and Agriculture website.