West Nile Virus

General Information

West Nile virus is a mosquito-borne virus that was first detected in the United States in 1999 and in Arizona in 2003. It is not known how the West Nile virus was introduced into the United States, but it may have entered in an infected traveler, bird, or mosquito. West Nile virus has been present in Africa, West and Central Asia, and the Middle East for a long time.

The virus can cause illness in people, horses, certain types of birds, and other animals.

Mosquitoes become infected with West Nile virus when they feed on birds that are infected with West Nile virus. After a few days, infected mosquitoes can then transmit West Nile virus to other birds, humans and horses. Only certain species of mosquitoes carry the virus. Culex mosquitoes are the main carriers of West Nile virus, and these mosquitoes are common throughout Arizona. Culex mosquitoes tend to bite from dusk to dawn.

People become infected with West Nile virus from the bite of an infected mosquito. Birds and other animals cannot transmit West Nile virus to people. In 2002, a small number of people in the United States became infected with West Nile virus when receiving blood products or organs from West Nile virus infected donors. In 2003, a blood testing program was initiated to screen blood products for West Nile virus prior to transfusion.

WNV infection has been reported in more than 280 bird species. Certain types of birds such as crows, ravens and jays appear to be most susceptible to WNV infection. Horses are also susceptible to WNV infection but a vaccine is available through veterinarians. WNV infections affecting the nervous system have also been reported in squirrels, goats, sheep and llamas, but the susceptibility of these animals to WNV is not well known. WNV illness in dogs and cats is rare. Contact your veterinarian for information about pets and livestock.

County and state health workers monitor and test mosquitoes at selected sites, dead birds and sentinel chicken flocks (17) for mosquito-borne viruses. Mosquito surveillance involves trapping mosquitoes, counting them, identifying the species involved, and testing the appropriate species for viruses. These surveillance methods are used to better identify areas where mosquito control efforts are needed. Detection and control of mosquito breeding sites depends upon integrated efforts among state, county, and tribal agencies as well as private citizens.

For more information, contact your local health department or Arizona's 24-hour bilingual Public Health Hotline at:

  • (602) 364-4500 in Metro Phoenix
  • (800) 314-9243 in other areas of the state

You can also contact the Arizona Department of Health Services Vector Borne & Zoonotic Diseases program.