West Nile Virus

West Nile virus (WNV) is found most often in birds and is spread to other birds, animals, and occasionally humans by mosquitoes (mosquito-borne) that have fed on infected birds. WNV is similar to other mosquito-borne viruses commonly found in Arizona such as the St. Louis virus and the Western equine virus. WNV is feared most for its threat of the rare, but serious and sometimes fatal illness West Nile encephalitis (swelling of brain tissue). Even in areas where West Nile virus is circulating, few mosquitoes are actually infected with the virus. However, once mosquitoes are infected, they may transmit the disease by biting birds, animals and humans.


transmission cycle of West Nile Virus

Most people infected by West Nile virus have no obvious symptoms. For others, mild to moderate symptoms may appear 3-15 days following infection. Mild symptoms may include:

  • fever
  • head and body aches
  • skin rash
  • swollen lymph glands

Less than 1 percent of the population bit by an infected mosquito is likely to become severely ill. Persons age 50 years and older have the highest risk of severe disease caused by the virus. More severe symptoms may include:

  • headache
  • high fever
  • neck stiffness
  • disorientation
  • coma
  • tremors
  • convulsions
  • muscle weakness
  • paralysis

In very rare cases, severe illness may cause death.


Supportive treatments for serious cases include hospitalization, use of intravenous fluids, and respiratory support to help prevent secondary infection. Persons who suspect that they have symptoms of West Nile virus should contact their physician as soon as possible. Vaccine for human use does not currently exist.


Arizona has established a surveillance network in order to detect and monitor the appearance of WNV statewide in humans, mosquitoes, birds, horses and other animals.