Zoonotic Diseases: Get the Facts

West Nile Virus (WNV) is a mosquito-borne disease caused by a virus in the Flaviviridae family.

Transmission and Diagnosis

  • The first documented case of WNV in the U.S. was in 1999.
  • Transmission to humans typically occurs after a bite from an infected mosquito. Though much less common, WNV may also be transmitted to humans through contact with other infected animals, their blood, or other tissues, and can pass between humans via blood transfusions or breast milk.
  • WNV requires laboratory testing to confirm a diagnosis. There are multiple tests available to clinicians; most require blood or cerebrospinal fluid.
  • WNV antibodies can usually be detected for three to eight days after the onset of illness, but can persist for 30 to 90 days or longer.
  • There is no vaccine or treatment for WNV in humans.

Impacts to Animals

  • WNV primarily affects birds, which serve as reservoirs for the disease and produce high enough levels of the virus to infect mosquitos.
  • WNV can also infect horses, alligators, racoons and fox squirrels, among others, but these animals are unlikely to serve as a source of infection for other hosts.
  • Neurological signs of WNV in birds include loss of coordination, head tilt, tremors, weakness, apparent blindness and lethargy. Some birds die of infection, while others show no clinical signs of WNV. Juveniles are more susceptible to life-threatening infections than adults, and WNV has caused population declines in some species.
  • Most mammals do not show clinical signs of WNV, but horses can develop flu-like symptoms, such as fever or vomiting, or neurological signs as described for birds. There is a WNV vaccine for horses.

Impacts to People

  • Most people infected with WNV do not have symptoms and are unaware they are infected.
  • Around 20% of people with WNV develop flu-like symptoms, such as fever, muscle pain, headache, fatigue, nausea and vomiting, that disappear within days without treatment. However, some people experience symptoms for weeks or months following infection.
  • Less than 1% of people infected with WNV develop West Nile
  • Neuroinvasive Disease, which includes brain swelling, meningitis and altered levels of consciousness.
  • This condition is life threatening and requires urgent medical attention.


  • Wear bug spray when outdoors, especially during warmer months and following rain. Reapply as directed.
  • Wear pants and long sleeves in locations with a lot of mosquitos. Cover strollers and baby carriers with mosquito netting.
  • Remove standing water to eliminate mosquito breeding sites from around your home.
  • Install and maintain window or door screens.
  • Use air conditioning during summer months, if possible.

More Information Decorative illustration of a mosquito.

  • Centers for Disease Control and Prevention
  • Louisiana Department of Health
  • Louisiana Department of Wildlife and Fisheries
6/2/2023 4:25:59 PM
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