West Nile Virus

In experiments, the Asian Tiger Mosquito was found to transmit West Nile Virus

Classified by the Center for Disease Control as part of the Japanese encephalitis strain of flavivirus (yellow fever) and also similar to dengue fever, West Nile Virus first came to widespread public attention in the United States following an outbreak in Queens, New York in 1999 and gained further notoriety during the epidemic of 2002. The virus is transmitted to humans principally through mosquito bites, though it can affect other mammals including horses, dogs, cats, and rabbits (among others) as well as a variety of birds including crows and robins.

Infection, Incubation, and Symptoms

Infection is humans has proven to be largely asymptomatic; the Louisiana Department of Health and Hospitals reports that 90% of people infected with West Nile Virus never present symptoms. Others contract West Nile Fever, a condition which includes feverishness, fatigue, sore muscles and rash.

Some extreme cases develop into one of several neuroinvasive diseases called West Nile Meningitis, West Nile Encephalitis, and West Nile Meningoencephalitis. Encephalitis is an inflammation of the brain; meningitis is an inflammation of the membrane around the brain of of the spinal cord. Meningoencephalitis is an inflammation of both.


The virus is transmitted from mosquitoes to birds, who act as incubators and who pass the strengthened virus to mosquitoes via further bites. According to the CDC, the disease is not transmitted from human-to-human contact, although one documented case involved a mother passing it to her infant child through breast milk. People older than 50 and patients with compromised immune systems are at higher risk of becoming seriously infected.

In 2009, U.S. health officials recorded 663 confirmed cases of West Nile Virus infection among humans. Of these, less than one percent of those infected contracted one of the neuroinvasive diseases.


Mosquito Magnets

An electronic microscope image of the West Nile Virus

The CDC┬árecommends applying EPA-approved insect repellent to both skin and clothing, or wearing long-sleeved shirts and full-length pants (not shorts) when weather permits. Insect netting should be placed over children’s cribs and strollers.

Homeowners can reduce the presence of mosquitoes in their home and on their property by installing window and door screens to keep mosquitoes from getting into the house. As mosquitoes tend to congregate around standing water, draining all flower pots and pans, storm drains and gutters will also discourage mosquitoes from lingering around the house and yard. Finally, a variety of mosquito traps are available which act to replicate human breathing and temperatures, working to divert mosquitoes from attacking human targets.

Where to Get More Information

Additional information is available at the CDC’s West Nile Virus information page