West Nile Virus
West Nile Virus (WNV) is a mosquito-borne flavivirus that causes mild to
severe illness. It was first introduced to the United States in 1999 in New
York and reached Florida in 2001. Since its initial detection, human cases of
WNV have been reported in all U.S. states with the exception of Alaska, Hawaii
and Maine. The virus is now considered endemic in the U.S., with annual
epidemics in some parts of the country, peaking in the late summer months.
In Florida, WNV activity has been identified in all 67 counties and peaked in
2003. In recent years, there has been a decreased level of activity,
possibly due to drought-like conditions throughout much of the state.
The natural cycle of WNV involves several species of Culex mosquitoes and
wild birds. It can cause high rates of mortality in certain families of birds,
especially crows and jays. Occasionally, an infected mosquito will bite a human
or animal (particularly horses) and cause disease. The virus can also be
transmitted to humans via contaminated blood transfusions and a few cases have
also been reported involving intrauterine transmission. Since 2003, all
blood donations are screened for the presence of WNV prior to transfusion.
Most West Nile Virus infections (approximately 80%) are asymptomatic.
In those people that do develop symptoms, most experience a mild illness termed
West Nile Fever (WNF) that is characterized by headache, fever, pain, and
fatigue. Less than 1% of infected people develop the most severe form of
disease, neuroinvasive WNV, which may involve meningitis and encephalitis
and can cause irreversible neurological damage, paralysis, coma or death.
Symptoms typically appear between 2 and 14 days after the bite of an infected
mosquito. People over the age of 50 and individuals with weakened immune
systems (especially transplant recipients and HIV infected individuals) seem to
be at increased risk for severe disease.
There is no specific treatment for WNV, and most mild infections are typically
overcome with little or no medical intervention within a matter of weeks.
Treatment of symptoms is common though, and recent research has shown efficacy
of retroviral drugs in treating severe neuroinvasive WNV patients. An effective
vaccine has been developed for horses, however research for a human vaccine
For mosquito-borne disease prevention tips:
For more information about WNV, see the CDC's West Nile Virus page:
Hayes E, Sejvar J, Zaki S, Lanciotti R, Bode A, Campbell G. Virology,
Pathology, and Clinical Manifestations of West Nile Virus Disease.
Emerging Infectious Diseases. 2005. 11(8) 1174-1179