West Nile virus (WNV) is an infection transmitted to humans and animals by mosquitoes which have fed on infected birds. According to the Centers for Disease Control and Prevention (CDC), between 1999 and 2008, 28,961 confirmed and probable cases of WNV were reported. Though 80 percent of individuals infected with WNV show no symptoms, approximately one in 150 develop severe illness with symptoms including high fever, vision loss, and paralysis. Though there is no vaccine or cure for WNV, it is important for individuals to have an understanding of how best to protect themselves from infection.
First identified in Uganda in 1937, WNV occurs when Culex pipiens mosquitoes feed on infected birds, which serve as vectors for the virus, and then pass the virus on to humans. WNV cannot be passed from one individual to another through close personal contact, and the primary method of transmission is through the bite of an infected mosquito. Mild infections (West Nile fever) are seen in approximately 20 percent of individuals, and are marked by symptoms including fever, headache, and body aches. Individuals with severe infections (severe West Nile disease) may experience inflammation of the brain (encephalitis) or of the brain and surrounding membranes (meningoencephalitis), and three to 15 percent of WNV infections result in death. According to CDC statistics, in 2009 there were a total of 663 cases of WNV in the United States, and 30 of these resulted in death.
The CDC indicates that WNV is an established seasonal epidemic in North America, beginning in the summer and continuing into the fall. In addition to documented cases in the United States, both sporadic cases and major outbreaks of encephalitis related to WNV have been reported in Africa, West Asia, Australia, Europe, and the Middle East. Recent reports have documented outbreaks in Greece and British Columbia, Canada; as well as in New York, Pennsylvania, and northwestern Indiana.
Like malaria, dengue, and other mosquito-borne illness, rates of WNV infection increase during periods of warm weather when mosquito populations are most active. Torrential rains in early summer and persistent high humidity in many regions of the United States have created the ideal breeding climate for mosquitoes. Experts recommend taking preventative measures to avoid infection including the “5 D’s”:
- Dusk and Dawn – avoid being outdoors during these periods which are when mosquitoes are most active
- Dress – wear clothing that limits exposed skin
- DEET – use repellents containing DEET (picaridin and oil of lemon eucalyptus are other repellant options)
- Drainage – eliminate areas of standing water in which mosquitoes lay eggs.
In addition, dead birds, particularly crows, which display signs of illness should be reported to authorities for proper disposal. As health officials continue to track outbreaks of WNV, individuals can take appropriate measures to limit their risk of infection and keep their community healthy.