West Nile Virus

West nile virus (wnv) is a vectorborne infectious disease of the family Flaviviridae and is found in both tropical and temperate regions. WNV normally transmits among birds, mosquitoes, and mammals (especially humans and horses). The virus circulates in the blood of birds (reservoir hosts) for a few days after infection. Mosquitoes, particularly Culex pipiens, C. restuan, C. tarsalis and C. quinquefasciatus, become infected when they feed on infected birds. Infected mosquitoes (vectors) can then transmit WNV to humans and horses through their bites. They also infect other birds when they bite again. The virus is injected from the mosquito's salivary glands into the blood stream where it can multiply and cause illness. It was initially believed that direct human-to-human transmission was impossible and that humans are dead-end “hosts.” However, in 2002, the Centers for Disease Control and Prevention (CDC) discovered the transmission of WNV through blood transfusion and organ transplants as well through breast milk, prenatal infection, and occupational exposure.

The typical incubation period for West Nile virus is 2–6 days, although it can be as long as 15 days. Most people (80 percent) infected with West Nile virus will be asymptomatic or experience a flu-like illness. In others, the virus causes West Nile fever. Very few infected people will develop the more severe form of West Nile, i.e., West Nile encephalitis (inflammation of the brain), and meningitis (inflammation of the lining of the brain and spinal cord), both of which can be fatal.

The geographic distribution of this virus has expanded since its discovery in Uganda in 1937 and now includes Africa, Asia, Europe, North America, central and south America, and the Caribbean. In the United States, the virus first appeared in the Bronx borough of New York City in 1999. Since then, it has spread rapidly west and south from its initial focus. By 2003, 45 states and the District of Columbia had reported human cases of WNV. From 1999 through 2001 the CDC confirmed 149 cases of human WNV infection, including 18 deaths. In 2002 the count increased to 4,156 cases and 284 fatalities. WNV outbreaks reached a peak in 2003 when the CDC reported 9,862 case and 264 deaths. At least 30 percent of those cases were considered severe, involving meningitis or encephalitis. However, in 2004, there were only 2,539 reported cases and 100 deaths. In 2005 there was a slight increase in the number of cases, with 2,949 cases and 116 deaths reported. Canada, Israel, and Romania also experienced outbreaks of WNV in recent years.

The distribution of WNV is dependent on the occurrence of susceptible avian reservoir hosts, competent mosquito vectors, and abundance of preferred hosts (humans and horses) for the infected mosquitoes. These factors are in turn affected by potential environmental and social factors. Several studies have highlighted land use and land cover change, elevation, abundance of vegetation, physiographic regions, stagnant water bodies, temperature, precipitation, spatial and temporal differences in periods of drought and rain, close proximity to dead birds and mosquito vectors, and farmland created by irrigation as potential environmental factors. Social factors influencing the distribution and transmission of WNV are human population density, age, income, race, age of housing, mosquito control activities, location of scrap tire stockpiles, and location of organic wastes from chemical industries.

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