West Nile Virus (WNV) and Related Mosquito Control and Prevention
West Nile fever is a mosquito-borne illness with fever, headache, muscular pain, and rash (mainly in young children). The virus, WNV, was named after the district of Uganda where it was first isolated in 1937, and it is closely related to St. Louis Encephalitis virus (SLE). It is usually mild, but serious complications of the liver or nervous system (e.g., encephalitis or meningitis) can occur especially in very young or older persons.
Throughout most of its distribution, WNV causes only a very small percentage (1- 3 %) of infected people to require medical attention. The WNV strain currently in the U.S. has so far caused death only in older persons (50 to 70+ years old) or those with immune system problems. Only about 20% of infected persons in the U.S. develop symptoms, and only one-in-150 will develop serious medical conditions that need hospital care.
Female mosquitoes lay their eggs in or on water or near the edges of natural or artificial sites which usually hold water for 7 to 10 days. The eggs need liquid water to hatch, and the larvae (“wigglers”) and pupae (“tumblers”) cannot survive very long out of water (usually a few hours at most). They develop through 4 larval stages and a pupal stage before adults emerge. This requires at least 7 to 10 days for most species under even the best conditions. Adults may live from a few days to several months. Some species live through the winter as adults (in basements, under large culverts, in large storm drains, or in caves).
Only adult female mosquitoes feed on blood, which they use to produce their eggs. Both sexes feed on nectar or plant juices, and are attracted to light. Each species has its own main daily activity period. Culex and Anopheles spp. are mainly active at dusk or at night; Aedes and Ochlerotatus spp. are generally active during the day. When they are in a particular physiologic state, female mosquitoes are stimulated by CO2 (and other chemical or physical cues) to seek out a blood meal host. After they take their fill of blood, females of most species typically fly a short distance away (usually only a few feet) and rest (usually on a vertical surface), for an hour or longer, while they filter off and excrete a large part of the liquid (mainly water) from that blood meal. They may remain there or later fly farther away to wait as they finish digesting that blood and developing their next batch of eggs. Individual hosts vary in their attractiveness for mosquitoes.