Mosquitoes In Winter?
This past fall’s panic due to outbreaks of St. Louis Encephalitis (SLE) and West Nile Virus (WNV) in New York has led to questions from homeowners. In order to give you the facts, we decided to reprint portions of the National Pest Control Association’s Library Update. Don’t worry…mosquitoes are not active in the winter, but at least you will be armed with the facts.
This summer, at least two cases of malaria were caught by children on Long Island who had not been outside their own country in more than a year. The CDC estimates about 1,000 cases of malaria are reported each year in the U.S., more than 20 of these are caught locally. The Northern House Mosquito, Culex pipiens pipiens L., is the main U.S. vector of SLE to humans. This is the species that probably infected the 48 cases reported in the New York City area that have led to four deaths so far.
St. Louis Encephalitis (SLE).
SLE, first isolated in St. Louis in 1933, is transmitted by the bites of infective mosquitoes, which pick up the virus from infected birds, mainly songbirds. It is not transmitted person to person, nor is it transmitted to, or by, pets. Infections vary widely in severity, and the disease in not as serious as Eastern Equine Encephalitis (EEE). Most people who are infected have no symptoms or have only a fever and headache before fully recovering. In some elderly persons, infants, or people with compromised immune systems, SLE can cause permanent nerve damage or be fatal. Symptoms occur five to 15 days after an infective bite and range from a slight fever or headache to rapid onset of severe headache, high fever, muscle aches, stiff neck, disorientation, coma, tremors, muscle weakness, paralysis, and death. There is no vaccine or specific drug therapy for either SLE or WNV, but severely affected patients may need life support.
Northern and southern house mosquitoes are closely related and hard to separate. They are medium-sized, brown mosquitoes with bands of white scales across their abdominal segments, but have no other prominent markings. Cx. P. pipiens, the northern form, occurs throughout the northern U.S. to as far south as GA and OK. Culex pipiens quinquefasciatus Say, the southern form, occurs in all southern states from coast to coast; northward to NE and OH. One or both of these mosquitoes are found in every state. In many published articles from outside the U.S., this mosquito is referred to as Culex fatigans Weidemann. House mosquitoes usually fly only short distances (less than ½ km). When adults are present, larvae will usually be found nearby. These species are active mainly at dusk or night time, and may be found resting during the day in and around shelters near their breeding places. They are attracted to light traps, but the numbers collected may not reflect their actual abundance, unless the trap is augmented with CO2 in some way.
Culex p. pipiens usually prefers to feed on birds but readily feeds on humans and large animals like cattle or goats. This species is an annoying biter and produces a high-pitched buzzing sound that can easily be heard. Members of the Cx. pipiens complex feed and rest indoors or outdoors. Larvae of the Cx. pipiens complex generally prefer ground pools with high concentrations of organic matter or swamps with emergent vegetation. Polluted water from septic systems is an ideal breeding site for them. Larval development requires 7 to 9 days at 25 to 30 degrees C. Adults show two small population peaks in temperate countries, one from May to June, and another from September to October. Reduction of adult populations by ULV spraying may be useful as a means of disease control. The most effective long-term control involves reducing breeding by eliminating or treating larval habitats.
West Nile Virus (WNV).
West Nile fever is a mosquito-borne illness with fever, headache, muscular pain, and rash. It is usually mild, but serious complications involving the liver and nervous system can occur in very old or very young persons. The etiological agent, WNV, was named after the district of Uganda where the virus was first isolated in 1937. It is closely related to the virus that causes SLE. This virus has been isolated from many areas of Africa, Europe, India and Pakistan, but has never before been reported from the Western Hemisphere.
Birds, such as crows and various songbirds, are the primary reservoirs for WNV and may reintroduce the virus during seasonal migrations. Infection in most mammals fails to produce enough virus for vectors to pick it up again from them. It has been isolated from several species of mosquitoes in nature, especially Culex spp. which are its major vectors. Infected mosquitoes can transmit WNV for life.