The Scoop on West Nile Virus

From PEEC Newsletter, Summer 2003
Michele Altherr

West Nile virus is a mosquito-borne disease. Currently 14 of the 176 species of mosquitos in North America are known to be infected by the virus, with the Culex species predominating. Mosquitos acquire the disease by feeding on infected birds and then transmit the disease to its subsequent hosts, be they other birds, animals, or humans. Over 130 bird species have been documented as having been infected with the virus, among them Corvids, Orioles, Robins, Warbling Vireos, Turkey Vultures, and Bald Eagles. It is the mortality of Corvids (crows and jays) that has become the sentinel hallmark of WN virus in the US. Almost all dead birds are found singly and not as part of a mass die-off. Interestingly, not all infected birds die.

Humans can also become infected with the virus as a result of the bite of an infected mosquito. The majority of people who are infected with WN virus are only mildly affected and show either no signs of illness or mild flu-like symptoms. In rare cases the virus multiplies into the central nervous system and results in an inflammation of the spinal cord (meningitis) or the brain (encephalitis). These cases sometimes result in death, with the majority being people over 50 years old. While in 2002 there were 4,156 reported human cases of WN virus encephalitis that resulted in 284 deaths, there have been zero human cases reported so far for 2003 (as of 7/2/03). Up-to-date maps of WN virus monitoring categories can be found at http://westnilemaps.usgs.gov.

The Culex mosquito species feeds primarily at dusk and dawn. During feeding time the mosquito is attracted to humans by skin odors and exhaled carbon dioxide from breathing. Therefore, at these times of day it is recommended to take preventive measures such as wearing long sleeves and pants. Also, spraying the chemical repellent N,N-diethyl-m-toulamide, DEET, on top of clothing (not under) is recommended. The chemical does not work by killing the mosquito, but rather by making it difficult for the mosquito to locate a person by odor detection. The percent of DEET in a repellent gives varying lengths of protection with 4.75% DEET providing 1 1/2 hours of protection and 20% DEET providing four hours of protection. Similarly, many mosquito traps lure the insect with carbon dioxide. Besides attracting mosquitos, these traps also kill measurable numbers. Mosquito trap information is at: www.mosquito.org/MosqInfo/Traps.html.

Source reduction of breeding areas is the most effective means of eliminating mosquitos and reducing a person’s risk of contracting WN virus. Pockets of standing water found in something as small as a bottle cap or as large as an unfinished building foundation can serve as mosquito breeding areas. Common sources of standing water and thus potential mosquito breeding habitats around home include Jacuzzi covers, clogged rain gutters, pet water dishes, flower pots, and birdbaths. These should be drained and cleaned several times a week. Another preventive measure is to use well-repaired screens on all open windows and doors.

Water management methods play an important role in mosquito control. "Impoundment management" is a technique developed to minimally flood mosquito marshes periodically during the summer. "Open marsh water management" is a method of connecting mosquito breeding areas to deep water through shallow (less than three feet deep) ditches. Mosquito broods are further controlled by allowing larvivorous fish access to mosquito-producing depressions. Draining occurs before the adult mosquito can emerge.

Several biological controls have been tested but are not widely used. The most commonly known control is Gambusia, a minnow-size predacious fish that feeds on mosquito larva. Other biocontrol agents are the predacious mosquito Toxorhynchite, the parasitic nematode Romanomermis, the fungus Lagenidium giganteum, and predacious copepods.

Chemical treatments of larvicides and adulticides by air or ground are less permanent than source reduction methods and are usually used only as a last resort. Larvicides act to control mosquito populations by killing the immature stages at their breeding site and before they disperse as adults. Adulticides are the least effective control technique as the chemical must drift through the habitat while the mosquito is flying through. As with all pesticide use, rapidly evolving resistance causes chemicals to lose their effectiveness.

With a grant from the CDC, New Mexico has set up surveillance sites around the state. The department of public health is testing selected birds only from counties where the virus has not been found. Residents who observe a freshly dead crow, raven, or jay can contact the Office of Epidemiology at 1-800-879-3421 or the Los Alamos Public Health Office. The Center for Disease Control and Prevention has set up a WN virus hotline for anyone with questions, 1-888-246-2675.