West Nile virus is a mosquito -borne infection that can cause encephalitis (inflammation of brain tissue) and/or meningitis (swelling of the tissue that encloses the brain and spinal cord). Native to parts of the Old World, WNV was first detected in North America in the summer of 1999, in New York City, NY; a dead crow at the Bronx Zoo was one of the first harbingers of what was to come. Within three months, WNV had spread to Connecticut and New Jersey, leaving tens of thousands of dead birds in its wake. Over the subsequent five and a half years, it has continued to spread across the continent, and by the end of 2004 had been detected in 48 U.S. states, the District of Columbia, and seven Canadian provinces.
By far, the absolute best thing you can do to reduce the presence of West Nile virus in your neighborhood is to limit mosquito breeding— get rid of places for females to lay eggs. This is one of the few things that you can actually do that will really make a difference.
Different species of mosquitoes have different feeding preferences— some prefer to bite birds, some prefer to bite mammals, and others prefer animals of other kinds. West Nile virus is primarily a bird pathogen (primarily crows), and is transmitted among birds by ornithophilic (bird-biting) mosquitoes. Until now, it was presumed that humans are exposed to the virus upon being bitten by “bridge-vector” species of mosquitoes— those that usually bite mammals but occasionally bite birds.
In a recent analysis of 10 northeastern species of mosquitoes, researchers found that up to 80% of the risk of human WNV infection comes from two species of ornithophilic mosquitoes, both of which breed primarily in backyard and neighborhood sources of stagnant water, such as discarded tires, unused swimming pools, catch basins, gutters, and birdbaths. The surprisingly high risk comes from the two species’ abundance, infection prevalence, and ability to replicate and transmit the virus, which compensate for the relatively small fraction of blood meals taken from mammals by these bird-biting mosquitoes.
These types of mosquitoes don’t travel far, so, unless you live near a freshwater wetland or salt marsh, most of the mosquitoes on your property probably hatched there, or somewhere nearby. Females lay eggs in wet areas that contain decaying organic material (e.g., artificial containers and puddles containing even small amounts of leaves, grass clippings, animal wastes, etc.). There doesn’t have to be much water and the water doesn’t have to be around for very long— in warm weather, mosquitoes can complete their development in any puddle or other source of standing water that lasts more than 5 — 6 days. Eliminating mosquito breeding habitat will limit the number of mosquitoes produced locally.
In most cases, humans infected by West Nile virus experience only mild, flu-like symptoms, but the virus also can trigger dangerous, sometimes lethal cases of meningitis and encephalitis.