West Nile Virus may still be concern
By Robert A. Hedeen, Naturalist
Now that the mosquito season has ended for the year in the Rock River Valley and the rest of the northern United States, there is still the slight possibility a mosquito harboring the West Nile virus may bite you.
Although at least a dozen species of mosquitoes have been incriminated in the transmission of this pathogen to humans, by far the most important carrier in our region is the northern house mosquito, Culex pipiens. Pipiens is the pestiferous gnat that frequently makes sleep impossible during the warm months of the year when it invades our bedrooms seeking a meal of blood.
This biting fly is also referred to as the domestic mosquito (as well as a number of unprintable names) as it breeds in almost any artificial container of watertin cans, clogged roof gutters, bird baths, overflow from septic systems, wading pools, and a host of other water receptacles apt to occur around our homes.
C. pipiens is especially important in the chain of transmission of the West Nile virus as it readily feeds on both birds and mammals, and birds serve as the primary reservoir of the virus in nature.
Most mosquitoes in the northern part of the country pass the winter in the egg stage, but this species survives the cold months as hibernating, adult females. As winter approaches, inherited survival instincts in the females of this species come into play. A females first instinct is to find a male with which to mate and to store the acquired spermatozoa in two pouches connected to her reproductive tract. The sperm cells will remain viable but in a dormant state, throughout the winter. After inseminating the female, the male dies.
The females next instinct is to have a final blood meal of the year that will provide a gradual source of nourishment during the coming months. When these necessary instincts have been responded to, she will then seek out a protected place to pass the winter.
When warm weather finally arrives, the female awakens from her prolonged period of inactivity and immediately looks for another blood transfusion. This protein-rich meal is necessary for the final maturation of the hundreds of eggs developing in her ovaries. When the eggs are ripe, they are passed down the reproductive tract. And, as they pass the openings of the ducts from the sperm sacs, male cells are released and fertilize each egg as it passes to the outside.
As the female lays her eggs, she cements them together to form a raft-like structure that floats high on the surface of the water. The rafts are light in color when laid but quickly darken, giving them the appearance of a fleck of soot. The fertilized eggs will hatch in a few days and the resulting larvae constitute the next generation. Now that the female has fulfilled her primary biological obligation, the perpetuation of the species, she dies.
Unfortunately for us, in the fall, when the female seeks a place to hibernate, she sometimes chooses a place inside our dwellings. Basements, sumps, crawl spaces, garages and a variety of other nooks and crannies may be selected as hibernation sites. If, during the course of the winter, we get the urge to tidy up the basement or put something in the crawl space, the female may be stimulated to awaken, and, thinking it is spring, immediately seek a meal of blood. How many times have you been bitten in your home by a mosquito during January or February or been amazed to find mosquito larvae in water in the basement sump?
Scientists know at least one thing about the epidemiology of the West Nile virus, and that is there is a lot they dont know about it. For example, it has recently been demonstrated that the micro-organism may be transferred from human to human via a blood transfusion. Whether or not the virus maintains its viability for several months in a hibernating mosquito has yet to be definitely determined, but preliminary studies indicate that it does. If it is definitely proven the virus can overwinter inside the mosquito, we could be in danger of acquiring West Nile during the cold months of the year.
Though the danger of contracting West Nile Fever during the winter is remote, it behooves us to take simple precautions. All secluded places in the home should be treated with an aerosol-type formulation of an insecticide labeled for use against flying insects. As these types of insecticides rapidly deteriorate after they leave the dispenser, the residual effect that is characteristic of some other household products is minimal. Of course, one should avoid inhaling the air-borne particles of an aerosol spray in an enclosed area.
Though it may seem a bit ludicrous to worry about mosquito bites and West Nile Disease when the temperature hovers around zero and the ground is covered with snow, it is better to be safe than sorry.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.