Fighting the West Nile Virus: Prevention Works Best

The West Nile virus (WNV) is a mosquito-transmitted disease recently introduced into North America. It is very similar (and closely related) to the native St. Louis encephalitis, also a mosquito-transmitted virus. Both WNV and St. Louis encephalitis are primarily diseases of birds. People can become infected when there is an outbreak of the disease in bird populations and a mosquito that fed on an infected bird subsequently bites a person.

Unlike St. Louis encephalitis, WNV has had a devastating impact on some native bird populations. Crows, blue jays, and some species of hawks are especially susceptible to infection by WNV, and are usually killed by the virus. Other common species of birds such as house sparrows appear to be carriers of the disease, but rarely become victims.

Asian tiger mosquito USFWS/Mike J. Higgins

Contrary to media descriptions of "the deadly West Nile virus," WNV is rarely fatal in humans. Less than one percent of people who acquire the disease will experience severe illness. Within this small proportion, the fatality rate is about 10-15%. Individuals most at risk from WNV are the elderly and persons with compromised immune systems. Most people who become infected will have no symptoms at all.

WNV can be transmitted by a number of different mosquito species. Mosquitoes that breed in containers appear to be very important in the transmission cycle of WNV, in both causing outbreaks in birds and, later, transmitting the disease to humans. Most marsh mosquito species are unlikely to become involved in the transmission cycle unless there is an outbreak of the disease in local bird populations.

Spraying to Eliminate Mosquitoes:
Chemical sprays or "fogs" are sometimes used to kill adult mosquitoes. Such chemicals may be effective in reducing the number of infected mosquitoes during a disease outbreak. However, these chemicals only kill adult mosquitoes, so populations recover quickly as larvae hatch and new adults emerge. In addition, all chemical pesticides used to kill adult mosquitoes are broad-spectrum insecticides that can potentially kill many other kinds of insects, most of which are either harmless or beneficial to humans. Insects, including mosquitoes, serve as important food items for a wide variety of wildlife, including dragonflies, fish, frogs, birds, and bats. In addition, many insects are important pollinators as well as predators of pests. The potential for mosquito pesticides to disrupt important ecological functions has led many communities in the eastern United States to take a more cautious approach to WNV spraying this year, applying broad-spectrum insecticides only when abundant virus-positive birds are discovered in an area with large numbers of mosquitoes. What communities are discovering is that prevention works best and is the least toxic way to control mosquitoes.

Protecting Yourself From West Nile Virus:
Preventative measures include such things as eliminating mosquito breeding places and treating breeding places with larvicides to eliminate the larval mosquitos. Because container-breeding mosquitoes appear to be important in the transmission cycle of WNV, people can reduce their potential exposure to the disease (and reduce the amount of pesticides in the environment) by eliminating standing water. Get rid of old tires and other debris. Regularly empty the water in backyard containers, birdbaths, wading pools, flower pot dishes and pet water bowls or add a fountain to your ponds or bird baths to keep the water circulating. If necessary, homeowners can use mosquito dunks or other products that contain the microorganism Bacillus thuringiensis israelensis (BTI). BTI is lethal to the larvae of mosquitoes, black flies, and a few other species of aquatic flies when ingested, but harmless to other wildlife such as fish, frogs, birds, mammals and most insects. Finally, reduce your exposure to mosquito bites by making sure screens on windows and doors are in good repair and wearing long, light-colored clothing or insect repellent if you are outside when mosquitoes are active. When using mosquito repellents, apply them to clothing rather than skin whenever possible. If you do apply them to skin, only apply them to exposed areas and wash them off as soon as you go indoors. Read the product label carefully to ensure proper use of the repellent and look for special precautions if you are considering using the product on babies or infants.

You may not think that you can make a difference by taking these simple precautions, but caring for yourself and your yard in an environmentally sensitive way can have a bigger impact than you might think. Your yard may only be a small piece of land, but all the yards across the country add up to a lot of land. That means that your activities, along with everybody else's, can make a big difference. You can help protect the environment by following these simple tips. If everyone does their part, fish, wildlife and people will have a cleaner, healthier place to live.

Links:

Harvard School of Public Health. Environmental Risk Analysis Program - West Nile Virus.

US Geological Survey. West Nile Virus.

Center for Disease Control, Division of Vector-Borne Infectious Diseases. 2006 West Nile Virus Activity in the United States.

U.S. Department of Agriculture, Animal Health Monitoring & Surveillance - West Nile Virus.