West Nile Virus & Mosquito Control

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What is West Nile Virus?
Viruses and bacteria can cause encephalitis (an inflammation of the brain) in humans, horses, birds and other animals.  West Nile Virus infection is caused by West Nile Virus (WNV), a flavivirus previously found only in Africa, Eastern European Australia and West Asia.  WNV is closely related to St. Louis encephalitis virus (SLEV), which is found in the United States, and to Kunjin virus (KV), which is found in Australia, some Western Pacific islands and parts of Southeast Asia.  West Nile Virus was first reported from the United States in 1999 when it caused an outbreak of human encephalitis near New York City.  This disease has since become distributed as far south as Maryland, and north to the Canadian border.

West Nile Virus Encephalitis is an arbovirus (carried by certain species of mosquitoes) and is transmitted to humans when infected female mosquitoes obtain a blood meal.  Mosquitoes may first become infected with the virus when they feed on birds that carry the virus in their blood.  After 10 to 14 days, the mosquito’s salivary glands become infected and they can then transmit this virus to humans and other animals.  

Many people infected with WNV show no symptoms of illness, while others may become ill 3 to 15 days after being bitten by an infected mosquito.  Symptoms indicative of this virus include fever, headache, body aches, and sometimes a skin rash with swollen lymph glands.  More severe infection (encephalitis) may be marked by headache, high fever, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis.  In a few cases, mostly the elderly, death may occur.  Persons with severe or unusual headaches should seek medical care as soon as possible.  

There is no vaccine for WNV so the best way to protect yourself is to avoid mosquito bites.  Treatment for WNV may involve hospitalization and good nursing care.  Researchers are hoping to develop and make available a vaccine by 2005.  


How can I reduce the risk from contracting West Nile Virus?
Vaccines against West Nile Virus are not available.  Elimination of areas where mosquito larvae breed is one of the most effective measures for preventing disease.  The primary mosquito vector for WNV is the Northern House Mosquito, Culix pipiens, a mosquito that prefers to breed I stagnant water.  Other prevalent mosquito species that may carry or transmit the West Nile Virus in Maine include Aedes japonicus which specializes in catch basin breeding and Culex Salinarus (Salt Marsh Mosquitoes).  There are over 200 subspecies of mosquitoes and not all carry or transmit the disease or pose equal risk. See tips for removing breeding sources below.

Avoiding mosquito bites is another significant way to reduce your risk of disease.  Each time a mosquito feds on , it injects saliva which is responsible for the itch and it is this saliva which may contain WNV.

Use Insect repellants.  With caution, you can apply insect repellents directly to exposed skin.  There are many different types of repellents but the most effective formulations contain DEET (N, N-diethyl-m-toluamide) as an active ingredient.  Be sure to follow directions on ;the label since DEET can be harmful if overused.  The recommended concentration of DEET for children is less than 10% and for adults, less than 30%.  Avoid applying repellant to children less than two years of age and to the hands o0f young children since these chemicals may irritate the eyes and mouth.  Cream, lotion, or stick formulas are best.  Mosquito netting can be used to cover baby carriages.  

For more information on insect repellents safety, effectiveness, and applications, follow this link to the web site of the Center for Disease Control or log on to http://www.cdc.ncidod/dvbid/westnile/qa/insect_repellent.htm .  

Although the CDC recommends DEET repellents, some people prefer repellents which are DEET-free.  There are products on the market such as Buzz-Off  which are available options.


Keep mosquitoes outdoors.  You should check all windows and doors carefully to make sure that screens are installed and that they are intact.  Gaps in the screen will only serve to keep mosquitoes inside.  Screen repair kits can be obtained from local hardware stores.  

Clothing and skin covering.  To minimize exposure to biting mosquitoes, wear clothing that will protect more of your body during the evening and night when mosquitoes that transmit West Nile Virus are actively seeking blood meals.  Head nets, bug socks and special pants and jackets are designed to keep mosquitoes at bay.  In situations where mosquito bites cannot be avoided because of outdoor work or enjoyment, Permethrin or DEET can be applied to clothing for additional protection.

Traps and Zappers of limited value.  Under most situations, mosquito traps by themselves are not expected to reduce the number of biting mosquitoes significantly.  Traps are normally used for surveillance to detect mosquito populations and to test for the presence of West Nile Virus.  The typical outdoor electric bug “zappers” will trap many more insects other than mosquitoes.  Use caution when purchasing expensive, highly advertised mosquito traps with sensational claims.  Ultrasonic traps may capture more money than mosquitoes.

Tips for removing mosquito breeding areas around your home.  Mosquitoes are not long travelers, so elimination of breeding areas in your home and community is one of the best control strategies.  One of the most effective measures you can take to reduce mosquito bites is to prevent mosquitoes from breeding in your back yard.  You can reduce the number of mosquitoes in areas outdoors where you work or play, by draining sources of standing water.  In this way, you reduce the number of places mosquitoes can lay their eggs and breed.  


At least once or twice a week, empty water from flower pots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, cans, fountains, wading pools, cemetery urns, plastic covers and toys.  Be sure to check for containers or trash in places that may be hard to see, such as under bushes or under your home.  Check for clogged rain gutters and clean them out.  Remove discarded tires and other items that could collect water.  Drill holes in the bottom of containers, including trash barrels to allow any water to drain out.  Drain or fill temporary pools with dirt.  Keep swimming pools treated and circulating and gutters unclogged.

Are There Options to Spray of Treat my Yard with Pesticides to Kill Mosquitoes?
There are two basic pesticide approaches available which should be considered and applies only with extra care and precaution, and only where other strategies have not been effective.  Standing water that cannot be removed could be treated with larvicides…chemical or organic agents that keep mosquito larvae from becoming flying and biting insects.  Adulticides are generally aerial or spray applications of chemical or synthetic organic agents that kill adult mosquitoes.  Adulticides are much more expensive, and are effective for only a short period, and of much less impact.  Adulticides are a measure of last resort only and should be avoided if possible.  

A secondary caution is on order as well:  mosquitoes are and always have been a nuisance and a bother…but it is justifiable to apply pesticides to control mosquitoes only if there is a clear indication of a significant presence of threat of West Nile Virus.  Although the virus has been detected in Maine through the testing of dead crows, there have not been any reported cases of West Nile Virus disease in humans in Maine up to this point (April 2004).  However, nationally there were 4,200 reported cases and 268 deaths in 2003 and Maine may experience greater exposures and more cases in 2004.

Larvicides can be applied to standing water, storm water catch basins and certain forest areas where Culex pipiens and japonicus like to breed.  The mosquito larvae ingest the agent which causes the larvae to stop eating and they die before they can mature into flying and biting insects.  The most frequently used larvicide agent is known as Bti (Bactillus thruingiensis israelensis, if you really must know) and is sold under a variety of trade names including Aquabac*, Teknar*, Bactimos*, and Vectobac*.  Although environmental laws and regulations provide exemptions for household applications by citizens, these agents should be used only by licensed professionals.  Commercial contractors with training, licensing, and required permits should be consulted foe application of larvicides or adulticides.   

Two more cautions are in order here.  (1) Individual home use of some pesticides is exempt from licensing requirements by the Maine DEP and the Maine Pesticide Control Board- but only if applied in accordance with the product labeling.  The label is the law.  (2) Whether a license is required or not, pesticides may not be applied to any of the waters of the State of Maine without a permit from the Maine DEP.  This means not only is it illegal to apply pesticides without a permit to the Cousins or Royal River or any of their tributaries of to a coastal/inter-tidal   lands, but also to any drains, pipes, ditches or swales that will carry those pesticides to the waters of the State.  So great caution should be used when applying pesticides (or herbicides and  fertilizers while we are at it) that will allow those agents to be washed away and carried into receiving waters.  The best approach to guard against such unintended pollution is to read and follow the package application procedures very carefully and to avoid over application.  It may be human nature to assume that if a little pesticide (or fertilizer, or herbicide) is effective then a lot more must be more effective…not so.

More detailed information can be found at the web site of the US Environmental Protection Agency and the National Pesticide Information Center. For those interested in learning more about pesticide application concerns, a detailed article was published online in 2003 by the Maine environmental Policy Institute at  “Overkill: Using pesticides to control West Nile virus mosquitoes in Maine may do more harm than good”.  

What’s the Deal with dead Crows?
Crows and Blue Jays die readily from West Nile virus, so the presence of an unusual number of dead crows is an indicator of the virus in the region.  The crows contract the disease from mosquito bites in ;the same way the other birds and mammals do, but for some reason the virus is quite lethal to that family of birds (corvids).  The carcasses (if fresh enough, usually within 24 hours of death) can be tested for West Nile virus which is only an indicator of  the presence of the virus in the area.  Once the presence of WNV has been established in an area, additional testing of more birds is unnecessary.  Dead crows which tested positive for West Nile Virus were found in parts of coastal Maine in 2003.  

The State of Maine, Bureau of Health has a lot of information on line about West Nile Virus and you should check out that web site and links for more information.  

The Bureau of Health is the receiving center for reports of dead crows and arranges for the testing of such birds.  If you spot a dead crow, call the Bureau’s Reporting Line at 1-888-697-5846.  The Bureau’s web site advises that you can leave a message at any time on the bird reporting line (1-888-697-5846).  You will be asked the following information about the dead bird:

        The type of bird
        The date you first found the dead bird
        Your name, town, county, and zip code
        Your phone number

Based on the information you provide, the public health staff will decide whether the bird should be tested.  If you find and report a dead crow, blue jay, or raven, please properly store the bird for no more than 24 hours or until you are contacted, whichever comes first.  Proper storage means:

        DO NOT handle the dead bird with your bare hands.  Use gloves or a shovel
        Double bag the bird (a clear plastic bag is fine for this purpose)
        Store the bag in a cool dry place, like your garage or basement (DO NOT store the bird in your refrigerator, freezer or cooler).

If you find a dead bird that is not a crow, blue jay or raven, or if we have not contacted you within 24 hours, please dispose of the bird as follows:

        DO NOT handle the bird with your bare hands, use gloves or a shovel
        Bury the bird 18 inches deep (or contact your local town official to see if you dispose of the bird in your trash)



The Maine Bureau of Health greatly appreciates the assistance of all residents who report dead birds.  The information is vital to their efforts to understand WNV.  For more information about mosquitoes, West Nile virus-related illness and related concerns, contact the Bureau of Health, Division of Disease Control at (207) 287-5301.

This information was taken extensively from the following web-based sources without specific attribution but with great appreciation:

State of Maine (www.maine.gov)
State of Washington (www.doh.wa.gov)
Harvard University (www.uos.harvard.edu)
Westchester County (www.westchestergov.com)
U..S. Environmental Protection Agency (www.epa.gov)
National Pesticide Information Center (www.npic.orst.edu)
Maine Municipal Association (www.memun.org)
Municipal Pest Management Services, Inc.
City of Cambridge, MA (www.cambridgema.gov)
Maine Environmental Policy Institute (www.meepi.org)
Centers for Disease Control and Prevention (www.cdc.gov)