West Nile Virus

What is West Nile virus?

How to Protect from West Nile virus

Health Effects of Pesticides Used to Combat Mosquitoes

Biology of Mosquitoes

Guidelines for Spraying

What is West Nile virus?

West Nile virus (WNV) is a mosquito-borne disease. The bite of a mosquito infected with the virus can cause illness or inflammation of the brain, known as encephalitis.

Who is at risk?
Anyone that lives in or visits an area with infected mosquitoes is at risk. People older than 50 years have the highest risk of severe encephalitis. However, according to CDC, "Less than 1% of those infected with West Nile virus will develop severe illness…Human illness from West Nile virus remains rare in areas where it has been reported, and the chance that any one person is going to become ill from a mosquito bite is low." Some animals are also at risk, including birds, horses, cats, and domestic rabbits. Current evidence shows that only mosquitoes can spread the disease, humans or other animals cannot.

What are the symptoms?
Symptoms of a mild infection include fever, head and body aches, skin rash, and swollen lymph glands. If you have high fever, experience confusion, muscle weakness, and severe headaches, call your health care provider immediately. It may take 3 to 15 days for any of these symptoms to show.

Where is WNV found?
WNV has spread throughout most of the United States. The disease is also found throughout the world, including Africa, West Asia, Eastern Europe, and the Middle East.

When is it most common?
Late summer and early fall is when mosquitoes infected with WN virus are most common in the United States and other temperate regions such as Europe. In warmer regions of the world, this disease can occur year-round. top

How to Protect from West Nile virus

What can you do?

  • Habitiat Modification - To get rid of mosquitoes, get rid of their breeding sites and exclude them from your immediate area. Because mosquitoes don't travel far from where they hatch, you can have a dramatic impact on local mosquito populations by cleaning up standing water on your property. Get rid of unnecessary debris on your property; empty water from toys, buckets, birdbaths, swimming pool covers and any other areas water may be collecting; drill holes in the bottom of recycling bins and other containers that must be kept outside; and, clean out rain gutters and make sure that they are draining properly. Add mosquito-eating fish to ponds or birdbaths and screen all doors and windows.
  • Avoid the Bite - If going outside in the early morning and evening when mosquitoes are most active, use a hat and wear light-colored long sleeves and pants. If you choose to use insect repellents, try a natural product especially when choosing for children. Botanical skin repellents may contain geraniol (MosquitoGuard or Bite Stop), citronella (Natrapel), a combination of soybean and coconut oils (Bite Blocker), herbal extracts (Beat It Bug Buster), Avon's Skin-so-Soft, or other all essential oils (All Terrain). These are effective mosquito repellents that are safer than products that contain DEET (N,N-diethyl-meta-toluamide), but should be reapplied often. Several cases of DEET poisonings have been reported by EPA, including three fatalities. Always wash off repellents with soap and water once indoors. Pregnant women should consult a doctor before using essential oils. top

What can your community do?

  • Public Education - West Nile virus prevention begins with a strong public education program. Community officials must educate the public on how to reduce the chance of mosquito bites through the elimination of breeding habitats and the proper use of repellents (see above).
  • Surveillance - Tracking larval and adult mosquito populations, species types, breeding locations and virus outbreaks is an essential part of any mosquito management program. Knowing when and where the virus is likely strike allows for precise, targeted control techniques.
  • Habitat Modification - As on personal property, community land should be cleared of all standing water that could serve as potential breeding habitat (see above).
  • Larviciding - Because not all breeding sites can be eliminated, it is a good idea to use larvicides, particularly products containing B.t.i. (Bacillus thuringiensis var. israelensis), in places such as storm drains and sewer treatment plants. B.t.i. is proven to be effective and is virtually non-toxic to humans. Methoprene (Altosid) is an endocrine disruptor, has been linked to wildlife deformities and should not be used. Aside from traditional larvicides, biological controls, like mosquito-feeding fish of the Gambia genus, have been used nationwide with great success. To avoid ecological problems, these fish should only be used in enclosed bodies of water.
  • Protect from Pesticide Exposure!- If your community insists on spraying, take all necessary precautions to avoid exposure.
    • Leave the area*
    • Close the windows
    • Turn off air intake on window unit air conditioners
    • Take toys and lawn furniture inside
    • Remove shoes before entering homes to avoid tracking in residues
    • Cover swimming pools
    • Don't let children play near or behind truck-mounted applicators

Health Effects of Pesticides Used To Combat Mosquitoes

The Centers of Disease Control and Prevention (CDC) state that spraying adulticides, pesticides intended to kill adult mosquitoes, is usually the least efficient mosquito control technique. Aside from adverse health effects posed to humans, adulticiding is not proven to reduce the risk of WNV in communities and may actually increase the number of mosquitoes by destroying their natural predators. Additionally, mosquitoes that survive the spraying may become resistant and have an increased prevalence of the virus within their bodies.

Synthetic Pyrethroids
Synthetic pyrethroids, which include resmethrin (Scourge) and sumithrin (Anvil), are adulticides patterned after pyrethrum, an extract from the chrysanthemum flower. While similar to pyrethrum, synthetic pyrethroids have been chemically engineered to have greater toxicity and longer breakdown times . Additionally, almost all synthetic pyrethroid mosquito products are combined with synergists, which increase potency and compromise the human body's ability to detoxify the pesticide. Symptoms of exposure include: dermatitis and asthma-like reactions, nasal stuffiness, headache, nausea, incoordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensations . Synthetic pyrethroids are endocrine disruptors and have been linked to breast cancer . Deaths due to exposure have resulted from to respiratory failure. People with asthma and pollen allergies should be especially cautious. Breakdown times range from a few hours in direct sunlight, to several months in damp, dark environments.

Organophosphates, which include malathion (Fyfanon), naled (Dibrom) and chlorpyrifos (Mosquitomist), are a highly toxic class of pesticides that affect the central nervous, cardiovascular and respiratory systems. Symptoms of exposure include: numbness, tingling sensations, headache, dizziness, tremors, nausea, abdominal cramps, sweating, incoordination, blurred vision, difficulty breathing, slow heartbeat, unconsciousness, incontinence, convulsions and fatality . Some organophosphates have been linked to birth defects and cancer. Breakdown times range from a few days in direct sunlight, to several months in damp, dark environments. top

Biology of Mosquitoes

In the United States, WNV is primarily associated with the Culex mosquitoes, also called the "backyard mosquito." Within this genus, three species, namely C. pipiens, C. restuans, and C. salinarius make up the majority of those mosquitoes found to be infected with WNV. Only female mosquitoes take blood meals; males do not bite humans or animals and usually die one or two days after emerging as adults. In contrast, adult females may live 2-4 weeks or more, depending on climate, species, predation, and a host of other factors.

All mosquitoes go through a four stage life cycle called "complete metamorphosis" from egg to larva to pupa and finally emerging as an adult. The length of time that each stage lasts depends on a number of variables with temperature having the greatest impact.

Eggs are laid in "rafts" on standing bodies of water. The eggs require one to two days in water before hatching into first instar larvae.
Larvae, or wigglers, develop as four instars. They molt three times during ten to twelve days before pupating.
Pupae, or tumblers, metamorphose over one to two days into adults.
Adults emerge from their pupal cases approximately twelve to sixteen days after being laid as eggs by their mother.

After mating, the female requires a blood meal in order to produce over 250 eggs. It takes her three to four days to digest the blood and produce the eggs. Females transmit diseases when they live long enough to spread infection from the first blood meal victim to the second blood meal victim. Only a very small percentage of females live this long. Culex mosquitoes are generally weak fliers and do not move far from their larval habitat, although they have been known to fly up to two miles. top

Guidelines for Spraying

The risks of pesticide exposure to humans and the environment caused by spraying pesticides are great. Spraying also kills natural predators, including other insects, birds, and fish. It may also result in mosquito resistance, which in turn will lead to a larger problem in the future. Should the community insist on spraying, local decision makers and mosquito control should be pressed to protect the public and environment from unnecessary exposure using the following guidelines:

  • Identify and locate the infected mosquito pool(s). As discussed above, an adult Culex mosquito can fly no further than a two-mile radius. All trapped mosquitoes should be speciated (to know if mosquitoes are potential vectors), sexed (only females bite), and tested for the virus (to know the actual prevelence of the virus). Clear and distinctive thresholds that determine when a spray may be triggered should be determined with involvement from the community. Some communities have higher thresholds for the threat of WNv than others.
  • Choose the least dangerous pesticides. Never use highly toxic pesticides such as Dursban™, Sevin™, or Dibrom. Organophosphate pesticides (OPs), such as malathion and synthetic pyrethroids such as resmethrin and permethrin, are neurotoxins that can create chronic health effects. Botanical-based chemicals, including synthetic pyrethroids, are linked to breast cancer and endocrine disrupters. However, pyrethroids are applied in small amounts and have shorter residual lives than OPs like malathion.
  • Know when to spray. Mosquitoes take refuge in grass and brush during the day, so spray at dusk when they are active and most vulnerable. Spray should never be conducted in winds above 10mph.
  • Ensure that the person spraying is certified. They should use protective clothing and equipment.
  • Alert the Community 72 hours in advance. Utilize the media and/or send notices to every household, school, hospital, and business in the community to tell them when the spraying will occur so they will have ample time to protect themselves. Alert the public that pesticides are not safe.
  • Monitor pesticide equipment calibration and application procedures. Verify that there is strict compliance with any label instructions, including prohibitions on spraying and drifting of certain pesticides over bodies of water. Also, be sure to comply with requirements for storage, disposal, and equipment cleaning. Mist blower and aerial application of these materials to populated areas will result in human exposure. We do not recommend aerial spraying. Rather, spray the pesticides from vehicles or use professional applicators on foot.
  • Provide public with precautionary measures. Everyone should receive guidelines on how to reduce exposure to pesticides. Manufacturers and distributors of pesticides are prohibited by law by ever calling a pesticide safe. The U.S. Environmental Protection Agency also states that no pesticide can ever be considered safe. As leaders in the community, never tell the public that the pesticides are safe, rather, give them information on how and when to avoid exposure.
  • Advise hospitals and schools. Also notify other buildings with especially vulnerable populations to take extra precautionary measures to prevent pesticides from entering buildings.
  • Monitor public and environment for adverse effects from the spraying. If WNv is a problem, the last thing to do is create another problem by not monitoring potentially adverse effects. Set up a hotline for receiving reports, collecting hospital records, and requiring physician reporting of incidents. Use wipe tests of outdoor and indoor surfaces, check air conditioner filters, evaluate water samples, and conduct soil and food residue tests from gardens and farms.top

* Infants, children, pregnant women, the elderly, and individuals with compromised immune systems and chemical sensitivities are especially vulnerable and must take extra care to avoid pesticide exposure.

For a more detailed report by Beyond Pesticides see Public Health Mosquito Management Strategy- For Decision Makers and Communities (Revised August 2004). If you are concerned about improper mosquito management in your community, learn what you can do by visiting Tools For Activists.