Mosquito Control and West Nile Virus

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Photo by Fotolia/Henrik Larsson
West Nile virus is carried by mosquitoes, which can infect birds, horses and humans with the disease.

This article was posted with permission from Northwest Center for Alternatives to Pesticide.

West Nile virus, a mosquito transmitted disease, was first documented in North America in the summer of 1999 when cases of human illness occurred in New York City. Since that time, West Nile virus has progressively moved across the country.1 By 2007, the virus was reported from all states except Alaska, and Hawaii.2

The Joint Statement on Mosquito Control from the U.S. Environmental Protection Agency and the U.S. Centers for Disease Control and Prevention states that:

“The underlying philosophy of mosquito control is based on the fact that the greatest control impact on mosquito populations will occur when they are concentrated, immobile and accessible. This emphasis focuses on habitat management and controlling the immature stages before the mosquitoes emerge as adults. This policy reduces the need for widespread pesticide application in urban areas.”3

NCAP encourages communities to make every effort to avoid resorting to mosquito sprays because of the hazards that pesticides pose — hazards for our health and the health of the environment. Communities should strive to control mosquito populations by reducing breeding habitats and take common-sense steps to protect themselves from mosquito bites. Focusing on these strategies will help prevent the use of pesticides that may add to the problems caused by this disease.

What is West Nile Virus?

West Nile virus was first identified in the West Nile region of Uganda in 1937.1 Until 1999 when the first documented cases occurred in New York City, the virus was found only in parts of Africa, Asia, and Europe.1 In areas where the virus is endemic, it is a childhood disease with an immune adult population.4

West Nile virus is closely related to St. Louis encephalitis, a virus historically found in the U.S.4 Most people infected with West Nile virus show no symptoms of illness or only mild ones. On rare occasions, however, infection can result in potentially fatal illnesses: encephalitis, meningitis or paralysis.5

Only about 1 in 5 individuals infected with the virus develops flu-like symptoms 1 such as fever, headache, painful muscles and joints, fatigue, and sometimes swollen lymph glands and a rash.6 About 1 in 140 infected persons develops one of the more serious symptoms.1 Age is a significant risk factor for severe disease, although paralysis from West Nile virus has occurred in patients of all ages. Diabetes and high blood pressure are other risk factors. 7

There were 124 reported deaths from West Nile virus in the United States during the year 2007.8 While these deaths are clearly a significant public health issue, in comparison approximately 36,000 deaths per year are attributed to influenza in the U.S.9

How Does West Nile Virus Affect Animals?

Birds, horses, and other animals are at risk of infection from West Nile virus. The virus has infected birds from 284 species.1 Crows and jays appear to be the most susceptible species. Other bird species usually survive infection.10 Wild bird mortality is one method that is used to monitor the geographic spread of West Nile virus.10

West Nile virus does not often cause severe illness in dogs and cats. When researchers experimentally infected dogs and cats with the virus, the virus could be measured in their blood, but the dogs did not show any signs of disease and the cats developed only a mild illness.11 There are, however, a handful of instances when dogs have developed serious illness.12 During a recent West Nile virus epidemic in Louisiana, about 1 in 4 dogs were infected with the virus, and about 1 in 10 cats.13

Of the mammals documented with infection, horses appear to be the most vulnerable. While many (90 percent) of horses infected with West Nile virus do not develop any illness, of those that do, between 40 and 60 percent die or need to be euthanized.14 Clinical signs of disease in horses include stumblin, incoordination, partial paralysis, muscle twitching, and behavior changes such as depression or apprehension. Vaccines are available for horses.15

According to the U.S. Department of Agriculture, no clinical signs of illness have been reported in poultry in the U.S.15a

How is West Nile Virus Transmitted?

West Nile virus is spread by mosquitoes. Birds serve as the host for the virus, and mosquitoes spread it through their bites. A mosquito first acquires the infection by feeding on a bird infected with the virus. The virus is then transmitted when the mosquito bites a person or animal.5

Current evidence shows that “the vast majority of human infections are caused by the bite of an infected mosquito.”A few cases have been caused by blood transfusions, organ transplants, breast feeding, and during pregnancy. The virus is prevalent when adult mosquitoes are abundant. “with a peak between mid-July and mid-September.”5

Migrating birds are an efficient means of geographic spread of the virus.7

Mosquito Biology

Mosquitoes play an important role in many ecosystems as a major food source for wildlife, including fish, birds, bats, frogs and other insects.16

Mosquitoes pass through four life stages — egg, larva, pupa, and adult. Female mosquitoes require a blood meal to obtain the nutrients required for development of their eggs.17

Most female mosquitoes lay their eggs directly on the surface of still or slow moving water. After passing through the aquatic stages of egg, larva, and pupa, adult mosquitoes emerge from their water nurseries. Mosquitoes breed in many different kinds of habitats and some can complete their life cycle in a few days.17

Breeding sites are usually still pools of water that offer shelter from wind and water flow. Predators such as fish and other insects in permanent ponds, lakes, and streams usually keep larvae populations under control. Lagoons, ditches, puddles, and artificial containers are potential breeding sites.18

Which Mosquito Species Transmit West Nile Virus?

Many species of mosquitoes do not transmit the disease. Out of the almost 175 species of mosquitoes found in the U.S.,7 about 60 have tested positive for West Nile virus.1

In the U.S., West Nile virus is primarily associated with Culex mosquitoes.7 Culex pipiens is a widespread species that is an important vector of the disease, but other Culex species can also be important.7 19,20

According to studies conducted by California entomologists, most Culex females travel less than 1 kilometer (0.6 miles); the maximum distance traveled was 13 kilometers (8 miles).21

Mosquito Control

The most important action you can take to control mosquito populations is to reduce mosquito breeding sites around your home and neighborhood, often referred to as “source reduction.” The U.S. Centers for Disease Control and Prevention’s (CDC’s) evaluation of source reduction is that it “remains the most effective and economical method of providing long-term mosquito control in many habitats.”22

Some “annoying and potentially dangerous mosquito species” 23 breed in places like tree holes, old tires, buckets, toys, plant trays, plastic covers, and tarps. These are good places to focus your efforts.

To reduce mosquito breeding sites around the home try these important steps:

  • Remove, turn over, or cover any container that can hold water, such as tin cans, buckets, ceramic pots, plastic covers, and toys. 23
  • Store unused tires inside a garage or shed, or drill drainage holes in them. 24
  • Change water often (at least twice a week) in birdbaths, fountains, wading pools, potted plant trays, and watering troughs. 23,26
  • Keep water off swimming pool covers. Maintain water quality in the pool. Drain wading pools regularly. 25
  • Make sure roof gutters are draining properly. Clean any debris out once a year. 25
  • Drain (or fill with dirt) unneeded or unwanted pools or puddles.25
  • Do not dispose of garden debris into catch basins or storm drains. Clean obstructions to promote flow. 25
  • Fill tree holes with sand or mortar. 25
  • Avoid  over irrigation of your lawn. 25
  • Fix leaky outdoor faucets and sprinklers. 26

Protection from Mosquito Bites

You can further reduce your risk of infection by taking some pesticide-free steps to protect yourself from mosquito bites:

  • Check that your window and door screens are “bug tight.” Repair or replace if needed. 23,26
  • Replace outdoor lighting with yellow “bug” lights.23
  • Avoid places where there are lots of mosquitoes and don’t plan outdoor activities during times when mosquito activity is high.25
  • If you go to places with lots of mosquitoes, wear a long sleeved shirt and long pants. Head nets can also be helpful.23

Protection for Horses

There are several steps you can take to protect horses from West Nile virus:

  • Reduce mosquito breeding sites by the steps identified above.15
  • Reduce exposure to mosquitoes by housing animals in structures with well maintained screening.15
  • Stable horses at night.15
  • Use fans to reduce the ability of mosquitoes to feed on horses.15
  • Vaccinate horses before infectious mosquitoes are likely to bite.15

Using Pesticides to Control Mosquitoes

A variety of experts have noted that preventing West Nile virus by using pesticides to kill adult mosquitoes is not an effective technique. Howard Shapiro, a Canadian medical officer, noted that “one criticism of adulticides [pesticides designed to kill adult mosquitoes] is their transient effect: mosquito numbers return to pretreatment levels within a few days without repeat applications.” 28 The University of California supports this conclusion, writing that outdoor sprays have “no lasting effect.”25 Recent research from the University of Kentucky showed that insecticide spraying reduced the total number of backyard mosquitoes for about six weeks after the spraying, but did not reduce the number of Culex mosquitoes that are the primary vectors of West Nile virus.29

Community-wide Efforts

West Nile virus makes headlines in late summer or early fall when mosquito populations and the number of reported virus cases is high. The best time to prevent mosquito problems, however, is much earlier in the year when source reduction efforts can be effective. If your community is facing West Nile virus, don’t procrastinate. Find out if there is an existing mosquito control program. If not, encourage your local government to start a prevention program. Take an active role in shaping the management strategies. If your community has a control program, find out what techniques they use. Encourage mosquito managers to act before a crisis develops and focus on long term nonchemical source reduction efforts instead of chemical treatments. Source reduction efforts are most effective when government, individuals, and businesses cooperate; be sure your community educates all stakeholders.


Reduction of mosquito problems around homes and neighborhoods can be successfully achieved with a few simple steps. Focus on the reduction or elimination of mosquito breeding habitats, any place or container that collects standing water. Individuals and communities can have a large impact on reducing the risk of West Nile infection without using pesticides.

Guidelines For Mosquito Control Programs

NCAP is committed to promoting alternatives and does not recommend the use of pesticides. However, we recognize that many communities faced with West Nile virus outbreaks will begin or expand mosquito spray programs. NCAP recommends the following guidelines for pesticide programs developed in response to West Nile virus:


  • Communities should act carefully and thoughtfully. Planning should begin early so that crisis actions are not necessary. Educate the local community about West Nile virus.
  • No pesticides should be used unless there are also “source reduction” efforts to reduce mosquito breeding habitat. In addition to management of public areas, source reduction should include cooperative efforts with residents and businesses. Comprehensive public education about source reduction is essential.


  • Focus on programs to kill mosquito larvae. Larval control is more targeted than mosquito sprays aimed at adults.
  • Applications of pesticides (larvicides or adulticides) should target areas where mosquitoes are abundant. Before using a pesticide, vector control districts should conduct systematic monitoring to identify where mosquito population levels are high.
  • Pesticide applications should be conducted when they are most effective.
  • Choose application techniques to minimize exposure of people and the environment. Ground applications can be more precisely targeted than aerial applications.
  • Choose pesticides that selectively target mosquitoes.
  • Choose pesticides that minimize hazards to people and the environment.
  • Community-level assessments of health and environmental hazards associated with spray plans help identify local conditions that warrant specific accommodations to protect people and wildlife. More general state or national-level assessments may not address local needs.
  • Pesticide products contain “inert” ingredients that are often kept secret from the public. Insist on disclosure of all ingredients in the pesticide products used for mosquito control.
  • Community residents and businesses should be notified at least 72 hours prior to any pesticide application.
  • Community residents should be informed about what they can do to minimize pesticide exposure.


  • Monitor mosquito populations before and after pesticide applications in order to target applications and document their efficacy.
  • Monitor pesticide contamination of the environment to identify potential problems.
  • Establish a “hotline” to collect reports of any suspected pesticide-related illnesses from residents and physicians.

No Spray Registry

  • Provide community residents the option to be excluded from the mosquito abatement program’s pesticide treatments. This option assists certified organic farmers and others wishing to avoid pesticide exposure.
  • Require community residents participating in the no spray registry to commit to managing mosquitoes on their property.
  • Notify community residents of the no spray registry option.


1. Petersen, L.R., and E.B. Hayes. 2004. Westward ho? — The spread of West Nile virus. New Engl. J. Med. 351:2257-2259.

2. Centers for Disease Control and Prevention. 2008. 2007 West Nile virus activity in the United States (reported to CDC as of April 1, 2008). [2007 Map]

3. U.S. EPA. 2007. Joint statement on mosquito control in the United States from the U.S. Environmental Protection Agency (EPA) and the U.S. Centers for Disease Control and Prevention (CDC). (current as of: April 10, 2007)

4. Campbell, G.L. et al. 2002. West Nile virus. Lancet Infect. Dis. 2:519-529.

5. Tyler, K.L. 2004. West Nile virus infection in the United States. Arch. Neurol. 61:1190-1195.

6. Centers for Disease Control and Prevention. 2003. Epidemic/Epizootic West Nile virus in the United States: Guidelines for surveillance, prevention, and control. (p. 67)

7. Granwehr, B.P. et al. 2004. West Nile virus: Where are we now? Lancet Infect. Dis. 4:547-556.

8. Centers for Disease Control and Prevention. 2008. 2007 West Nile virus activity in the United States. [2007 Case Counts]

9. Fiore, AE, et al. Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007. MMWR Recommendations and Reports July 13, 2007. vol.56 no.RR-6.

10. Ref. # 6, pp. 5-13.

11. Austgen, L.E. et al. 2004. Experimental infection of cats and dogs with West Nile virus. Emerg. Infect. Dis. 10:82-86.

12. Read, R.W., D.B. Rodriguez, and B.A. Summers. 2005. West Nile virus encephalitis in a dog. Vet. Pathol. 42:219-222.

13. Kile, J.C. et al. 2005. Serologic survey of cats and dogs during an epidemic of West Nile virus infection in humans. J. Am. Vet. Med. Assoc. 226:1349-1353.

14. Castillo-Olivares, J. and J. Wood. 2004. West Nile virus infection of horses. Vet. Res. 35: 467-483.

15. U.S. Dept. of Agriculture. Animal and Plant Health Inspection Service. 2004. West Nile Virus: Protecting your horses.

15a. U.S. Dept. of Agriculture. Animal and Plant Health Inspection Service. 2004. West Nile virus: Veterinary Services.

16. National Park Service. 1998. National Park Service integrated pest management manual: Mosquitoes.

17. Univ. of Florida and American Mosquito Control Assoc. 2002. Public-health pesticide applicator training manual: Chap. 3. Mosquitoes.

18. North Carolina State University. Dept. of Entomology. North Carolina Cooperative Extension. 2004. Mosquito control around the home and in communities.

19. Kilpatrick, A.M. et al. 2005. West Nile virus risk assessment and the bridge vector paradigm. Emerg. Infect. Dis. 11:425-429.

20. Turell, M.J. 2005. An update on the potential of North American mosquitoes (Diptera: Culicidae) to transmit West Nile virus. J. Med. Entomol. 42:57-62.

21. Reisen, W.K., M.M. Milby, and R.P. Meyer. 1992. Population dynamics of adult Culex mosquitoes (Diptera: Culicidae) along the Kern River, Kern County, California, in 1990. J. Med. Entomol. 29:531-543.

22. Ref. # 6. p. 28.

23. U.S. EPA. 1998. Mosquitoes: How to control them. Similar information at:

24. Cornell Univ. Environmental Risk Analysis Program. 2005. West Nile virus: Frequently asked questions. (link no longer active)

25. Univ. of California. Div. of Agriculture and Natural Resources. 1998. Mosquitoes. Pest Notes Publ. 7451.

26. Washington State Dept. of Health. 2007. West Nile virus questions and answers.

27. Washington Dept. of Fish and Wildlife. 2003. West Nile virus, fish, and wildlife.

28. Shapiro, H. and S. Micucci. 2003. Pesticide use for West Nile virus. Can. Med. Assoc. J. 168: 1427-1430.

29. Hubbard, J.L. et al. 2005. Do backyard mosquito sprays work? Pest Cont. Technol. (May): 44-56.

The original version of this article was published as: McKinney,D. 2002. Meeting the challenge of West Nile virus without poisons. Journal of Pesticide Reform 22(4):2-8.