Introduction. The introduction and spread of West Nile virus in the United States has reawakened an appreciation of mosquitoes as vectors of diseases. The term "reawakened" is used advisedly, for mosquito-borne diseases were once quite prevalent in the United States and, indeed, played a major part in shaping our nation's destiny. Dengue Fever, long a scourge in the tropics worldwide, was in fact first described by Dr. Benjamin Rush in Philadelphia in 1780. Additionally, Yellow Fever caused over 100,000 deaths in 135 separate epidemics in the United States from 1793 until 1900, and as late as 1934, there were 125,566 cases of malaria. These diseases no longer claim victims in the United States as a matter of course largely due to the exemplary efforts of organized mosquito control agencies, in conjunction with an enlightened and effective public health infrastructure. Indeed, the mosquito control profession enjoys a long and proud legacy of community service in its pursuit of improved quality of life and a society relatively free from the ravages of mosquito-borne diseases that have afflicted our country in times past.

As of 1 August 2005, a total of 61 human cases have been reported to the CDC. Of these, 2 have been fatal, 18 exhibited neuroinvasive symptoms, and 40 were classified as West Nile fever. In 2004, a total of 1508 human cases have been reported. Of these, 45 have been fatal, 532 (35%) exhibited neuroinvasive symptoms, and 622 (41%) were classified as West Nile fever. In 2003 a total of 9862 human cases were reported. A total of 264 of these were fatal, 2866 (29%) were diagnosed as neuroinvasive, and 6830 (69%) were classified as West Nile fever. The 2002 outbreak constituted the largest documented outbreak of mosquito-borne meningoencephalitis in the history of the western hemisphere.

The costs these cases entail are extraordinary and extend far beyond medical and vector control expenditures. It has been estimated by CDC that the average cost per patient hospitalized with WNV infection in Louisiana in 2002 was $51,826, with the total cost of treatment and control exceeding 69 million dollars. However, these numbers fail to address the additional emotional cost to families of victims of mosquito-transmitted disease, a radically-changed quality of life of the victims and similar issues.

West Nile virus has wrought havoc with wildlife as well. More than 200 avian species and 30 mammalian species have been found infected. Although accurate counts of absolute numbers of birds and mammals fatally infected are problematic, the toll for corvids (crows, jays, etc.) is estimated to be in the millions. Horses suffer a 40% mortality rate from infection with this virus. The cost to the horse industry in vaccinations, medical costs, prevention/control measures, and mortality is estimated to exceed one billion dollars.

Transmission. Great strides have been made in defining the transmission dynamics of West Nile virus. However, considering that it is a comparatively recent epidemiological phenomenon, there remains much to learn in order to establish and verify baseline data. The cycle involves birds as a reservoir of infection and means of spread through migration, avian-feeding species of mosquitoes amplifying the virus among bird populations, and bridging species of mosquitoes that feed upon both birds and mammals transmitting the virus to humans and equines. At present, 60 of the 174 species of mosquitoes currently recognized in the United States have tested positive for the virus. Of these, generally one species is primarily responsible for transmitting the disease in a particular area. The extent to which other species contribute to the problem is often poorly understood. Each species utilizes preferred aquatic habitats within which to breed. These habitats vary widely, from salt marshes to used car tires. Virtually any collection of stagnant water is fair game, with some species successfully utilizing even soda bottle caps. Factors favoring choice of breeding habitat depend upon the mosquito species involved, topography, climate and human use patterns.

From http://www.cdc.gov/ncidod/dvbid/westnile/cycle.htm

Many mosquito species have been found infected by WNV in nature. Those species are listed below:

1. Aedes aegypti
2. Aedes albopictus
3. Aedes cinereus
4. Aedes vexans
5. Anopheles atropos
6. Anopheles barberi
7. Anopheles crucians/bradleyi
8. Anopheles franciscanus
9. Anopheles freeborni
10. Anopheles hermsi
11. Anopheles punctipennis
12. Anopheles quadrimaculatus
13. Anopheles walkeri
14. Coquillettidia perturbans
15. Culex coronator
16. Culex erraticus
17. Culex erythrothorax
18. Culex nigripalpus
19. Culex pipiens
20. Culex quinquefasciatus
21. Culex restuans
22. Culex salinarius
23. Culex stigmatasoma
24. Culex tarsalis
25. Culex territans
26. Culex thriambus
27. Culiseta impatiens
28. Culiseta inornata
29. Culiseta melanura
30. Culiseta morsitans
31. Deinocerites cancer
32. Mansonia tittilans
33. Ochlerotatus atlanticus/tormentor
34. Ochlerotatus atropalpus
35. Ochlerotatus canadensis
36. Ochlerotatus cantator
37. Ochlerotatus condolescens
38. Ochlerotatus dorsalis
39. Ochlerotatus dupreei
40. Ochlerotatus fitchii
41. Ochlerotatus fulvus pallens
42. Ochlerotatus grossbecki
43. Ochlerotatus infirmatus
44. Ochlerotatus japonicus
45. Ochlerotatus melanimon
46. Ochlerotatus nigromaculis
47. Ochlerotatus provocans
48. Ochlerotatus sollicitans
49. Ochlerotatus squamiger
50. Ochlerotatus sticticus
51. Ochlerotatus stimulans
52. Ochlerotatus taeniorhynchus
53. Ochlerotatus triseriatus
54. Ochlerotatus trivittatus
55. Orthopodomyia signifera
56. Psorophora ciliata
57. Psorophora columbiae
58. Psorophora ferox
59. Psorophora howardii
60. Uranotaenia sapphirina

Nonetheless, the vast majority of these are not important vectors for any number of reasons, e.g. their feeding preference, time of feeding, resting habits, breeding proclivities, etc. In most cases, a few species are thought to comprise the preponderance of vector activity. Thus, in the northeastern United States, Culex pipiens and Culex restuans account for the majority of transmission, whereas in the western parts of the country, Culex restuans is replaced by Culex tarsalis as the rural vector.

Control. Successful West Nile virus control programs as practiced nationwide today rely upon principles of Integrated Pest Management (IPM). IPM, as the name implies, utilizes a variety of physical, chemical, mechanical, cultural, biological, and educational measures, singly or in appropriate combination, to exploit the mosquito's vulnerabilities and attain the desired level of mosquito control consistent with community needs. Application of these measures is predicated upon surveillance data indicating a need for intervention. In this light, the sine qua non of effective, sustainable West Nile virus control is a sound, comprehensive surveillance program driving intervention efforts. Knowledge of the target mosquito vector allows efficient allocation of control resources specifically tailored to safely counter each stage of the mosquito life cycle. Larval control through water management, vegetation management and source reduction, where compatible with other land management uses, is a prudent pest management alternative - as is use of the environmentally friendly EPA-approved larvicides currently available.

When source elimination or larval control measures are clearly inadequate, or in the case of imminent disease, the EPA and CDC have emphasized in a published joint statement the need for considered application of adulticides by certified applicators trained in the special handling characteristics of these products. The extremely small droplet aerosols utilized in adult mosquito control are designed to impact primarily on adult mosquitoes that are on the wing at the time of the application. Degradation of these small droplets is rapid, leaving little or no residue in the target area at ground level. These special considerations are major factors that favor the use of very low application rates for these products, generally less then 4 grams active ingredient per acre, and are instrumental in minimizing adverse impacts.

Components of contemporary West Nile virus control programs include the following, adapted from "Public Health Confronts the Mosquito: Developing Sustainable State and Local Mosquito Control Programs", Interim recommendations of the Mosquito Control Collaborative, a project of the Association of State and territorial Health Officials in partnership with The National Association of County and City Health Officials and Centers for Disease Control and Prevention.

Prevention

Surveillance - A sustained, consistent surveillance program targets vector species, identifies and maps their larval habitats by season, documents the need for control through larval and adult trapping regimens. It thus also monitors the effectiveness of the control program. Appropriate and timely response to surveillance data is the key to preventing human and animal disease associated with WNV. Detection of epizootic transmission of enzootic arboviruses Control activity should be intensified in response to evidence of virus transmission, as deemed necessary by the local health departments.

• Virus Surveillance of Mosquitoes/Birds - Detection of WNV in bird and mosquito populations appears to be the most sensitive early detection system for WNV activity, typically preceding detection of human cases by several days to several weeks. Early-season detection of WNV activity in birds and mosquitoes appears to be correlated with increased risk of human cases later in the season.
  • Surveillance programs based upon dead birds are the most sensitive method of detecting WNV presence in an area.
  • Captive sentinel surveillance typically utilizing chickens and programs based upon free-ranging bird surveillance have both been used. Both of these approaches requires extensive knowledge of local transmission dynamics and may require animal use and care protocols and other authorization permits.
  • Mosquito surveillance based upon trapping remains the primary tool for quantifying the intensity of virus transmission in an area. In addition, these techniques can monitor efficacy of control programs.
    • Light traps and gravid traps remain classical methodologies
    • If appropriate, biting/landing counts can be used to establish accurate data regarding mosquitoes questing for human meals.
• Human Surveillance - Human case surveillance, both passive and active, alone should not be used for the detection of arbovirus activity, except in jurisdictions where arbovirus activity is rare or resources to support avian-based and/or mosquito-based arbovirus surveillance are unavailable.

Public Information and Outreach - The public has concerns about problems related to mosquito populations and insecticide spraying. Addressing these concerns is critical to maintaining support of the citizenry. Successful programs having the have multi-phase communications plans that educate the public about preventing the breeding of mosquitoes, personal protection guidance, and the various activities of the agencies involved.

Studies have shown that information programs, while crucial to the overall prevention/control strategy, have a moderate effect on modifying population behaviors related to personal protective measures. About half of the population actively attempts to reduce breeding habitats aroound their domiciles. A smaller percentage use repellents due to perceived risk and other complex demographic factors. Nevertheless, programs should include strategies to facilitate protective actions and to address barriers that hinder preventive actions. Effective programs include developing a community task force, interventions to improve access to window screening materials or repellents, and social marketing to reinforce preventive behaviors. These are critical components of any mosquito control program, but cannot, in and of themselves, replace established prevention/control methodologies.

Source Reduction - Source reduction involves the elimination, where possible, or modification alteration of water sources to make them unavailable for mosquito breeding. Removing breeding habitat is the most effective long-term mosquito control where allowed, but modification through the selective use of herbicides to make the habitat unsuitable for breeding is also extremely effective. Source reduction includes activities as simple as the proper disposal of used tires, paint cans and trash, in addition to the cleaning of rain gutters, bird baths, and unused swimming pools by individual property owners. This can also include extensive regional water management projects conducted by mosquito control agencies on state and/or federal lands, where permitted. Source reduction activities can be separated into the following two general categories:

• Sanitation - Cleanup of peridomestic stagnant water sources provides a substantial reduction in biting activity. Educational information about the importance of sanitation in the form of videos, slide shows, and fact sheets distributed at press briefings, fairs, schools and other public areas can be effective in reducing these as breeding habitats. Considering that mosquitoes breeding in these containers tend to feed upon humans in close proximity, they constitute an important disease risk.
• Water Management - Proper stormwater management and both fresh and saltmarsh management are critical and resource-intensive forms of source reduction of important nuisance and vector species. Included in this strategy is vegetation management through physical removal or herbicide applications within potential habitats to remove means for larvae to escape predation.

Control

Surveillance results drive all facets of the control program. Control ultimately consists of reducing the contact between the vector mosquito and humans. This is accomplished through removing, modifying or treating larval habitats; modification or removal of adult mosquito resting areas, adulticide treatments when indicated; use of repellents. Most Best Management Practices (BMP) utilized in mosquito control districts employ a phased response based upon surveillance data, using only those measures likely to be most effective based upon a variety of bionomic, atmospheric and environmental factors. Such programs should consist of public education emphasizing personal protection and residential source reduction; municipal larval control to prevent repopulation of the area with competent vectors; adult mosquito control to decrease the density of infected, adult mosquitoes in the area; and continued surveillance to monitor virus activity and efficacy of control measures.

The following components may be used concomitantly or at intervals determined by target bionomics, host demographics or environmental factors.

• Larval Control - Mosquito larvae, although air-breathers, require a source of reasonably stagnant water in which to feed and ultimately metamorphose into adults. Larval control is extremely efficient, in that the larvae are confined within the aquatic habitat and are usually concentrated. While this makes possible a variety of strategies to effect control, environmental considerations are of paramount concern.
  • Biological Control - this may involve augmentation of natural predator species such as mosquitofish, but may also include cannibalistic species of mosquito larvae, viruses, fungi, bacteria and predaceous aquatic invertebrates.
    • Fish, most notably Gambusia, are extensively used throughout the country but their use must generally be cleared with local Fish and Wildlife officials.
    • Augmenting or introducing aquatic predators of mosquito larvae alters the local ecosystem in often unforeseeable ways, and should be done with great caution.
• Chemical Control - Because chemical larvicides are to be used in sensitive aquatic environments, they are specifically designed to minimize their impact on non-target organisms. They must be applied, by law, only to a predefined target site whose guidelines are specified on the label. To ensure its effectiveness, the application rate for each larvicide is calculated on the basis of its toxicity profile and degradation characteristics. For example, the application rate for methoprene is calculated to achieve a final concentration in water of between 0.22 to 1.1 parts of product per billion (ppb). This would be equivalent to an initial dose of roughly one drop in an Olympic sized swimming pool. Chemical larvicides roughly fall into the following categories:
  • Bacteria such as the various species of Bacillus are widely used and extremely effective means of control. They must be ingested by the larvae and therefore are less effective in habitats with high organic loads serving as competeing food sources.
  • Insect growth inhibitors constitute insect metamorphosis hormone analogs that prevent the mosquito larvae from molting eventually to the adult stage.
  • Surfactants reduce surface tension of the water, making it impossible for the larvae to attach their breathing apparatus, drowning them.
• Adult Mosquito Control - Adult mosquitoes, being active fliers in a three dimensional space, present a unique challenge for their control. Control methodologies vary with the species involved, their peaks of activity, known resting areas, and other environmental factors.
  • Elimination of resting areas - Eliminating brush and high grass removes places where mosquitoes avoid dessication during their non-active periods. This makes the immediate vicinity less hospitable for questing female mosquitoes.
  • Personal protective measures - Measures to reduce biting include alteration of schedules to avoid peaks of mosquito activity, proper dress when outside, and use of repellents.
  • Encouragement of natural predation on adult mosquitoes - Use of bats and certain bird species has great public appeal, but has been disappointing in terms of reducing mosquito populations.
  • Chemical control - Modern pest management strategies endorsed by EPA and the Centers for Disease Control and Prevention recommends application of adulticides when surveillance indicates that larval control measures have proven inadequate to prevent imminent disease outbreaks. Certified operators trained in the special handling requirements of adulticides apply them after dusk under specified atmospheric conditions when mosquitoes are most active and non-target species are generally not at risk. Adulticides are usually applied in aerosol form of extremely small droplets (10 million of the standard 20-micron droplets could fit inside of a BB) so that they remain airborne to impinge upon mosquitoes in flight at the time of application. The minute droplet size also ensures that products dissipate and degrade quickly, to minimize any deposition of active ingredient on the ground or other surfaces. The low application rates of these aerosols-generally less then ¾ ounce of insecticide per acre treated-further minimizes environmental risk. 
    
    There is a large body of scientific literature demonstrating significantly reduced trap counts after adulticide applications. Since the size of questing female mosquito populations is crucial to disease transmission, it would be prudent to utilize all approved means to reduce these populations below transmission threshold. Adulticide applications should not be the sole means of control in an urban setting. But that is not to argue that adulticides should not be used at all. Even a 30% kill rate would still have a significant impact on disease transmission. 
    
    Adulticides used in the United States fall into two general chemical categories, organophosphates and pyrethroids. The pyrethroids and organophosphates are rotated at specified intervals in mosquito management programs to prevent the mosquitoes from becoming resistant after long-term exposure to a single group of pesticide.
    • Only two organophosphates, malathion (Fyfanon) and naled (Dibrom, Trumpet), are in general use for adult mosquito control. Malathion is a popular choice because of its low price, proven efficacy and low level of toxicity (it's less toxic than table salt). Naled is an extremely effective adulticide when applied aerially.
    • Pyrethroids constitute the other class of adulticides. Four products currently on the market, pyrethrins (MGK), resmethrin (Scourge), sumethrin (Anvil) and permethrin (Aqua-Reslin) are produced from a highly potent chrysanthemum extract. These synthetic derivatives have both a longer shelf life and are as much as 50 times less toxic than the natural insecticide, while performing the same function.