Environmental Health Reports

Overkill: Why Pesticide Spraying for West Nile Virus in California May Cause More Harm Than Good

8/21/2003

Executive Summary

Since its emergence in New York City in 1999, West Nile Virus (WNV) has spread rapidly across the United States. The disease, borne by wild birds and transferred to humans by bird-biting mosquitoes, is likely to reach California shortly. If and when WNV does arrive, California communities must be prepared to respond in a manner that prevents harm to human health and the environment. In doing so, California can and should avoid the massive pesticide spraying programs that have been triggered in other states at the first sign of West Nile Virus.

Broadcast pesticide spraying, by truck or aerial application, has not been proven effective in curbing WNV:

• The Centers for Disease Control and Prevention have stated that ground and aerial spraying targeted at adult mosquitoes is one of the least effective mosquito control techniques.

• Northeastern communities (Boston, NYC) that first responded to WNV with massive spraying subsequently scaled back their use of adulticides, prioritizing preventative measures and establishing stricter criteria to limit adulticide spraying.

• Despite three years of widespread spraying to control WNV, no scientific studies have demonstrated that such spraying has effectively reduced the human risk of infection.

Spraying may cause more harm than good:

• Pesticide spraying may actually increase the number of mosquitoes by killing off insect predators such as dragonflies that feed on mosquitoes and their larvae.

• Pesticide spraying may increase infection rates by leading mosquitoes to develop resistance, live longer, exhibit more aggressive biting behavior, and become more susceptible to infection by WNV.

• Pesticide spraying may create a false sense of security, diminishing public participation in preventative public health measures that are necessary to effectively reduce the risk of contracting WNV. Such measures include wearing protective clothing and helping reduce mosquito habitat by eliminating stagnant water that serves as a breeding ground for mosquitoes. Pesticide spraying will expose human beings and nontarget organisms to chemicals known to affect human health and the environment:

• For spraying to be effective at all, it must be timed during the hours when the mosquitoes are most active (for most species, the early evening). However, these same times entail the greatest risk of exposure to the general population.

• The chance of any one individual becoming seriously ill from exposure to West Nile may be significantly lower than an individual’s chance of becoming ill from pesticide exposure. For example, in 1999 there were 59 known cases of meningitis due to WNV infection in New York City, and 187 individuals who reported experiencing illness after malathion exposure. California’s current West Nile Virus Response Plan is overly permissive of dangerous and ineffective pesticide spraying:

• Current pesticides approved for mosquito control in the state include organophosphates (malathion) and pyrethroids (Pyrethrin, Sumethrin, Resmethrin) known to have serious human health impacts.

• Human health risk assessment studies, conducted to show these pesticides are theoretically "safe" if applied correctly, routinely fail to account for errors in application rates and vulnerability of certain populations, such as infants and the elderly. To ensure minimal environmental and human health impact, and maximum effectiveness in mosquito control, the state plan should be revised to:

• Include strict parameters limiting the use of health-threatening pesticides.

• Include specific benchmarks to help promote public outreach, communication, and education activities essential for a preventative public health strategy.

In addition, local mosquito and vector control agencies, which will have significant decision-making power to choose among mosquito control options, should immediately initiate a public process in which concerned community members can be involved in outreach and education about mosquito prevention activities as well as the establishment of strict local thresholds to reduce or eliminate the use of pesticide sprays in mosquito control.

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