PANNA: Keeping Perspective on West Nile Virus


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Keeping Perspective on West Nile Virus
August 22, 2005

As the West Nile Virus (WNv) has moved across the U.S., a number of communities have responded to the threat of illness and officials’ plans by joining coalitions against pesticide spraying. Until recently, the U.S. Centers for Disease Control and Prevention (CDC) and many local mosquito abatement districts emphasized public education and the control of larval populations as the first line of defense against the virus. But as current experience in the California Central Valley shows, spraying of hazardous pesticides to kill adult mosquitoes is still being used as a primary WNv control mechanism. In these cases, community members are forced to scramble as spray programs are hastily implemented. Local groups need better information if they are to decide if the plans of their local public health and mosquito abatement authorities are truly in the best interests of the health and wellbeing of their communities. Unfortunately that information is not coming from the CDC, and what is offered is highly arguable.

There are a few key facts about the virus and its control. First, CDC estimates that 80% of those infected never show any symptoms. Twenty percent will develop West Nile Fever with flu-like symptoms such as a headache, fever, and muscle aches, but fewer than one percent of those infected will develop the West Nile Neuro Invasive Disease, which can cause inflammation of the brain and spinal cord, and can be life threatening. Those at highest risk for serious illness are the elderly and people who have weakened immune systems.

Second, the most effective virus prevention involves personal protection and control of larval populations (occurring in ponds or other stagnant water sources) rather than the use of pesticides against adult mosquitoes. Effective public health strategies have focused on educating the public on how to help eliminate mosquito breeding grounds (eliminate standing water nearby) and how to reduce exposure to insects by wearing sensible clothing (e.g., long-sleeve shirts), limiting outdoor activity at certain times and by using insect repellents. (Many repellents are available; Pesticide Action Network North America recommends those not containing DEET [N,N-diethyl-meta-toluamide]. See “Ways to Beat DEET” on the PANNA website [ ]. Pesticide treatments for larval control involve much smaller quantities of pesticides with fewer known health effects than those used against adult populations.

In a troubling turn away from the emphasis on larval treatment, public health authorities in some California counties are increasingly using adulticides, arguing that ultra low volume sprays result in no substantial (our emphasis) human exposure. While there is some recent evidence that adulticide applications may decrease mosquito populations, there is still no clear evidence of decreased incidence of West Nile Neuro Invasive Disease. In fact, a survey by a Nashville No Spray Coalition notes that some communities that have not used adulticides as part of their mosquito control programs have reported no significant difference in number of WNv cases compared to cities and counties that have used adulticides.

The main pesticides used against adult mosquitoes are organophosphate pesticides such as naled or malathion, and pyrethroid pesticides including permethrin and d-phenothrin (Anvil). Organophosphates are nervous systems toxins; both naled and malathion have been linked to cancer and malathion has been associated with blood, vision and reproductive disorders. Synthetic pyrethroids can cause dermatitis and asthma-like reactions, are suspected disruptors of human hormone function and possible carcinogens. These pesticides may be sprayed alone, in combinations with one another, or together with “inert ingredients” such as the synergist piperonyl butoxide (a possible carcinogen).

A recent report cited in support of the “no substantial exposure” argument appeared in the June 3, 2005 issue of Morbidity and Mortality Weekly Report of the CDC. The CDC claimed that the levels of naled, permethrin, and d-phenothrin detected in small samples from the three states of Mississippi, North Carolina and Virginia (sample size of exposed individuals were 125, 75 and 83 respectively) “might be too low to cause important human exposure.”

However, the data do not support this claim for several reasons. First, a lack of difference between exposed and unexposed individuals may simply result from having sample sizes that are too small to detect differences. When variation in pesticide exposure is substantial among individuals in a population (for example, the subjects in Mississippi who treated pets with permethrin had much higher levels of the permethrin metabolite, 3-phenoxybenzoic acid, than those who did not), then much larger samples sizes are needed to determine whether differences between “exposed” and “unexposed” groups exist. In the North Carolina study, for example, food was found to be a substantial source of naled metabolite (DMP). By comparison, the CDC’s 2001/2002 study of chemical exposure in the U.S. population tested more than 2,500 individuals for pesticides in their urine and had sample sizes for different compared groups ranging from 500 to 1300 individuals.

Second, the levels of 3-phenoxybenzoic acid (3pba) in the Mississippi study were about four times higher than the average levels measured by the CDC in their 2001/2002 study. The notably high levels in these communities may mask any differences due to spraying. One should ask, “Why is everyone so highly exposed in the Mississippi study?” Similarly, the 3pba post-spray levels in the Virginia study were double the values in the CDC national study.

Third, and perhaps most importantly, no one can claim that application of these pesticides “is safe.” The known health effects of these pesticides are enough to negate that claim. Furthermore, full information on the health effects of these pesticides is unknown. The pesticides in question have not yet been evaluated for several chronic or long-term health effects. No information is available on whether naled can impact the human hormone system. Permethrin and d-phenothrin have yet to be evaluated regarding their toxic effects on reproduction and development. We know that the pesticide synergist piperonyl butoxide is listed as a possible carcinogen, but it has yet to be evaluated regarding its effects on human development or on human hormonal or reproductive systems. Furthermore, in the real world we are never exposed to single pesticides alone. We know virtually nothing about the health impacts of either these chemicals together or of these chemicals in combination of the myriad chemicals we already carry in our bodies.

In the light of the many unknown risks of these hazardous pesticides, activists are working in many U.S. communities to protect the public and the environment from unnecessary exposure. The National Alliance for Informed Mosquito Management (AIMM) represents more than 30 organizations and individuals promoting safer, least-toxic methods of managing mosquitoes. Community activists have successfully worked with local mosquito control boards to suspend aerial spraying and to implement safer control methods. Often mosquito control boards will provide the options for residents to join “no spray” lists that may (or sometime may not) be honored when spraying occurs. Usually this applies to ground-level sprays and not aerial sprays.

For more information, see Beyond Pesticides and Californians for Pesticide Reform,

Sources: PANNA website (; CDC, Human Exposure to Mosquito-Control Pesticides- Mississippi, North Carolina, and Virginia, 2002-2003 , Morbidity and Mortality Weekly Report (MMWR), June 3, 2005,; CDC; The National Report on Human Exposure to Environmental Chemicals , July 2005 ,; No Spray Nashville,; AIMM,
Contact: PANNA

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