CDC Offers Weak Numbers on West Nile Spraying

Before we blanket ourselves with pesticides, we should clear some things up.

Wait. Take five. That's all I'm asking. As we approach mosquito season in Dallas, the conversation about aerial spraying for West Nile once again begins to sound like free beer day at the ballpark.

Last week The Dallas Morning News published a story with a triumphal tone saying, "Aerial spraying of insecticide last summer was effective in reducing severe illness from the West Nile virus in North Texas counties" according to "a federal report released Tuesday."

But knowledgeable people are reading the same report and coming to very different conclusions. The report was released last week by the U.S. Centers for Disease Control and Prevention, an arm of the Department of Health and Human Services.

Jared Boggess


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First: Why do we care? We care because the risks may be enormous. Science is coming to a finding that the kind of spray disseminated over Dallas last summer brings with it serious health hazards. Earlier this year Dr. David Bellinger of the Harvard School of Public Health and Boston Children's Hospital spoke to a Dallas teleconference on West Nile to warn Dallas officials about the substances sprayed here.

Bellinger said pesticides can have stealth effects on brain development in young children that may not show up for decades. He said these effects, "do not necessarily bring a child to medical attention, but can nevertheless limit their futures by causing intellectual deficits and abnormal behavior."

Last year I spoke with Sheldon Krimsky, a professor of urban and environmental policy planning at Tufts University in Massachusetts and author of several books dealing with public policy and environmental risk. He told me the most dangerous elements in what was sprayed here last year are not the active mosquito-killing chemicals but additives designed to shut down a mosquito's endocrine system.

The danger is that these substances do the same thing to our own endocrine systems, at least temporarily, and it's the endocrine system that minutely controls brain development in early life. For that reason, any decision to spray these chemicals from airplanes on residential areas must be carefully balanced against the risk. He said, "It's not a free ride in other words."

The CDC report released last week found a "measurable impact in preventing West Nile neuroinvasive disease." But the question asked by the critics is how measurable.

Neuroinvasive is the kind of West Nile that causes paralysis and often death. Non-neuroinvasive is another type of the disease that causes flu-like symptoms but not paralysis, lasting for a day to several weeks.

Some of the experts I spoke with last week said that on close examination the only measured impact they could find in the report was so slight as to seem almost like statistical sophistry. Gene Helmick-Richardson, a Ph.D. entomologist and pest control consultant in Dallas who has been a skeptic of the spray campaign here, said he did some work to dig out hard numbers on effectiveness in terms of human infections, something the report did not explicitly provide.

By drawing from one table in the report, Helmick-Richardson says he found that "you had a reduction of two tenths of 1 percent per hundred thousand [population], which means you might have five cases of neuroinvasive disease that were prevented by spraying three million dollars worth of pesticides."

Even this scant finding of a positive effect on human health is in doubt, some experts say, because the CDC tossed out one entire type of West Nile disease, the non-neuroinvasive form, from its computations. Because some numbers in the report seem to show the non-neuroinvasive form may have been more resistant to spraying than the other kind, critics told me they think tossing it out raises the possibility that including it would have wiped out the spray campaign's overall very narrow margin of success.

I took that question to Susan Hills, one of the authors of the report. She said by email: "We did not conduct an analysis that included neuroinvasive and non-neuroinvasive disease cases so I can't tell you what the results would be."

Dr. Laurence Boyd, a Dallas entomologist who has expressed skepticism about the spray campaign in the past, told me that the CDC's stated reason for tossing out non-neuroinvasive cases doesn't add up.

The report cites as its reason a "possible increase in the diagnosis of non-neuroinvasive disease in the treated areas after aerial spraying." It's a simple idea. Attendant on the spray campaign is a whole raft of publicity. More doctors are looking for West Nile, so more cases are found instead of being misdiagnosed as cold or flu.

But Boyd said that doesn't fly as a reason for not including non-neuroinvasive cases. The report is based on a comparison of last year's West Nile infection rates in sprayed areas with rates in areas that were not sprayed. Boyd points out the effect of publicity would have been the same for sprayed and non-sprayed areas.

"The publicity in the areas that were sprayed is mostly television," he said. "The television stations also reach the areas that were not sprayed."

So if TV stories increased the vigilance of doctors in the sprayed areas, it would have increased their vigilance by the same amount in non-sprayed areas. The comparison would still have been valid. So why was non-neuroinvasive tossed out?

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Bellinger is half right when he warns that pesticides can have "stealth" effects on brain development in young children. Meanwhile, prevenable diseases like West Nile do most certainly have permanent effects on children, death for instance.


Actually you are quite fast and loose not to mention inconsistent with your use of "can" and "do." West Nile does not have these effects in everyone as some develop the milder form of the disease and more importantly there are safer, more effective ways to control mosquitoes according to entomologists, like the ones noted in this article. It is this very seriousness of West Nile that calls on us to use the most effective mosquito control strategies, which also happen to be the safest. The immature stages of the mosquitoes represent as much as 98 percent of the mosquito life cycle during the mid months and are not affected by adulticide pesticides that are used to aerial and truck spray. Leading entomologist generally agree that targeting the biggest part of the life cycle represented by the eggs, pupae and larvae is the most effective way to control mosquitoes. BTI larvicide kills larvae before they ever turn into adult mosquitoes that can bite and infect humans with West Nile. When mosquito breeding hot spots are aggressively targeted with larvicide it cuts the problem off at the quick. BTI Larvicide has also been used for decades and has shown no insect resistance. The Entomologist at Rutgers University who regularly review the New Jersey Mosquito Plan and make recommendations have said insect resistance is comparable to bacterial resistance to antibiotics, a serious public health threat.  (  They state that spraying should be "avoided" particularly over large "contiguous areas" because of the threat of insect resistance and the public health effects. And yet the county here sprayed almost an entire densely populated county. Sacramento, which has been held up here as an example of successful aerial spraying, specifically avoids aerial spraying populated areas because of the health effects and they ONLY aerial spray because of the 50,000 acres of rice patty fields, which makes it difficult to larvicide. They also emphasize larviciding and public education over spraying. Even though they have sprayed less than Dallas did last summer, they are already experiencing significant insect resistance in Sacramento and have had to start using much more toxic organophosphate pesticides that are in the same chemical family as nerve gas and have been associated with autism, ADHD, lower IQ and cancer in children. And so your dismissiveness of the health effects is inappropriate. The Rutgers entomologist also note that pyrethroids, which were sprayed here last summer, are endocrine disruptors, which have been associated with cancer, and should be used "rarely."   While  BTI larvicide, which is the most effective way to target mosquitoes before they can even bite you, has negligible human health effects, there is a substantial growing body of evidence showing long term health effects in children opposed to low level pesticides. Two major reports were released just in the last couple of months, including one by the American Academy of Pediatrics, offering stern warnings on the effects of pesticides in children. See these links for copies of the reports:   There is also the public health threat of creating a level of resistance where no pesticide is effective in controlling the adults. Spraying also can pose the risk of increasing the larval population, which can lead to more mosquitoes, which is exactly what happened according to the CDC report. The only long term study of aerial spraying in New York's  Cicero Swamp found 11 years of sprayng there led to a 15 fold increase in the disease carrying species of  mosquitoes.  There can be a time and a place for using adulticides, but they are best used in an extremely targeted and limited way so as to not lead to resistance, killing off of predator insects, disruption of the ecocystem that is driving the explosion of diseases like West Nille according to leading research, and insect resistance. (  Leading integrated pest management experts say targeted spraying should only be a last line of defense after implementing an intensive program of larviciding, source reduction, code enforcement and education, which is the best, most effective, safest approach. It's also important to consider the secondary public health effects of spraying that can result from insect resistance. IF we eventually create a mosquito that no longer responds to the limited pesticides we have, we can end up in a situation where we have no effective adulticides to use in the middle of an outbreak, much like we are seeing happening with certain bacterial infections that no longer respond sufficiently to the current arsenal of antibiotics.