When Bugs Carry Disease: Anticipating the Return of the West Nile Virus

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With the advent of Spring, we can expect the return of the mosquito an insect which can be just plain bothersome or it can be a vector, or carrier, of disease. Mosquitos right now are laying their eggs in salt marshes, backyard puddles, abandoned water-filled buckets and anywhere else they find stagnant water. In a matter of weeks, these insects will mature and the biting of humans and animals will begin. Mosquitos are of particular concern this year in the Northeast because there is evidence that they may carry the potentially deadly West Nile encephalitis virus. The virus killed seven people and caused brain infections in 62 others in the New York metropolitan area last year.

As part of their broad approach, public health authorities in the New York area are already urging residents to eliminate all stagnant water from their yards, to change birdbaths frequently and to make sure their swimming pools are adequately chlorinated. Further efforts are being made to control the mosquito population by using larvicides. Yet some environmental activists want to tie the hands of public health authorities by prohibiting the next step in this integrated pest management approach: widespread aerial spraying of pesticides. But sound science dictates that officials will render those activists silent this spring, and protect the public health by responsibly spraying pesticides including malathion to kill mosquitos. Does the responsible and regulated use of these pesticides pose a risk to human health as some advocates have claimed?

Actually, malathion is one of the safest and most beneficial pesticides currently in use. It is precisely because it is so useful that it is drawing attention from those who oppose the use of chemicals simply because they are "chemicals." Although there have been sporadic media reports that the chemical is "carcinogenic", there is no evidence that it increases cancer risk in humans. The "carcinogen" label comes from animal experiments. However, a substantial number of natural food chemicals (like hydrazines in mushrooms and safrole in spices) may also cause cancer in animals but pose no human cancer risk.

Mosquito control is what pesticides like malathion do best and they have been used safely and effectively for years. But are pesticides risk-free? Certainly not. It depends upon how they are used. Used irresponsibly, they can be hazardous or deadly. After all, pesticides are designed to kill pests, so common sense dictates that "the dose makes the poison." And the limited amounts sprayed to protect us from mosquitos are the low doses that do not endanger human health. With its regulated, approved use malathion has never posed any human health hazard. And what are the risks of not using malathion? Clearly, as we have seen with the West Nile encephalitis virus, the health risks are real, not merely hypothetical.

When and if widespread malathion spraying is ordered late this spring and summer, there will inevitably be those who will argue that the "precautionary principle" should be applied, that is, "just in case" malathion spraying could pose a health threat, it should be withheld. But how would one explain such "precautionary" policies to the dozens, hundreds or even more people infected with the West Nile virus because the most effective mosquito control measures were not instituted? Public health measures need to be based on scientific and medical realities, as well as prudence.