Preliminary Notice Concerning the Selection of an Adulticide for the Control of Mosquitoes to Prevent the Transmission of West Nile Virus in Quebec and Elsewhere in Canada

In the event that West Nile virus (WNV) enters Canada, the control of the principal vector would be one of the key components of the efforts to limit the spread of this virus. At the present time, it appears that mosquitoes are the primary vector of WNV to humans and other animals.

The use of insecticides for this purpose is not without certain human health risks. For that reason, it is critical to select the product that offers the highest level of safety, both for the public and for workers who apply the treatments.

To determine what adulticide offers the highest level of safety, we evaluate the principal toxicity indices of the potential products identified for this type of work namely malathion, resmethrin, permethrin, propoxur and dichlorvos.

Given the lead time required to draft this notice, we focussed primarily on the most recent studies and those that most effectively met the new toxicity assessment requirements.

In order to take account of the most recent data concerning the inherent toxicity of malathion, our evaluation is based primarily on two documents: the most recent revision to the preliminary risk assessment for the eligibility of the product for reregistration by the United States Environmental Protection Agency (EPA) and the most recent toxicological and environmental assessment document produced by the Joint Food and Agriculture Organization (FAO)/World Health Organization (WHO) Committee on Pesticide Residues.

In addition to the toxicological data specific to malathion, we also present a summary of the principal data for malaoxon, the metabolite primarily responsible for the toxicity of malathion.

The data concerning pyrethroids (resmethrin and permethrin) are more limited because these products are not currently considered priorities for toxicological reassessment. The data presented comes largely from the evaluations conducted by the World Health Organization.

In the case of propoxur and dichlorvos, the data are mainly drawn from decision documents of the U.S. EPA.

On the basis of a comparison of the available toxicological data, we were able to recommend an adulticide for control of mosquitoes. In our view, this recommendation will make it possible to ensure greater safety of mosquito control operations, should such a need arise.

Whenever possible, resmethrin should be considered the adulticide of choice for the control of mosquitoes potentially infected with WNV.

Immediate steps should be taken to obtain emergency registration for resmethrin for the control of mosquitoes using aerial-based ULV equipment.

Regardless of which product is selected, effective preventive measures will have to be taken to ensure safe and effective use of the adulticides with a minimal impact on the environment (e.g., communications plan, protection measures for the public and insecticide applicators, programs to monitor for potential health effects in the general public and applicators, non-target effects, etc.).

Onil Samuel
B. Sc., expert et conseiller scientifique, Institut national de santé publique du Québec
Louis St-Laurent
M. Sc., conseiller scientifique, Institut national de santé publique du Québec
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