Modified Response of Anopheles Albimanus to DDT Residual House Spraying in Panama

SummaryFrom observations conducted in 1944 to 1946 in a group of native villages on the middle Chagres River in Panama it had been concluded that the DDT residual spraying of dwellings initially reduced the malaria transmission by Anopheles albimanus in houses in three ways. During the first four months of 1952, following the twentieth spraying of one of these villages and the thirteenth spraying of another, it was found that two of these effects are no longer apparent. The gross numbers of A. albimanus in dwellings are no longer drastically reduced as was formerly the case, and the selective killing of engorged albimanus is no longer evident. A large reduction in the percentage of albimanus which successfully engorge with blood in DDT treated houses is still present to about the same degree as at the time of the initial observations in 1944 to 1946. This last appears to be the principal way in which albimanus is affected by DDT to modify the malaria transmission potential at the present time.A possible explanation is given of how this change in the way albimanus responds to DDT residues may have resulted from the selection of a population which has become resistant and/or hyperirritable to DDT. Laboratory experiments to measure the contribution of each of these factors quantitatively are in progress.The modification in the response of A. albimanus to DDT residual house spraying has not as yet been reflected in any rise in malaria rates at these villages.