Impact of Pre-seasonal Focal Treatment on Population Densities of the Mosquito Aedes Aegypti in Trinidad, West Indies: a Preliminary Study

A new pre-seasonal focal treatment strategy against Aedes aegypti L. (Diptera: Culicidae) mosquitoes is described for the first time. It was evaluated for 25 weeks using ovitraps, and larval and pupal surveys in the townships of Curepe (treated) and St. Joseph (untreated), Trinidad, West Indies. Both townships were similar with respect to number of houses, size of human populations, the number of Ae. aegypti infested houses and containers. In March 2003, a total of 9403 containers were inspected, of which 1.4% in Curepe (63/4499) and 1.3% in St. Joseph (64/4904) were positive for Ae. aegypti immature stages. Curepe had a lower percentage (12.7%) of the main type of breeding container present (small miscellaneous containers) than St. Joseph (28.1%). Following focal treatment during the month of April (2-3 weeks before the onset of the rainy season), the Ae. aegypti population declined significantly (P>0.01) from a Breteau index (BI; proportion of containers positive for larvae and pupae) of 19.0 to a minimum of 6.0 and a pupae/person index (PI) of 1.23 to a minimum of 0.35 in May, while in the untreated town of St. Joseph, the BI steadily increased from 23 to 38 and the PI rose from 0.96 to 2.00 in August. Similar declines in other measures of population density (the number of positive houses and number of eggs collected in ovitraps) were observed in Curepe, while St. Joseph maintained PI of >1.50 and BI of >28. Furthermore, the Curepe Ae. aegypti population did not return to pre-treatment levels until 9-11 weeks after treatment, far beyond the 6 weeks normally expected during vector suppression campaigns. The results suggest that timely application of pre-seasonal focal treatment with temephos together with standard control measures, such as source reduction of the most productive containers, can reduce the Breteau index to <5 and the pupae/person index to <0.71 (i.e., below the suggested dengue transmission thresholds for Trinidad), and extend the duration of vector suppression.