Investigations on the Density and Breeding Habitats of Aedes Mosquitoes in Dengue Epidemic Areas in Taiwan

Immediately following the discovery of dengue epidemics in southern Taiwan from December 1987 to October 1991, investigations on the vector mosquitoes and their ecology, especially related to the epidemiology of dengue, were undertaken in nine localities in Taiwan in which a number of indigenous dengue cases were found Results indicated that the density of Aedes aegypti was markedly higher than that of Ae. albopictus in the nine areas under investigation in southern Taiwan. During an epidemic which occurred in November 1989 in the Sanmin district of Kaohsiung city, no Ae. albopictus were found. The data obtained from the present study suggests that Ae. aegypti was involved in the transmission of dengue fever in Taiwan. Another finding in the same district in October 1991 revealed that Ac. aegypti with a low density figure of two (house index of 5%, container index of 3%, and Breteau index of 6) could still maintain the transmission of dengue in its endemic area. This is the threshold density of Ae. aegypti in relation to dengue transmission in Taiwan. However, transmission of the virus is enhanced by a dense human population. Surveys made in the nine localities with dengue epidemics reveal that Ac. aegypti breeds primarily in flower vases and buckets, while Ac. albopictus breeds mainly in discarded automobile tires and earthenware jars. The percentage of breeding sites used by both Aedes species was 32.3% in ornamental containers, 30.0% in discarded containers, 24.6% in water storage containers, 10.4% in refrigerator receptacles and 2.7% in flooded basements. More breeding sites of Ac. aegypti and Ac. albopictus occur in outdoor containers than in indoor ones.