Aedes Aegypti Surveillance in Tobago. West Indtes

During 1983-88 Aedes aegyptiwerc collected using larval surveys and ovitraps in Tobago, W.I. The main artificial containers used by Ae. aegypti were drums (35.4%) and small miscellaneous containers (23.4%). From a total of 15,600 ovitraps exposed, 183 ovitraps (1.3%) were positive, with 25 containing Ae. aegypti eggs and 158 with Haemagogus equinus eggs. No Aedes albopict&s eggs were detected in Tobago during this study. Tobago maintained its Aedes aegypti (Linn.) eradication status between 1960 and 1982, despite episodes ofreinvasion (Chadee et al. 1984). The Insect Vector Control Division (IVCD), Ministry of Health detected Ae. aegypti at the Scarborough Wharves and at Lambeau, Tobago, in 1982 but quickly brought these reinfestations under control by ULV adulticiding (malathion), residual spraying (fenthion) and focal treatment (temephos). By the end of 1982, Tobago was again considered free of. Ae. aegypti (Chadee et al. 1984). Although Ae. aegypti was eradicated from the island, 14 other mosquito species were found in artificial containers, including the primary vector of malaria, Anopheles aquasalis Curry, and 2 sylvan vectors of yellow fever, Haemagogus celeste Dyar and Nunez Tovar and Hg. equinus Theobald. No cases of dengue or yellow fever have been reported from Tobago despite urban (dengue) and sylvan (yellow fever) outbreaks of these diseases in Trinidad (Hamilton 1979, Chadee 1984, Tikasingh et al., in press). This paper reports on the Ae. aegypti surveillance program conducted in Tobago from 1983 to 1988. The geographical distribution and abundance of Ae. aegypti were studied throughout the island of Tobago, West Indies, on a house to house basis. To assess the distribution of Ae. aegypti, and to evaluate its association with other species occurring in drums and other types of artificial and natural containers, all containers were examined and immature mosquitoes collected folIowing the PAHO (1968) guidelines. All mosquitoes encountered were collected using strainers and dippers, placed into vials, labeled according to container tlpe and locality and sent to the IVCD laboratory for rearing and identification. Twice each year, vigilance surveys were conducted by IVCD Trinidad staff as outlined in Chadee et al. (1984). Fifty modified ovitraps (Fay and Eliason 1966) were used to detect the presence and distribution of Ae. aegypti and, Aedes albopictus (Skuse) at all ports ofentry and several vulnerable areas on the island. The placement of ovitraps was in accordance with the criteria suggested by Jakob and Bevier (1969). Ovitrap surveys were carried out for 6 years (1983-88) and paddles collected each week. At each inspection the paddle (labeled according to site number and Iocality) was removed and the contents of the ovitrap were examined for larvae. If larvae were present, they were collected and then the water was discarded. The ovitrap was washed, refilled with fresh water and an egg-free paddle added. Upon removal, each paddle was placed in a plastic bag and sent to the IVCD laboratory, where paddles were examined under a microscope (at x40) for eggs. Egg-bearing paddles were soaked in water for hatching. The larvae obtained were reared to the adult stage to ensure correct identification. Figure 1 shows the 6-year average monthly collections of Ae. aegypti immatures and rainfall measured in Tobago. Fluctuating patterns of container usage were observed and this coincided at times with the rainfall pattern (June, July, September and November). The geographical distribution of Ae. aegypti in Tobago is shown in Fig. 2. The results show most of the island from Charlotteville to Bon Accord infested at one time or another with Ae. aegypti. The associated container breeding mosquitoes collected revealed that Limatus durhamii Theobald, Cx. quinquefasciarlus Say and Hg. equinus were the 3 most common mosquitoes in Tobago. Aedes aegypti larvae were found in many types of artificial containers and were associated with other mosquito species, namely Ae. fuluithorax (Lfiz), An. aquasalis Cwry, Anopheles sp., Cx. quinquefasciatus, Haemagogus celeste, Hg. equinus, Lm. durhamii, Trichoprosopon digitafum (Rondanll and Toxorhynchites sp. The collection of Anopheles species in artificial containers is rare, but the foci were in close proximity to their natural habitats (see Chadee et al. 1984). The containers most frequently used by Ae. aegypti werc steel drums (35.4%) and small misMARCH 1990 Opnnauoull AND ScTENTIFIc NorES r49 cellaneous containers (23.1%); followed by tins (Ll.l%), vases (8.5%), flower pots (7.07o), tftes (4.8%), water tanks (3.8%),l ive plants (3.3%) and brick holes (3.0%). Of interest were the collections of Ae. aegypti from live plants (natural containers) including tree holes, papaw stumps (Carica pepa.ya (Linn.)), wild tania, (Xanthosoma saEitofoliunx (Linn.) Schott) and water Iilies (Lilium sp.). The 2 latter natural containers may represent new habitat types for Ae. aegypti in the Caribbean region. An analysis of the frequency of usage of small, J F M A M J J A S O N D