Field trials of different traps for malaria vectors and epidemiological investigations at a foot-hill basin in Kanchanaburi, Thailand

Abstract

Malaria endemicity and vector prevalence were investigated by using biting collections and trapping systems in a malaria endemic area in Kanchanaburi, Thailand, in February 1987-January 1988. A stream-bed in the village of Ban Tha Lam Yai was selected as the study area. Buffalo bait caught the highest number of species (16) and individuals of anopheline mosquitoes; 4 times more adults of Anopheles minimus and 3 times more of those of A. maculatus were collected than obtained using human bait. High parous rates of the 2 vectors throughout the year, 50% in September and 90% in May for A. minimus, 43% in October and 74% in March for A. maculatus, seemed to account for the high Plasmodium infection rates of these mosquitoes and the stability of malaria endemicity in the village. The vectors were efficiently attracted to sound traps when these were used with a hamster bait and dry ice (carbon dioxide). The number of vectors trapped was 3 times more than that with the human bait, but nearly half of that with the buffalo bait. Sound traps segregated A. minimus into 2 groups according to the attractiveness at different sound frequencies: one at 500-600 Hz and the other at 800-1000 Hz. A. maculatus was, however, attracted over a wide range of frequencies (between 350 and 1000 Hz), though numbers were smaller than for the other species. ADDITIONAL ABSTRACT: In a village in Thailand, in a foothill area, where malaria was common (the parasite rate from active case detection was 29% in 1986 and 24% in 1987, with Plasmodium falciparum slightly in excess of P. vivax), several mosquito-catching techniques were employed including unusual methods utilizing wing-beat sounds of various frequencies with and without animal (hamster) bait and dry ice. With buffalo-bait traps 20 different species were caught (if you consider Anopheles maculatus a complex of several sibling species). Only 4 of them were caught biting man, however- An. minimus and forms A, B and G of An. maculatus (form A was the most common form of An. maculatus caught). Parity in An. minimus varied from 0.5 in September to 0.9 in May and in An. maculatus [forms not specified] from 0.43 in October to 0.74 in March (in the years 1987 and 1988). It is pointed out that the lower values coincided with an influx of newly emerged females. The wing-beat frequencies (along with hamster bait and dry ice) attracting the most An. minimus were 500-600 Hz and 800-1000 Hz. An. maculatus [forms again not specified] was attracted to a more limited frequency range. [There is evidence that wing-beat frequency increases with age within a single species.] G. Davidson