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Severe outbreaks of malaria occurred in the coastal villages of the Candolim Primary Health Centre (PHC) of Goa, India, in 1993 and 1994. These outbreaks were associated with accelerated construction activity with an influx of migrant laborers. The weekly application of Bacillus thuringiensis var. israelensis (B.t.i.) strain 164 at 1 g/m2 and introduction of the indigenous larvivorous fish Aplocheilus blocki in major breeding habitats of Anopheles stephensi replaced ongoing DDT spraying and pyrethrum fogging in June 1994. The objective of this study was to evaluate the effects of B.t.i. and larvivorous fish on An. stephensi and subsequent transmission of malaria in the Candolim PHC, Goa, India. In 1995 the populations of an. stephensi in larger habitats (habitats with immatures: t = 5.19, P = 0.0017; immature density: t = 3.57, P = 0.007) and smaller habitats (habitats with immature: t = 3.86, P = 0.005; immature density: t = 4.93, P = 0.002) and malaria incidence declined substantially (malaria cases: chi 2 = 712, P < 0.001; slide positivity rate: chi 2 = 10.36, P < 0.001; annual parasite index; chi 2 = 15.1, P < 0.001), whereas the incidence of malaria continued to increase in other nearby towns.

Bacillus thuringiensis (H-14), strain 164 (Bactoculicide) when applied at 1 g/m2 surface area successfully controlled Anopheles stephensi breeding in construction sites, abandoned overhead tanks, and curing waters. Subsequent to application, no pupal production was observed in these habitats for 3, 18, and 21 days, respectively. Based on these findings, inclusion of Bactoculicide in the bioenvironmental vector control strategy is suggested and fortnightly spraying in construction sites at 1 g/m2 surface area is recommended for the containment of vector breeding. However, frequent retreatment of abandoned overhead tanks would be uneconomical and operationally impractical.

In a large malaria endemic area in Panaji city, Goa, India, the weekly application of the biolarvicide Bacillus sphaericus (Strain 101, Serotype H 5a 5b) at the rate of 1 g/m2 in the main Anopheles stephensi larval habitats, viz., curing waters, masonry tanks, and sump tanks (under construction), from April to December 1993 resulted in a sharp decline in the habitat positivity (range 0.13-8.0%) as compared with the rest of the Panaji (range 2.2-30.6%) where temephos (Abate) was used as the larvicide. Bacillus sphaericus spraying also led to a significant decline in anopheline densities in positive habitats (range 0-7.3/10 dips) as compared with control habitats (range 0.9-53.0/10 dips). Concurrently, malaria incidence observed in the experimental area (slide positivity rate [SPR] range 2.3-7.8%; monthly parasite index [MPI] range 0.18-1.44) was lower than in the control area (SPR range 14.3-25.5%; MPI range 1.75-6.12).

Adult and larval insecticidal tests conducted in Panaji, Goa, revealed that adults of An. stephensi were resistant to DDT (4.0%), dieldrin (0.4%) and malathion (5.0%). The larvae of An. stephensi were also resistant to DDT (2.5 mg/l) and malathion (3.125 mg/l). Cx. quinquefasciatus adults were resistant to DDT, dieldrin, malathion and fenitrothion (1.0%), and larvae were highly resistant to DDT but showed low resistance to malathion and fenitrothion (0.125 mg/I). Adults of Ae. aegypti were resistant to DDT and dieldrin and the larvae showed resistance to DDT but good susceptibility to fenitrothion and complete susceptibility to malathion. The triple resistance of An. stephensi to DDT, dieldrin and malathion is intriguing as malathion has never been used in the health sector in Goa. A very limited use of malathion was done in the agricultural sector in Chandor area, which is 50 km away from the study area.

A one-year longitudinal study conducted in 9 categories of breeding habitats in Panaji, Goa, showed that 1.1% of the 67,360 breeding sites contained Anopheles stephensi immatures and the overall positivity varied from 0.4 to 3.5% with a peak in June. The habitat-wise proportion of An. stephensi was: wells, 0-1.3%; fountains, 1.4-11.4%; masonry tanks, 0.8-6.1%; overhead tanks, 0.1-4.0%; curing water in construction sites, 0.6-9.0%; groundwater tanks, 0-1.4%; tyres, 0-8.9%; barrels and tins, 0-5.4%; and intradomestic containers, 0-1.9%. An. stephensi was breeding along with An. subpictus, An. vagus, An. barbirostris, Culex quinquefasciatus, Cx. vishnui, Ae. aegypti, Ae. albopictus and Ae. vittatus.