Studies on prevalence of anopheline species and community perception of malaria in Jaffna district, Sri Lanka.

BACKGROUND & OBJECTIVES: Over two decades of civil unrest and the conflict situation have had detrimental effects on vector control activities and management of malaria in Jaffna district which is an endemic region for malaria in Sri Lanka. With the background that only a few small-scale studies on malaria and its vectors have been reported from this district, a study was designed to explore the current status of malaria in the Jaffna district in relation to vector and community aspects. METHODS: Adults and larvae of anopheline mosquitoes were collected monthly from selected endemic localities. Species prevalence of the collected mosquitoes was studied while the collected adults of Anopheles subpictus, a potential vector in the district, was screened for sibling species composition based on morphological characteristics and exposed to common insecticides using WHO bioassay kits. Knowledge, attitude and practices (KAP) of the community were tested using a pre-tested structured questionnaire in high-risk and low-risk localities in the district. RESULTS: The anopheline mosquito species distribution in the district was--An. culicifacies (0.5%), An. subpictus (46%), An. varuna (4%), An. nigerrimus (44%) and An. pallidus (5.5%). Among the collected larvae the percent prevalence of An. culicifacies was 13% and other species follows as: An. subpictus (71%), An. varuna (4%), An. nigerrimus (10%) and An. pallidus (2%). Sibling species B, C and D of An. subpictus were present in the district with the predominance of B in both coastal and inland areas, while all members showed both indoor and outdoor resting characteristics, they were highly resistant to DDT (4%) and highly susceptible to malathion (5%). KAP study in the district showed a reasonable level of knowledge, positive attitude and practices towards malaria. CONCLUSION: An. subpictus, the reported major vector of Jaffna and a well-established secondary vector of malaria in the country, continues to be the predominant anopheline species. The distribution of sibling species of An. subpictus complex in the Jaffna district, revealed for the first time, has implications for future studies on its bionomics and malaria transmission pattern in this area and the planning of control strategies for this region. The community perception of disease, which revealed a satisfactory knowledge indicates the potential for better community participation in future malaria control activities in this region. As potential vectors are still present, health authorities need to be vigilant to prevent any future epidemics of malaria.

Insecticide cross-resistance spectra and underlying resistance mechanisms of Sri Lankan anopheline vectors of malaria.

Present status of insecticide resistance was investigated in two major vectors of malaria; An. culicifacies and An. subpictus, collected from a high malaria transmission area in Sri Lanka during 1996/1998. Adult and larval bioassays were carried out to obtain log-probit mortality lines for malathion, propoxur, permethrin and chlorpyrifos. Respective LD50 values were 4.45%, 0.002%, 0.16% and 0.001% for An. culicifacies and 0.66%, 0.004%, 0.004% and 0.04% for An. subpictus. Adults were also tested for WHO standard discriminating dosages of malathion, propoxur, permethrin, DDT, cypermethrin, deltamethlin and lambda cyhalothrin. Both populations were highly resistant to DDT. An. culicifacies was more resistant to malathion and An. subpictus was more resistant to chlorpyrifos. About 25% of both populations were resistant to permethrin. An. culicifacies was susceptible to propoxur, deltamethrin and lambda cyhalothrin and An. subpictus to cypermethrin and lambda cyhalothrin. Adult mosquitos were individually tested for their insecticide detoxifying enzyme activities and altered target-site, acetylcholinesterase. High general esterase activity indicated the presence of amplified esterase genes in both populations. Native gel electrophoresis resolved one elevated esterase isoenzyme, with high affinity to organophosphates, from each species. Malathion carboxylesterase mechanism was present in both populations. Higher glutathione-S-transferase activity was marked in An. subpictus. Synergistic studies showed the possible involvement of monooxygenases in resistance in both species. Acetylcholinesterase activity of approximately 80% of both populations was not inhibited by a standard dosage of propoxur. Low resistance to carbamates shows that the impact of agricultural pesticides is not significant in the development of resistance especially in An. culicifaies. Pyrethroids, other than permethrin, can be successfully used in vector control programs. Carbamates will be an alternative.