Status of insecticide resistance in malaria vector, anopheles culicifacies in chhattisgarh state, india

Successful eradication of malaria in different parts of the world was achieved mainly by chemical insecticides for vector mosquito control 1 . Generally, two types of vector control methods, namely indoor residual spraying (IRS) and long-lasting insecticide-treated nets (LNs) are used for controlling malaria transmission 2 . Development of insecticide resistance in malaria vectors to conventionally used insecticides has been a serious impediment experienced by many vector control programmes. Therefore, monitoring of insecticide resistance and its changing trend in target species are basic requirements to rationalize its use in malaria control programme. In Chhattisgarh state, Anopheles culicifacies (Diptera: Culicidae) is the dominant malaria vector species supported by An. fluviatilis in the hilly forested regions of the state. To date, DDT (organochlorine), and synthetic pyrethroids have been used for IRS in the national malaria control programme in the state. Continuous use of DDT for IRS since the inception of the programme has led to widespread resistance in An. culicifacies in the country 3–5 . Later, in 1970s malathion and in 1990s synthetic pyrethroids were introduced for IRS and the species developed resistance to these insecticides as well 6,7 . The use of pyrethroids for vector control further increased as it was used in IRS and also for the treatment of mosquito nets (ITNs). Chhattisgarh has a total population of about 25 million living in 16 districts and nearly 44% of the area is forested. Malaria is a major public health problem and the state contributes about 13% of the total malaria cases reported in the country 8 . The annual parasite incidence (APIcases/’000 population/year) during 2009 was 5.2 (Source: National Vector Borne Disease Control Programme). Due to high malaria endemicity in three southern most districts, IRS with two rounds of synthetic pyrethroids annually was introduced in the programme during mid-nineties. Rest of the districts with low (central) and high (northern) malaria endemicity continued to be sprayed with two rounds of IRS with DDT. In the present study, susceptibility tests on An. culicifacies were carried out in 11 districts of the state. Anopheles culicifacies mosquitoes were collected from one or more villages either within the district or in the congruent 2–3 districts with similar ecotype, vector prevalence and vector control activities to assess susceptibility status to DDT, malathion and deltamethrin in the year 2009 (May, July–September and November) and 2010 (January). Female mosquitoes were collected in the early morning hours using an aspirator and a torch light 9 from human dwellings and cattlesheds and transported to field laboratory in a cloth cage covered with a moist towel. Mosquitoes were identified to species based on morphological characters using standard identification key. Mosquitoes were exposed to insecticide-treated papers of DDT (4%), malathion (5%) and deltamethrin (0.05%) procured from Vector Control Research Unit, University Sans Malaysia. After an exposure to insecticide for 1 h, mosquitoes were held for 24 h holding period at 27±2oC temperature and 70–80% relative humidity and percent mortality was determined. Mortality was corrected using Abbott’s formula when the mortality in control replicates was between 5 and 20%. Tests with control mortality of >20% were discarded. The data are interpreted in three classes as per the WHO criteria of susceptibility, status namely susceptible – mortality >98 and, verification required (tolerant) – 81 and 97%, and resistant—mortality < 80% 10 .