Increasing insecticide resistance in Anopheles funestus and Anopheles arabiensis in Malawi, 2011-2015.

BACKGROUND: Susceptibility of principal Anopheles malaria vectors to common insecticides was monitored over a 5-year period across Malawi to inform and guide the national malaria control programme. METHODS: Adult blood-fed Anopheles spp. and larvae were collected from multiple sites in sixteen districts across the country between 2011 and 2015. First generation (F1) progeny aged 2-5 days old were tested for susceptibility, using standard WHO procedures, against pyrethroids (permethrin and deltamethrin), carbamates (bendiocarb and propoxur), organophosphates (malathion and pirimiphos-methyl) and an organochlorine (DDT). RESULTS: Mortality of Anopheles funestus to deltamethrin, permethrin, bendiocarb and propoxur declined significantly over the 5-year (2011-2015) monitoring period. There was wide variation in susceptibility to DDT but it was not associated with time. In contrast, An. funestus exhibited 100% mortality to the organophosphates (malathion and pirimiphos-methyl) at all sites tested. There was reduced mortality of Anopheles arabiensis to deltamethrin over time though this was not statistically significant. However, mortality of An. arabiensis exposed to permethrin declined significantly over time. Anopheles arabiensis exposed to DDT were more likely to be killed if there was high ITN coverage in the mosquito collection area the previous year. There were no other associations between mosquito mortality in a bioassay and ITN coverage or IRS implementation. Mortality of An. funestus from four sites exposed to deltamethrin alone ranged from 2 to 31% and from 41 to 94% when pre-exposed to the synergist piperonyl butoxide followed by deltamethrin. For permethrin alone, mortality ranged from 2 to 13% while mortality ranged from 63 to 100% when pre-exposed to PBO. CONCLUSION: Pyrethroid resistance was detected in An. funestus and An. arabiensis populations across Malawi and has worsened over the last 5 years. New insecticides and control strategies are urgently needed to reduce the burden of malaria in Malawi.

Risk factors for Anopheles mosquitoes in rural and urban areas of Blantyre District, southern Malawi.

BACKGROUND: Although urban malaria transmission is low and seasonal, it remains a major public health problem. This study aimed at demonstrating the presence of Anopheles mosquitoes and their potential to transmit malaria in urban settings. METHODS: Two cross-sectional surveys were carried out in Blantyre District, Malawi, during the dry and wet seasons of 2008 and 2010, respectively. A map of Blantyre was divided into a grid of 400 cells, of which 60 cells were randomly selected. Five households located within 100 m from the centre of each selected cell were enrolled, a standard questionnaire was administered, and indoor resting mosquitoes were sampled. RESULTS: In 2008 and 2010, a total of 960 and 1045 mosquitoes were collected, respectively. Anopheles funestus comprised 9.9% (n = 95) and 10.3% (n = 108) during the two surveys, respectively. Anopheles gambiae sensu lato (s.l.) was rarely detected during the second survey (n = 6; 0.6%). Molecular identification was performed on samples collected during the first survey, and An. funestus sensu stricto (s.s.) was the only sibling species detected. All the Anopheles mosquitoes were collected from households located in rural areas of Blantyre and none from urban areas. In univariate analysis, the presence of open eaves was associated with increased Anopheles prevalence, both during the dry (incidence rate ratio, IRR = 4.3; 95% CI 2.4 - 7.6) and wet (IRR = 2.47; 95% CI 1.7 - 3.59) seasons. Chances of detecting Anopheles spp. decreased with increasing altitude (IRR = 0.996; 95% CI 0.995 - 0.997) and during the dry season, but increased during the wet season (IRR = 1.0017; 95% CI 1.0012 - 1.0023). These factors remained significant following a multiple Poisson regression analysis. No association was found between insecticide-treated bednet ownership and the number of Anopheles mosquitoes detected. CONCLUSIONS: The presence of An. funestus s.s and An. gambiae s.l. in the periphery of Blantyre city was an indication that malaria transmission was potentially taking place in these areas.