Correlations between age, distance from aquatic habitats and pyrethroid resistance status of field populations of the malaria vector, Anopheles funestus in south-eastern Tanzania

Abstract Background: Malaria transmission can be highly heterogeneous between and within villages, and is influenced by factors such as survival and biting frequencies of Anopheles mosquitoes. This study investigated the correlations between biological age, distance from aquatic habitats and pyrethroid resistance status of Anopheles funestus mosquitoes in south-eastern Tanzania; and how these correlations may inform improved vector control strategies.Methods: Female An. funestus were collected in houses located 50-100m, 150-200m or over 200m from the nearest known aquatic habitats. The mosquitoes were exposed to 1×, 5× and 10× the diagnostic doses of either deltamethrin or permethrin, or to the synergist, piperonyl butoxide (PBO) followed by the pyrethroids, then monitored for 24hr-mortality. Ovaries of exposed and non-exposed mosquitoes were dissected to assess parity status as a proxy for biological age. Adults emerging from larval collections in the same villages were exposed to the same insecticides at 3-5, 8-11 or 17-20 days old to compare resistance. Findings: Mosquitoes collected nearest to the aquatic habitats (50-100m) had the lowest mortality compared to other distances, with a maximum of 51% mortality at 10× permethrin. For the age-synchronized mosquitoes collected as larvae, the percentage insecticide-induced mortality, assessed at both the diagnostic and multiplicative doses (1×, 5× and 10×) increased with mosquito age. The highest mortalities at 1× doses were observed among the oldest mosquitoes (17-20 days). At 10× doses, mortalities were 99% (permethrin) and 76% (deltamethrin) among 8-11 day-olds compared to 80% (permethrin) and 58% (deltamethrin) among 3-5 day-olds. Pre-exposure to PBO restored the potency of both pyrethroids. The proportion of parous females was highest among mosquitoes collected farthest from the habitats.Conclusion: In this specific setting, older An. funestus and those collected farthest from the aquatic habitats (near the centre of the village) were more susceptible to pyrethroids than younger ones and those caught nearest to the habitats. These findings suggest that pyrethroid-based interventions may remain at least moderately effective despite widespread pyrethroid-resistance by killing the older, less-resistant and potentially-infective females. Further studies should investigate whether fine-scale targeting of key interventions (e.g. preferentially spraying homes near the centres of villages) would be more impactful.