Living on the edge: a longitudinal study of Anopheles funestus in an isolated area of Mozambique

BACKGROUND: Understanding the survival strategies of malaria vectors at the edges of their distribution, where they are under stress from environmental conditions, may lead to the development of novel control techniques and may help predict the effects of climate change on these mosquitoes. METHODS: The population dynamics of an isolated population of Anopheles funestus from the peninsula of Linga Linga in southern Mozambique was studied over a period of 104 weeks from March 2009 to May 2011 by 917 light-trap and 390 exit collections, mostly in an area close to a seasonal pond. RESULTS: Over the sampling period, 3,684 An. funestus females were caught. Densities decreased with increasing distance from the pond. In 2009 and 2010, a single annual peak in An. funestus density coincident with the single annual peak in rainfall was observed, but a clear population peak was absent during the first 21 weeks of 2011. In between population peaks, An. funestus remained present at low densities. In light trap collections, the proportion of gravid mosquitoes was significantly higher during the ‘low season’ (the period between peaks) than during the peak season (RR = 4.3, p<0.001). In contrast, in exit collections, the proportion of gravid mosquitoes was significantly lower during low season than during the peak season (RR = 0.64, p<0.01). Also, in light traps, the proportion of part-fed females was higher during the low season than during the peak season (RR = 4.5, p<0.001), whereas this was inversed for engorged females (RR = 0.46, p<0.05). Thirteen out of 289 (4.5%) An. funestus tested positive in the sporozoite ELISA. The proportion of sporozoite positive females was higher during the low season (6.25%, six out of 96) than during the peaks (3.63%, seven out of 193), but this difference was not significant. CONCLUSIONS: It is suggested that a proportion of the mosquito population may become gonotrophically discordant during the long dry season resulting in enhanced mosquito survival and sustained malaria transmission.