The impact of host species and vector control measures on the fitness of African malaria vectors

Abstract

Many malaria vector mosquitoes in Africa have an extreme preference for feeding on humans. This specialization allows them to sustain much higher levels of transmission than elsewhere, but there is little understanding of the evolutionary forces that drive this behaviour. In Tanzania, we used a semi-field system to test whether the well-documented preferences of the vectors, Anopheles arabiensis and Anopheles gambiae sensu stricto (s.s.) for cattle and humans, respectively, are predicted by the fitness they obtain from host-seeking on these species relative to other available hosts. Mosquito fitness was contrasted, when humans were fully exposed and when they were protected by a typical bednet. The fitness of both vectors varied between host species. The predicted relationship between host preference and fitness was confirmed in An. arabiensis, but not in An. gambiae s.s., whose fitness was similar on humans and other mammals. Use of typical, imperfect bednets generated only minor reductions in An. gambiae s.s. feeding success and fitness on humans, but was predicted to generate a significant reduction in the lifetime reproductive success of An. arabiensis on humans relative to cows. This supports the hypothesis that such human-protective measures could additionally benefit malaria control by increasing selection for zoophily in vectors.

The fitness of African malaria vectors in the presence and limitation of host behaviour

“…However, as these recapture rates were generally similar across trials with different host species, it suggests there may be no significant variation in host-seeking mortality between host species. Previous work under the semi-field conditions suggest that mortality associated with host-seeking by these vectors is negligible [29], but further work is required to confirm this under natural field conditions.…”

“…arabiensis and An. gambiae s.s were estimated after they were released inside a chamber (9.1 × 9.6 × 3.7 m) of a netting – enclosed SFS that contained a vertebrate host situated within an experimental hut (3.5 × 4 × 2.5 m, [29]). On each night of experiments, an individual host that was either a human, cow, dog, goat, or chicken was randomly allocated for placement in the hut [30].…”

Eave tubes for malaria control in Africa: initial development and semi-field evaluations in Tanzania

“…The colony was held in a room within a semi-field screened structure under ambient temperature and relative humidity as described previously described [13]. Larvae were maintained on ground fish meal, adults were provided with sugar water (a 10 % glucose solution), and human volunteers provided blood meals for caged adult female mosquitoes.…”

“…Overnight survival has previously been used as a mosquito fitness measure in a similar screened compartment system [13], and it is a standard measure when testing vector control interventions in experimental huts [14, 15]. However, because of dirt floors in the screened compartments used for this set of experiments, it was difficult to find dead mosquitoes the following morning.…”

The transmission potential of malaria-infected mosquitoes (An.gambiae-Keele, An.arabiensis-Ifakara) is altered by the vertebrate blood type they consume during parasite development

“…Haematological properties such as biochemical composition, and red cell density and size vary between vertebrate species3435, and may affect the nutritional value of host blood to mosquito-stage Plasmodium. Anopheles may have adapted to optimize extraction of resources from the blood of preferred host species, resulting in enhanced longevity36, which indirectly benefits parasites by ensuring vectors survive long enough for them to complete sporogony. The differential blood feeding behaviour of An.…”