Partitioning the contributions of alternative malaria vector species.

Background: In many locations malaria is transmitted by more than one vector species. Some vector control interventions, in particular those using genetic approaches, are likely to be targeted against a single species or species complex, at least initially, and it would therefore be useful to be able to predict the epidemiological impact of controlling a single species when multiple vector species are present.

Methods: To address this issue, the classical Ross-McDonald model of malaria epidemiology is expanded to account for multiple vector species, giving expressions for the equilibrium prevalence, sporozoite rates and reproductive number. These allow one to predict when control of just one vector species will lead to elimination of the disease. Application of the model is illustrated using published data from a particularly extensive entomological and epidemiological survey before the rollout of bed nets in eastern Kenya, where Anopheles gambiae s.l. and An. funestus were vectors.

Results: Meta-analysis indicates that sporozoite rates were 38 % higher in An. gambiae s.l. than in An. funestus, and, according to the model, this difference could be due to An. gambiae s.l. having a higher frequency of feeding on humans, a higher human-to-mosquito transmission rate, a lower adult mortality rate, and/or a shorter incubation period. Further calculations suggest that An. gambiae s.l. would have been sufficient to maintain transmission by itself throughout the region, whereas An. funestus would not have been able to support transmission by itself in Malindi District.

Conclusions: Partitioning the contributions of different vector species may allow us to predict whether malaria will persist after targeted vector control.