Abstract
BackgroundWhether Plasmodium falciparum, the agent of human malaria responsible for over a million deaths per year, causes fitness costs in its mosquito vectors is a burning question that has not yet been adequately resolved. Understanding the evolutionary forces responsible for the maintenance of susceptibility and refractory alleles in natural mosquito populations is critical for understanding malaria transmission dynamics.MethodsIn natural mosquito populations, Plasmodium fitness costs may only be expressed in combination with other environmental stress factors hence this hypothesis was tested experimentally. Wild-caught blood-fed Anopheles gambiae s.s. females of the M and S molecular form from an area endemic for malaria in Mali, West Africa, were brought to the laboratory and submitted to a 7-day period of mild hydric stress or kept with water ad-libitum. At the end of this experiment all females were submitted to intense desiccation until death. The survival of all females throughout both stress episodes, as well as their body size and infection status was recorded. The importance of stress, body size and molecular form on infection prevalence and female survival was investigated using Logistic Regression and Proportional-Hazard analysis.ResultsFemales subjected to mild stress exhibited patterns of survival and prevalence of infection compatible with increased parasite-induced mortality compared to non-stressed females. Fitness costs seemed to be linked to ookinetes and early oocyst development but not the presence of sporozoites. In addition, when females were subjected to intense desiccation stress, those carrying oocysts exhibited drastically reduced survival but those carrying sporozoites were unaffected. No significant differences in prevalence of infection and infection-induced mortality were found between the M and S molecular forms of Anopheles gambiae.ConclusionsBecause these results suggest that infected mosquitoes may incur fitness costs under natural-like conditions, they are particularly relevant to vector control strategies aiming at boosting naturally occurring refractoriness or spreading natural or foreign genes for refractoriness using genetic drive systems in vector populations.
Highlights
Whether Plasmodium falciparum, the agent of human malaria responsible for over a million deaths per year, causes fitness costs in its mosquito vectors is a burning question that has not yet been adequately resolved
A reduction in fecundity was found in Anopheles gambiae s.l. naturally infected with Plasmodium falciparum in Tanzania in one of the rare field-based study of fitness costs [13]
The results of this study provide indirect and direct evidence suggesting that infection with Plasmodium interacts with hydric stress to impose fitness costs on An. gambiae females in terms of survival
Summary
Whether Plasmodium falciparum, the agent of human malaria responsible for over a million deaths per year, causes fitness costs in its mosquito vectors is a burning question that has not yet been adequately resolved. Determining whether Plasmodium falciparum, the agent of human malaria responsible for over a million deaths per year, causes fitness costs to its mosquito vector Anopheles gambiae has critical implications for the understanding of mosquito/malaria interactions. Reduced fecundity was shown in Aedes aegypti infected with Plasmodium gallinaceum [8,9] and anopheline mosquitoes infected with Plasmodium yoelii nigeriensis [10,11] Results from these laboratory-based model systems cannot be extrapolated to natural mosquito/Plasmodium populations because of unrealistic parasite loads and the lack of coevolutionary history between their vector and Plasmodium components (discussed in [12]). A reduction in fecundity was found in Anopheles gambiae s.l. naturally infected with Plasmodium falciparum in Tanzania in one of the rare field-based study of fitness costs [13]
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