QUESTION:
Do mosquitoes with malaria get sick like people do?
ANSWER:
That’s a great question! As far as I know, the jury is still out in terms of what overall effect being infected with malaria has on mosquitoes, and to a large extent, it appears to vary due to lots of factors, such as the compatibility between the mosquito and the particular type of malaria parasite, the intensity of malarial infection, and even environmental conditions, to name but a few. However, what is clear is that mosquitoes don’t seem to get ‘sick’ like we do, with fevers, chills and the like—their immune system is just too different from ours! Instead, with mosquitoes, it’s more a question of whether malaria decreases their lifespan, reduced their reproductive success, or other such effects.
A meta-analysis (Ferguson & Read, 2002, ‘Why is the effect of malaria parasites on mosquito survival still unresolved?’, in Trends in Parasitology) of studies that looked on the effect of malaria on mosquito survival demonstrated that 41% of studies reported a detrimental effect of malaria on mosquitoes, whereas 59% reported no effect (none reported a positive effect, which is interesting).
Also worth noting is that the length of the study seemed to have an effect on whether it would report a detrimental effect; this might be due to early stages of infection with malaria not having a negative impact on the host, while the malaria parasite is developing; then, once it is mature, it may be that the parasite’s virulence towards its host increases. Similarly, at this stage in its life cycle, the parasite might induce changes in the mosquito’s behaviour, encouraging it to feed more, which can result in higher mortality to the mosquito (think of swatting away annoying, biting mosquitoes, whereas you’re less likely to go for ones that leave you alone!).
More recently, a study by EJ Dawes and colleagues at Imperial College, London (2009, ‘Anopheles mortality is both age- and Plasmodium-density dependent: Implications for malaria transmission, in Malaria Journal) found evidence for the age of the mosquito and the intensity of malaria parasitism influencing mortality of the insects. Similarly, another research group found that infected female mosquites had significantly lower fecundity than non-infected mosquitoes (Gray & Bradley, 2006, ‘Malarial infection in Aedes aegypti: Effects on feeding, fecundity and metabolic rate’, in Parasitology). These examples begin to swing the balance of the debate in favour of malaria having a negative impact on mosquitoes, at least at certain points in the timeline of an infection (i.e. near the beginning, and then after parasite maturation).
You might ask why mosquitoes that are commonly infected with malaria don’t become resistant to infection, given that they seem to suffer ill-effects when they are parasitised—well, some recent research may have provided the answer to that one. Researchers looking at population growth rates in malaria-susceptible mosquitoes versus those resistant to infection noticed significantly slower growth rates in the resistant population, suggesting that in terms of reproduction, resistance at a population level might come at a cost to growth (Voordouw et al., 2009, ‘Rodent malaria-resistant strains of the mosquito, Anopheles gambiae, have slower population growth than -susceptible strains’, in BMC Evolutionary Biology). However, there may be ways in which this effect can be by-passed, in order to control malaria transmission at the level of the mosquito; one group of researchers recently published the results of a study, where they showed that when resistant and susceptible male mosquitoes rae released into a population of females (bearing in mind only the females feed on blood, and so males are not implicated in transmission), the females tend to mate first with the earlier-hatching resistant males, and moreover, lay a greater number of eggs with these males! So it may be that research on the effect of malaria on mosquitoes can be used in the future to decrease malaria transmission, which would be a hugely positive step for public health in many parts of the world.
It would be great if any malaria researchers out there would comment on the above answer, especially if there are more up-to-date examples of research on the effect of malaria on mosquitoes!