Welcome to the second day of World Malaria Day [week] at the Bug Blog! I’ve talked several times about the way in which different mosquitoes respond differently to pesticides for malarial control, but here’s a new twist.
In a recent study to compare different species of mosquito in their ability to be a malarial vector, there were very large differences!
As a review– a disease vector is an organism that does not cause disease itself, but transmits an infection by transporting pathogens from one host to another. The malarial parasite is alive inside the mosquito, although they don’t get malaria–but they can give it to us, the host. In this research, different mosquitoes were compared in their abilities to serve as a vector for malaria.
One of the claims frequently made by the “DDT will solve everything” crowd is that just spraying enough DDT will kill all the malarial mosquitoes. This ignores that mosquitoes vary widely from population to population, and species to species, in their ability to resist DDT. They aren’t all the same, and there is no one size-fits-all control method.
The research I discussed yesterday is another good example of the variability problem–predicting malaria using weather and other environmental data in different areas of Africa required different solutions.
In this experiment, you can get a sense of another layer of difficulty in controlling malaria. The experiments looked at two different strains of malarial parasite (Thai and Korean), and 3 different mosquito species. That’s a lot of potential variation!
I won’t go into the specifics–you can read the paper if you want to see technical words like “sporogony”–but basically, not all of the mosquito species were able to support the malarial parasites’ life cycle. (BTW, this diagram is probably the single greatest cause of drinking in invertebrate zoology students. The names! The stages! Ugh!)
Infecting a mosquito with the malarial parasite isn’t enough–the parasite has to change, migrate out of the gut of the mosquito, and into its salivary glands. Not all mosquito bodies are equally friendly to this process–there were large differences between the 3 mosquitoes tested in this experiment.
Hopefully this gives you a sense of the complex layers of difficulty surrounding malarial control–in addition to the environmental variability from location to location discussed yesterday, and variation in pesticide resistance which I’ve discussed before, there are also large genetic differences within mosquito species, mosquito populations, and malarial parasites. It’s a spaghetti tangle of variables, many of which we have no control over.
This is why there is no easy solution to malaria, and why after centuries, we are only now beginning to make progress.
But we are making progress! Just not as fast as we’d like.
Joshi, D., Choochote, W., Park, M., Kim, J., Kim, T., Suwonkerd, W., & Min, G. (2009). The susceptibility of Anopheles lesteri to infection with Korean strain of Plasmodium vivax Malaria Journal, 8 (1) DOI: 10.1186/1475-2875-8-42