- CDC/James Gathany
The deadliest animal on Earth would cause a lot less trouble if it preferred to feed on the blood of cattle instead of the blood of humans.
Many of the approximately 3,500 mosquito species actually do prefer cows – or pigs, dogs, or other animals. But it’s the small number that don’t that kill hundreds of thousands of people every year by spreading malaria, dengue, Zika, and other illnesses.
Figuring out why the mosquitoes that spread those diseases prefer humans helps us figure out how to control or kill those populations. And it turns out that a large part of that preference is genetic, which for certain deadly species in particular might point towards new mosquito control strategies.
For some mosquito species, like the Aedes aegypti that spread dengue and Zika, we’ve long known about the genetic preference that drives them to seek out humans.
Now, a new study published in PLOS Genetics reveals for the first time why certain populations of one species of Anopheles malaria-spreading mosquitoes prefer humans and other populations of the same mosquito prefer cattle. (Anopheles and Aedes populations split about 150 million years ago, long before humans walked the Earth, meaning they most likely independently evolved the traits that unfortunately lead them both to want to feed on us.)
In East Africa, the study explains, successful malaria control efforts have stopped much of the malaria transmission by Anopheles gambiae mosquitoes. They prefer to feed on humans and rest indoors, and so long-lasting indoor insecticides have managed to successfully wipe out many of them.
- Kiszewksi et al., 2004. American Journal of Tropical Medicine and Hygiene 70(5):486-498
That leaves Anopheles arabiensis as the main malaria vector in the region, and this sub-species is much more confusing. It seems to feed on animals and humans alike and to rest both indoors and outdoors, making it “more of a generalist,” according to the study, which makes it harder to control.
After collecting hundreds of specimens of arabiensis mosquitoes that had fed from cattle and people from three Tanzanian villages, the researchers conducted whole genome sequences on 48 of the bugs.
They were able to pin down certain genetic patterns that they think explain why certain mosquitoes prefer humans and why others prefer cattle. They didn’t find any genetic explanation for whether the bugs rested indoors or outdoors, and so they assume that these mosquitoes can adapt their napping behavior to wherever happens to be convenient. That adaptability makes it much harder to figure out how to vanqish them.
Other research has shown that if you keep cattle near mosquitoes who prefer to feed on cattle but can spread malaria to humans, it can reduce disease transmission rates up to 50%. In other words, if you make a cow available, the mosquitoes will bite the cow and leave the human alone.
The scientists behind the work write that they’ll need to confirm this finding based on a broader geographic sampling, but it could offer helpful strategies for the battle against malaria.
First of all, if they develop tests that can identify the preference for human-biting mosquitoes, these could be used to target high-priority regions for mosquito eradication.
Additionally, they suggest that it might be possible to genetically engineer more of these mosquitoes with a preference for feeding on cows, which could help spread this trait and hopefully reduce the frequency of the genes associated with a preference for human blood.