It’s easy to just think of malaria as a medical problem. It is caused by single-celled parasites—Plasmodium—that are spread through the bites of other parasites—mosquitoes. To beat the disease, we need to neutralise either Plasmodium or its mosquito carriers, using drugs, insecticides, nets, or even genetically-modified competitors.
But malaria is also an ecological problem. Mosquitoes aren’t static, unchanging targets. They move around. They mate. They breed in some areas and not in others. Their populations swell and contract throughout the year. They bite at varying times of day. We need to understand these subtle quirks of mosquito life, because they all have a huge impact on our strategies for fighting malaria.
Consider the Sahel—a belt of land that stretches across Africa’s waist, with the Sahara to the north and savannahs to the south. In this region, half a million people die from malaria every year—which is puzzling. Every year, between December and June, the Sahel goes through an intense dry season. Rain hardly falls. Stagnant pools and puddles, in which mosquitoes lay their eggs, evaporate. The adults ought to die before they can start a new generation. And yet, when the rains return, so do malarial mosquitoes, in huge numbers. How do they survive?
Scientists have puzzled over this ‘dry season paradox’ for more than a century. Some said that the insects persist through the dry spell in a dormant state, while others felt that they migrate over long distances to more habitable climes.
Now, a team of scientists led by Adama Dao from the University of Sciences in Mali, and Tovi Lehmann from the US National Institutes of Health, have finally found that both answers are right. Some species of malarial mosquitoes persist; others travel. And that has big implications for our attempts to stop malaria in this critical region.
Under Dao’s leadership, a team of researchers from Mali spent years counting the numbers of malarial mosquitoes in the Malian village of Thierola. They checked for larvae in every puddle, tree hole, or well they could find. They collected adults from every house in the village, and used a variety of traps to capture those flying outside. And they did this on every fourth day or so, for five years.
“How mosquitoes survive the dry season is a deceptively simple question, but it has been very difficult to answer,” says Jason Rasgon from Pennsylvania State University . “It takes the kind of heroic sampling effort that the authors performed to get a handle on this issue.”
The team found different patterns for each of the three different species of malarial mosquitoes in the area. Anopheles coluzzi is the most common of these. Its populations peak in September and October, at the height of the wet season, before plummeting in November as the larval sites dry up. They stay at low levels for most of the dry months, with two exceptions: huge short-lived population spikes in December and April, when numbers suddenly soar by 10 to 90 times. And once it starts properly raining in June, A.coluzzi bounces back almost immediately.
These patterns suggest that the adults somehow wait out the dry season in a dormant state. They can take advantage of any rare bursts of rain, and they’re ready and waiting when the wet season truly begins.
The other two species—Anopheles gambiae and Anopheles arabiensis—showed very different patterns. Neither of them had any peaks during the dry season. Once their populations fell, they stayed that way until the rains returned. Even then, it took a few months for them to bounce back. It seems that these two species survive by migrating to other areas, hundreds of kilometres away.
“Any mosquito paper that tries to unravel their complex ecology is a winner in my eyes,” says James Logan from the London School of Hygiene and Tropical Medicine. “There is much about malaria mosquito ecology and biology that we still don’t understand, so studies like this could have large implications in the control of diseases like malaria.”
For example, the team suspects that A.coluzzi’s ability to survive in a dormant state allows it to maintain cycles of malaria transmission that would otherwise break during the dry season. By peaking twice during the drought, it can continuously shuttle Plasmodium between humans at a time when the parasite should face dead-ends. When the wet season begins, A.coluzzi can immediately start ratcheting up these cycles of transmission. And when A.gambiae and A.arabiensis return in September and October, they kick things into even higher gear.
Lehmann’s team are now trying to break these cycles by finding A.coluzzi’s dry-season hide-outs and blitzing them with insecticides. They are testing this approach in a larger number of villages.
It seems counter-intuitive to go after the mosquitoes when they’re at their rarest, but those rare populations are critical—they are the seeds of the next wet season’s boom. “By hitting the late dry-season peak and the early wet-season surge, we think we’ll virtually eliminate the seed population,” says Lehmann. “We think we could potentially cut down transmission in those areas by 75 percent or more, and it would be very cost-effective.”
His results also have implications for other malaria control strategies. For example, some scientists are trying to develop genetically modified mosquitoes that cannot harbour Plasmodium, and that would outcompete local insects. But if these GM-mozzies cannot last through the dry season, their impact would be short-lived. And if A.gambiae and A.arabiensis return in the wet season, flying in from distant parts of the Sahel, they would reintroduce a fresh pot of parasites every year. “The long-term planning of the battle against malaria cannot ignore these phenomena,” says Lehmann.
Unfortunately, that’s exactly what people tend to do. Many historical attempts to control mosquitoes have failed dismally because they were build on shoddy ecological foundations. As Heather Ferguson from the University of Glasgow once wrote: “A lot of the knowledge gaps that hindered previous attempts still remain… We have made substantially more headway in understanding the reproductive biology of species with no direct public health or economic importance, such as Drosophila, fur seals and blue tits, than we have done for this vector that kills millions.”
I wrote about this in a piece for Slate in 2011:
“Crucial ecological research on mosquitoes is trapped in a financial no-man’s land. Organizations that fund basic research into issues like how insects behave assume that biomedical agencies will foot the bill, while these agencies are more likely to prioritize research with more obvious and immediate clinical impact. But the necessary ecological studies would not be expensive. Ferguson estimates that it would take just $500,000 to fund 10 students in the field, an act that “could easily quadruple our knowledge of this area within a few years.”
For example, in 2008, her student Kija Mg’Habi worked in an isolated, malarious part of Tanzania and discovered that among Anopheles gambiae (a species that carries malaria), the medium-sized males get the most sex. You might expect the biggest males to outcompete their smaller rivals, but they were actually six times less successful. This is exactly the type of information you need if you want your modified mosquitoes to outcompete their natural brethren… It may not be as sexy as modifying genes, but ecology is tantamount to knowing your enemy, and that surely is a cornerstone of victory.”
Reference: Dao, Yaro, Diallo, Timbine, Huestis, Kassogue, Traore, Sanogo, Samake & Lehmann. 2014. Signatures of aestivation and migration in Sahelian malaria mosquito populations. Nature http://dx.doi.org/10.1038/nature13987