Mosquitoes spread diseases to millions of people around the world, yet they remain poorly understood by most. Studying their behaviours can help us combat, and eventually eliminate, dangerous diseases like malaria.
There are nearly 3,500 species of mosquitoes. About 400 belong to a family called Anopheles, and of these, only about 50 can actually transmit malaria to humans.
In Africa, where malaria burden is highest, the most important are Anopheles gambiae, Anopheles funestus, Anopheles arabiensis and Anopheles colluzzi.
A female Anopheles lays about 500 eggs in her lifetime, usually in standing fresh waters, although some breed along rivers or in brackish waters. The eggs weigh just 4 micrograms each, and float like little pontoons.
When sunlight strikes, the eggs hatch into tiny wiggly swimmers, called larvae. This makes them vulnerable to application of chemicals to the water surfaces, an approach that eradicated Anopheles gambiae from Brazil in late 1930s, and substantially reduced malaria in Dar es Salaam in mid 2000s.
The larvae mature within 1-2 weeks and form pupae, inside which they grow wings, legs, mouthparts and antennae for smelling humans. Within 48 hours, the pupae rapture, releasing mature adult mosquitoes.
Emergent adults immediately begin seeking sugar, and mates. Mature males gather in swarms each evening for a 20-30 minute dance, against sunset-lit horizons. Locations of these swarms are marked by mosquito ‘grandpas’, and remain in use by multiple generations for years.
Virgin females, attracted by male songs enter the swarms to select their “Mr. Rights”. The couples can sometimes be seen leaving in tight-knit pairs. Female mosquitoes mate only once, for 20 seconds, but males are scandalously polygamous.
Only females feed on blood, to obtain protein for developing eggs, but both sexes take sugar for energy. Using their specialized sensors on antennae and mouthparts, they detect carbon dioxide and other vertebrate smells from a distance, and can accurately distinguish between individuals, based on breath, sweat and body smells.
This is why some people get more bites than others. At close range, mosquitoes also ‘see’ colors, and can distinguish warm from cold bodies. They also memorize, and can return to households where they last obtained blood.
In flight, hungry female Anopheles are like flying syringes. Weighing just 2.0 milligrams, they can double or triple their weight after a single blood-meal. During blood-feeding, mosquitoes inject saliva, sometimes loaded with infectious malaria parasites (called sporozoites) into humans.
They also pick immature parasites (gametocytes), from previously infected people. These ‘immatures’ come to the peripheral bloodstream only a few times in their lifetime, spending the rest of time hiding in the liver or inner blood vessels. Yet, even where few people carry malaria, Anopheles mosquitoes can identify the carriers and bite them at the right time.
Scientists can prick hundreds of people’s fingers without finding parasites at this stage, but mosquitoes easily find them. We still don’t know how they do it.
Once picked up, the parasites develop inside the mosquito gut, and in 10-12 days, begin lurking in the salivary glands, waiting to be injected into humans.
Mosquitoes have different biting preferences, which also impact disease control. Anopheles gambiae and Anopheles funestus primarily bite humans and prefer feeding indoors, so insecticide-treated bed nets, house spraying and mosquito-proof housing can be very effective.
Indeed, in parts of Kenya and Tanzania, Anopheles gambiae virtually disappeared when insecticide-treated nets were scaled up between 2005 and 2010, causing major reductions in malaria transmission. Other species such as Anopheles arabiensis readily bite humans and animals outdoors, and are less impacted by indoor interventions.
Unfortunately, long-term use of chemicals in public health and agriculture has led to widespread insecticide resistance, and is decelerating the anti-malaria war. Anopheles funestus, the tiniest of these death merchants, surreptitiously developed strong resistance to insecticides, and today rules most of east and southern Africa. In parts of Tanzania, it now transmits nine in ten new malaria infections.
Longer-term integrated strategies that complement insecticide-based approaches are needed to achieve and maintain ‘Zero Malaria’. This may include removing water sources suitable for Anopheles breeding, mosquito-proofing houses, strengthening health systems and educating people about mosquitoes and disease prevention.
So, what if we eliminated mosquitoes? Either using insecticides or genetic-engineering? Predators such as dragon flies and bats feed on various mosquitoes and other insects. Around Lake Victoria, one vampire spider feeds on vertebrate blood from abdomens of blood-fed Anopheles.
However, this spider also prays on other blood-fed mosquitoes. It is therefore unlikely that loss of the few dangerous Anopheles species would endanger mosquito populations or their natural predators.
The biology of malaria mosquitoes is truly an amazing garden of intrigues. The more we understand it, the closer we will get to “zero malaria”.
Fredros Okumu is director of Science at Ifakara Health Institute in Tanzania. He is a mosquito biologist and public health expert working on new ways to improve control and prevention of vector-borne diseases. https://twitter.com/Fredros_Inc