By Fredros Okumu, 19 August 2021
MOSQUITOES spread diseases to millions of people around the world, yet they remain poorly understood by most. Studying their biology and behaviors can help us combat, and eventually eliminate, dangerous diseases such as malaria and dengue fever.
There are nearly 3500 species of mosquitoes. About 400 belong to a family called Anopheles, and of these, only about 50-70 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. Often, only one or two of these dominate malaria transmission in any country. Effective malaria control can therefore be achieved by simply identifying, understanding and then targeting just the one or two dominant Anopheles species instead of trying to kill all mosquitoes.
Photo credit: Dr. Joachim Pelican, SwissTPH
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 on the water surfaces. 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. According to the WHO, removal or treatment of the water bodies where mosquitoes breed is most effective in areas where such water bodies are few, easy to find and are fixed throughout the year. A growing number of scientists however think that there are ways to expand these approaches cost-effectively, and in the process create jobs for local youths in malaria-endemic countries.
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. For several years, scientists have been investigating opportunities for targeting these mating swarms as a way to reduce populations of the malaria vectors.
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. For such species, effective control will require additional tools that target mosquitoes outside homes. Examples include the use of drugs such as ivermectin, given to humans and animals to indirectly kill mosquitoes that bite these hosts. Others are attractive toxic sugar baits, mass-trapping or use of mosquito repellents, particularly those that protect people over wide areas.
Today, the main malaria prevention tools still include insecticide treated nets and house spraying with insecticides. Unfortunately, long-term use of chemicals in public health and agriculture has led to widespread insecticide resistance, and is decelerating the anti-malaria war. In some parts of Africa, the levels of resistance has risen so high that we will need more than ten-fold doses of chemicals previously used to kill the mosquitoes. Anopheles funestus, the tiniest of these death merchants, surreptitiously developed strong resistance to insecticides, and today rules many parts of east and southern Africa. In parts of Tanzania, it now transmits nine in ten new malaria infections. It is increasingly obvious that malaria control strategies must gradually move away from current overreliance on insecticide-based interventions.
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. There are also a number of potentially transformative new technologies currently being developed, which could accelerate these efforts at far lower costs and effort.
One particularly exciting example is release of “protector mosquitoes”, which upon mating with the wild mosquitoes produce offspring that are incapable of either further reproduction or transmitting malaria to people. A naturally-occurring technology called gene drives has been optimized by laboratories around the world, and is being used to efficiently produce these kinds of mosquitoes. Versions of such protector mosquitoes are already proven to work efficiently inside laboratories and will hopefully undergo field evaluation in malaria-endemic countries, once necessary risk assessments and regulatory processes are complete. Though no field evidence exists yet, results from laboratory cages and in controlled large cages have been so powerful that experts now believe the protector mosquitoes would readily spread across communities, selectively targeting certain Anopheles species of concern, and cost-effectively curbing malaria transmission even in the most remote locations otherwise unreachable by organized health system networks. One leading experts has recently told me that the available evidence is so powerful that it would be unethical for us not to investigate the actual merits and demerits of this technology in real world settings.
So, what if we eliminated mosquitoes? Either using insecticides or the genetic-engineering such as used to produce Protector Mosquitoes? Yes, there are predators such as dragon flies and bats, which 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 preys on other blood-fed mosquitoes. It is therefore unlikely that loss of the few dangerous Anopheles species would endanger the overall 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 sustainable malaria control.
This article has been updated from a previous post (2019) on MalariaWorld
The Amazing Biology of Malaria Mosquitoes
Dr. 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