Mosquitoes are found virtually anywhere on Earth. They are annoying with their itchy bites and incessant buzzing at the most untimely moments, but there is more to them than the nuisance factor.
Mosquitoes are the vectors – living organisms that transmit infectious pathogens between humans or animals to humans – of some of humanity’s most deadly vector-borne diseases. Apart from killing more people than any other animal known, they are also a burden that disproportionately affects mostly children and the elderly in the poorest populations. In fact, these tiny transmitters of death are so medically important that they have a global day to commemorate their existence.
World Mosquito Day is observed annually on 20 August to honour the discovery made by a British doctor, Sir Ronald Ross, in 1897, linking the disease malaria to mosquitoes. The ancient Romans originally thought malaria disease came from the harmful fumes in swamps, hence the name malaria, taken from the words mal and aria, which literally means ‘bad air’ in Medieval Latin.
Mosquito facts that can be exploited for disease control purposes
Malaria comes to mind most often when people think of diseases transmitted by mosquitoes. Out of the more than 3 000 species of mosquitoes only the members of three genera – Aedes, Anopheles and Culex – are responsible for transmitting human diseases such as chikungunya, dengue, filariasis, Rift Valley fever, yellow fever, Zika, malaria, encephalitis and West Nile fever. These diseases result in more than 700 000 deaths annually, putting mosquitoes at the top of the ‘most deadly animals known to man’ list. Responsible for transmitting more than half of the mosquito-borne deaths reported annually, the Anopheles mosquito single-handedly caused around 405 000 reported malaria-related deaths in 2018 alone.
Not all mosquitoes bite but there is a specific reason why they do – and it is not for nutrients. Both male and female mosquitoes feed on plant sap and nectar for nutrition, but female mosquitoes bite and ingest blood for egg production. It is during this time when a female Anopheles species mosquito bites a person infected with malaria, a small amount of blood containing microscopic Plasmodium parasites is taken in and transmitted to another person about a week later when the mosquito feeds again.
Mosquitoes are highly skilled and adaptive to provocations in their environments. For decades, malaria vector control tools and strategies have focused on protecting people when indoors using indoor residual spraying (IRS) or long lasting insecticide-treated nets (LLINs). Natural (genetic) selection has seen some malaria vector populations start biting outdoors when people are not protected by vector control methods, making outdoor vector control very difficult with the use of insecticides. Prevention is better than cure and the best way to prevent getting malaria (when outside) remains to avoid being bitten by not being outside between dusk and dawn; wearing long-sleeved clothing; applying insect repellent on exposed skin and using anti-mosquito sprays or burn mosquito coils.
The mosquito is only drawn to certain people when in need of a blood meal. The body chemistry, or skin microbiome, of different people determines if a mosquito will bite you or not. Chemicals including carbon dioxide (released from the body when exhaling) and lactic acid (found in sweat) contribute to the mix that attracts mosquitoes. Highly specialised sensory organs situated on the mosquito’s antennae detect small amounts of chemical signals that females use to locate their hosts when they need to feed. These antennae are also used to detect pheromones allowing males and females to find each other in order to mate. Synthetic chemical signals can be used to repel mosquitoes and, in many cases, lure them towards traps where these killers can be killed.
Mosquitoes depend on water to breed, requiring only a small amount to accomplish this task. The mosquito has a complete metamorphosis life cycle consisting of four stages – egg, larva, pupa, and adult – and the cycle is semi-aquatic with all stages except for the adult being water-bound. A tablespoon full of water is enough and can easily collect in nooks in trees, post-rain pools, uncovered water containers often placed close to homes (in rural communities) and even trash including old tyres, empty cool drink cans, and crisp packets amongst others. It is therefore imperative to keep areas around your home litter free as this can also be considered an effective way to limit breeding habitats for malaria mosquitoes. The use of larvicides to kill the larva is a vector control technique that is increasing in popularity, enabling control of the vector outdoors.
Getting to know and understand vectors to aid in disease control
At the University of Pretoria Institute for Sustainable Malaria Control (UP ISMC), mosquitoes are the bread and butter of the researchers working in the Institute’s Vector Control research cluster. This group of researchers focuses on the development, promotion and appropriate integration of safer, sustainable, new and innovative tools, methods and policies for optimisation of malaria vector control strategies. The best way to understand how to exploit the vectors is achieved by studying the mosquito vector’s biology, habits, habitat preferences and other factors that determine their disease-transmission capabilities.
The Institute’s researchers have developed innovative tools, from mosquito repellent formulations (potent enough to kill mosquitoes) and textiles to produce protective clothing (socks and ankle bands) for outdoor vector control to insecticide-impregnated wall linings for indoor protection, as safer and more sustainable methods to control vectors. Habits of the Anopheles mosquito, including the acts of biting on the ankle or lower leg area and resting in high and dark spaces against walls after a blood meal, contributed towards the development of the control tools. The studying of the skin biome and how semiochemicals can be used in odour lures for outdoor vector control is fascinating. UP ISMC researchers are considering the variation in human attractiveness to mosquitoes and their preference to bite select regions of the human anatomy as prospective avenues to identify compounds effective for lure and kill strategies.
UP ISMC researchers also use their knowledge and understanding of mosquitoes to build capacity. Besides researching ways to control the malaria mosquito, the group is finalising a vector-borne disease surveillance and control course with a heavy focus on malaria. The course will be completely COVID-19 safety compliant through an online-only, virtual classroom that will include morphological identification skills and techniques that are invaluable when doing vector surveillance. The Institute also has its own insectary with a colony of Anopheles arabiensis that is not only used for vector control research but will soon be open for insecticide and repellent testing of commercially and private developed products.
There is an African proverb that says, “If you think you are too small to make a difference, you haven’t spent the night with a mosquito.” It is remarkable how much loathing there can be towards such a minuscule animal – albeit substantial in stature. But despite all the irritation and death caused by mosquitoes, the question remains whether it is ethically sound to completely eradicate them from the face of the planet. That is a tricky question to answer, especially when a mosquito keeps you up in the middle of the night!
Dr Taneshka Kruger is a senior researcher in the School of Health Systems and Public Health in the Faculty of Health Sciences at the University of Pretoria, and the Project Manager: UP Institute for Sustainable Malaria Control.