If asked to name the deadliest insect in the world, most people would search their minds for some sinister-looking spiders or scorpions, or exotic garden pests. However, if we define ‘deadly’ in terms of the number of people who die directly as a result of the insect, one of them leads the field, by far: the mosquito. As a blood-sucking pest, it transmits diseases to over 700 million people a year, killing a fair proportion of them in the process. No other insect comes even close to this.

Although all mosquitoes are nectar feeders, the females also need protein from a blood meal in order to produce eggs. To find this, they have a keen sense of smell, detecting the sweat and other organic compounds of mammals, such as the carbon dioxide they exhale. Scientific tests have proven that some people attract more mosquitoes than others, presumably having a better ‘scent profile’ — in fact, so adept are female mosquitoes at following these trails, they can infiltrate buildings through pipeways and air-conditioning ducts as they move inexorably towards their victims. Upon biting, they inject an anti-coagulating saliva into the flesh, and it is this fluid (and not their blood) which may contain the range of viral and parasitical nasties for which mosquitoes are notorious.

Yet even without such diseases, mosquitoes are an irritating nuisance which can occasionally cause serious injury. Upon being bitten, the body’s immune system is activated, and subsequent bites trigger antibodies which cause inflammation and itching, particularly with young children. More bites can increase such sensitivity, resulting in pronounced swelling and blistering — wounds which can occasionally become infected, particularly when scratched. Two famous victims of infected mosquito bites are Lord Carnarvon, the Egyptologist who played a role in the discovery of Tutankhamen’s tomb, and the British poet, Rupert Brooke, passing away in Egypt and Greece, respectively.

But the real danger will always be mosquito-borne diseases. Dengue fever, West Nile virus, and several encephalitis-type diseases are all modern day killers. A less deadly but more insidious example is filariasis, a disease named from the thread-like parasites which migrate to the body’s lymphatic system, causing parts of the body to permanently swell to grotesque proportions. Yet, as distressing as all this is, in terms of its death toll, the worst disease is undoubtedly malaria. Carried by the Anopheles mosquito, this parasite causes fever, shivering, joint pains, vomiting, and, if left untreated, a painful death. It infects over two million people a year, most of them children, killing over one quarter in the process.

The Aedes Aepypti mosquito is the species responsible for that other great killer: yellow fever. This is a viral disease, but limited to tropical areas, primarily in Africa, but also Central and South America. After high lever, nausea, and joint pains, the virus attacks the liver, causing the host’s skin to turn yellow (hence the name), with death following some days later. Its toll is much smaller than malaria, with about 200,000 infections and 30,000 deaths every year, mostly in Africa. Unlike malaria, there exists a vaccine, and extensive vaccination programs sponsored by the WHO have had some success, whilst travelers to disease-prone areas are usually similarly protected.

With such a death toll, it took a surprisingly long time before the link between mosquitoes and disease was realised. This is exemplified in the construction of the Panama Canal — that ambitious project to excavate a passageway for ships through that narrow Central-American nation. In the 1880s, the French struggled for eight years in insect-infested jungle, but the death toll from malaria and yellow fever made it very difficult to maintain an experienced work force. After the loss of 22,000 lives, work was abandoned, yet shortly afterwards, a British doctor in India, Ronald Ross, deduced the means of disease transmission, identifying the malaria parasite in the gastrointestinal tract of mosquitoes. He also realised that mosquito numbers could be reduced by limiting their access to water, providing two crucial insights which laid the foundations for controlling the disease.

Thus, in 1904, when America resumed work on the Panama Canal, they instituted a multi-million dollar mosquito-abatement program, consisting of many strategies. Houses for workers were built with screens on the windows, buildings harbouring mosquitoes were fumigated, and sick workers were isolated behind nets. Stagnant pools of water (where mosquitoes breed) were sprayed with oil and insecticide, and roads were paved to eliminate puddles. For this same reason, swamps were drained, and proper piping was used for the transmission of drinking and waste water. All this reduced the number of deaths from disease over the ten-year construction phase to less than 6,000 — a considerable number, but still considered a major success.

To this day, reducing the incidence of stagnant pools of water, however small, remains very cost-effective in combatting mosquito-borne diseases in urban areas. Many of the most dangerous species breed in incidental ditches, flowerpots, or discarded containers into which rainwater has pooled. By eliminating such sites, the insects’ numbers fall greatly, limiting bites to those mosquitoes which come from further afield, yet since they cannot travel far, the likelihood of being bitten (and infected) is greatly reduced.