The deadliest animal in the world is smaller than a pencil eraser and weighs around two-thousandths of a gram — less than the weight of a single raindrop. Every year, it kills an estimated 700,000 people by partaking in what scientists grimly call a “blood meal.” Mosquitoes do not kill the way a shark or a lion does. Instead, they are “vectors” for many painful and life-threatening diseases, from dengue fever to malaria to chikungunya. When a mosquito “bites” someone, by stabbing a needle-filled proboscis deep into a blood vessel — it both sucks out blood and leaves some of its own saliva behind.
That saliva, when contaminated by virus or parasite, can make people sick, often painfully so. Dengue fever is also known as “breakbone” fever; the name chikungunya comes from an African word meaning “to be contorted,” as patients often bend over from severe joint and muscle pain.
Malaria cases worldwide declined steadily for nearly two decades. But that progress stalled as cases have flat lined and even ticked up in some countries in the past few years. Cases increased to an estimated 247 million in 2021 from a recent low of 231 million in 2018, according to data from the World Health Organization.
It is to be noted that vector borne deadly diseases will spread as temperatures warm across the globe. Like all insects, mosquitoes are cold blooded and rely on ambient temperatures to sustain their body temperatures. They thrive, particularly, on temperatures between 50 and 95 degrees Fahrenheit — and, unlike most humans, who wilt under high humidity, mosquitoes love damp air.
Different mosquitoes thrive under different temperatures. The Anopheles mosquito carries malaria; the Aedes aegypti and the Aedes albopictus mosquitoes carry diseases like dengue and chikungunya. But the A. aegypti thrives at higher temperatures than the A. albopictus. As different parts of the world warm at different rates, some mosquito-borne diseases will thrive while others will be put under stress.
There are other factors that are also changing the patterns of disease transmission. As urban areas expand and populations increase, mosquitoes like the A. aegypti — which love living around humans, can find more places to live and more people to feed on.
CONTROL OF VECTOR BORNE DISEASES
Fogging largely kills adult mosquitoes, leaving the larvae untouched – it may cause a short-term dip in mosquito densities but it soon bounces back to its original levels. Insecticide resistance, observed in some parts of Asia, can spread swiftly to India if we continue to indiscriminately use fogging as the primary means of mosquito control. The Aedes aegypti mosquito, which is responsible for the transmission of the majority of cases of dengue and chikangunya, is well adapted to densely populated urban environments. It breeds in sheltered habitats, such as pots, rain water harvesting containers and tyres.
The mosquito is aggressive, often biting multiple people in a day. It can spread many times faster in cities where people tend to move to multiple locations compared to rural areas. Originally active during the day, some reports suggest that it has adapted to artificial light and is active at night. The key to mosquito control lies in increased attention to larva. Mosquitoes, like other aquatic insects, avoid laying their eggs in lakes, rivers or wetland ecosystems where they face the risk of predation by aquatic fauna. Maintaining healthy water bodies helps reduce the risk of mosquito breeding.
Building over stormwater drains or blocking them with piles of garbage creates fertile breeding spaces for mosquitoes in areas that are so polluted that they lack other natural predators like fish or tadpoles.
Ironically, “sustainable” urban practices such as gardening and rainwater harvesting provide habitat for the mosquito to flourish uninterrupted even during the dry summer months where it would otherwise have diminished.
A relatively small number of urban containers can account for the majority of urban cases of infection.
India needs systematic urban monitoring and education campaigns to ensure that rain harvesting containers are completely covered, and pots and compost pits are well drained. Urban heat island effects, including the increase in daytime temperature and day-night temperature fluctuation, are known to play an important role. They increase mosquito breeding, promote dengue virus replication, and make patients more vulnerable to infection – a triple whammy that greatly increases chances of severe outcomes. Areas of cities with greater green surfaces and reduced urban heat islands have lower disease prevalence.
Managing urban biodiversity could help. Some studies suggest that as bird populations decrease due to environmental impacts near cities, mosquitoes shift from feeding on birds to feeding on humans.
Dogs, pigs, goats and cattle can slow down disease transmission by acting as incompetent or dead-end reservoirs – they are bitten by mosquitoes, but do not permit virus replication. Urban ecological planning holds the key to the long-term management of mosquito borne diseases. Restoring ecologically diverse water bodies and extending green spaces has the added attraction of providing multiple co-benefits, including providing alternate biodiversity hosts, reducing air and water pollution, and making city climate more bearable.