The recent surge in dengue cases, linked to global warming, and reports of a new variant of the Chikungunya virus causing paralysis and even death, with Pune reporting over 2000 cases, are very alarming indeed. We have come a long way since Sir Ronald Ross on August 20th 1897, found that the female Anopheles mosquitoes transmit malaria. It earned him the Nobel prize and the day commemorated as “World Mosquito Day”. An estimated 249 million cases of malaria occurred in 85 malaria-endemic countries in 2022.
Three countries accounted for 98% of the total reported cases; India (58%), followed by Indonesia (30%) and Myanmar (10%). Since then, we have found that the mosquito is also responsible other diseases like dengue fever, Chikungunya, Zika, West Nile, Yellow fever, Eastern and Western equine encephalitis, Japanese encephalitis, La Crosse encephalitis., and St. Louis encephalitis. In the Indian context, malaria, dengue and Chikungunya, are the most worrying, though there have been sporadic reports of other diseases.
The reports of the diseases, and the misfortune of living next door to a construction in progress, with the fresh cement, stagnant water, and an exuberant monsoon triggered my concern.
Mosquitoes have been around for more than 100 million years, when dinosaurs still roamed the earth. The oldest-known fossils date back 130 million years ago. Their impact on human civilisation, has even shaped the outcomes of war, the spread of religion, and the development of modern culture. The Persians were affected as they navigated swampy terrain, leading to their defeat by the Greeks in the Greco-Persian Wars.
The Pontine Marshes provided a source of disease and served as a barrier to the enemies of the Roman empire. Christianity as a healing religion was also affected by the mosquito-borne diseases, so much so that the crusaders failed to capture the Holy Land being hit by Plasmodium-infected mosquitoes.
The life cycle starts with the female needing the protein in the blood to produce its eggs; and wreaks its havoc within the average 6 weeks of its life. This period may extend to 5 months with an adequate food supply. After obtaining a blood meal, it lays the eggs directly on or near water; though the eggs can survive dry conditions for a few months. The eggs hatch to release a mosquito larva or “wriggler”. “Hatching” time depends on water temperature, food and type of mosquito. The larva lives in the water, and develops into the third stage of the life cycle called, a pupa or "tumbler." Finally, the mosquito emerges from the pupal case after 2 to 7 days. Stagnant water is crucial to the life cycle of the pest.
Removing standing water from containers such as plant saucers, buckets, and gutters, eliminates their breeding grounds. A table-spoon of water is enough to sustain it through the cycle. Recent research has identified a new breed of mosquito, that survives even in saline water.
Having emerged from the pupa, it must now find a victim for a blood meal. Over the years it has acquired some interesting abilities to locate its victim. Mosquitoes use a combination of olfactory, sensory, and visual cues to select their hosts. Some mosquito species are more selective than others and some people attract mosquitoes more than others. Carbon dioxide which is present in the exhaled breath is a primary attractant to mosquitoes.
People with higher metabolic rates, such as those who are pregnant or exercising, exhale more carbon dioxide and attract more mosquitoes. Mosquitoes can detect exhaled carbon dioxide from 30 feet away, and are in fact “activated” by the gas to hunt for other cues that indicate a human’s presence. They seek certain colours that help them identify a host. A study published in 2022 suggests that mosquitoes are drawn to orange and red wavelengths, regardless of skin tone.
Hence one needs to wear lighter colours to keep mosquitoes away. In addition to carbon dioxide and visual cues, higher body temperature and a sense of smell which comes from normal odours and skin microbes attract them. Research has found that soaps change a person’s “odour profile,” which could make someone more or less appealing to the insect.
Hence using floral and fruity scented soaps is an invitation to mosquitoes. When you apply soap that is perfumed with other chemicals, you manipulate your odour signature to the mosquitoes, so the whole chemistry of how you smell, changes. Curiously, research has shown that the consumption of even one pint of beer provides an invitation to the mosquito; though this is believed to arise from the cutaneous vasodilatation and slight increase in body temperature. DEET containing repellents like skin creams also help. Citronella, oil of lemon grass and eucalyptus either in the form of incense sticks, candles or mats are very effective in keeping the insects away; the additional advantage is, being naturally occurring elements they are biologically more acceptable to humans.
Though prallethrin impregnated mats are the most widely used, there is some debate about their safety in young children. The insect does not tolerate low temperatures, and are immobilised at 100C. Hence the efficacy of air conditioning. The popular panchayat remedy of fogging eliminates the adult and does not affect the larvae or pupae. It is therefore of limited value in mosquito control.
Long term research seeks to develop genetically modified mosquitoes to stop the spread of disease; and a promising pilot project is already in progress in Florida.
On a lighter note, the classical gin and tonic owes its origin to malaria. Quinine was used to prevent malaria; and because of its bitter taste, it was blended with sugar, water and gin creating a multi-tasking, malaria-fighting, accidentally delicious sundowner; widely distributed both by the Royal Navy and British army officers for its anti-malarial properties.
Cheers! And take care not to get bitten.
(The author is a past
President of the IMA Goa State Branch, a founder member of VHAG and a healthcare activist)