Mosquitoes: The menace that kills a million a year, and why it is difficult to control it

CHENNAI: They are only 0.5 to 1 cm in size, but kill around a million people around the world every year. Little wonder, then, that mosquitoes have remained a major threat to the human race for several millennia.

The civic authorities in Chennai and elsewhere in the country face flak for the rise in the mosquito menace after every sporadic rain. While the complaints about a lack of preventive measures are valid, what is not evident is that the strategies to prevent and eradicate them have not been fully successful, as mosquitoes have prevailed, evolved over time, and continue to pose a danger to the health and well-being of mankind.

According to the World Health Organisation’s World Malaria Report 2023, the global tally of malaria cases reached 249 million in 2022. More than six million die due to malaria as per estimates. That is just one communicable disease. Add dengue, chikungunya, Zika, yellow fever, West Nile fever, and lymphatic filariasis, we have an animal that kills more people than any other creature in the world, enough for the Centres for Disease Control, US, to term it the world’s deadliest animal that kill about a million people every single year.

To put this in perspective, sharks kill just five people globally, while tigers, another apex predator, claimed around 100 lives in 2022 in India.

As one of humanity’s most formidable adversaries, thanks to their role in disease transmission, mosquitoes are one of the most intensely studied creatures on the planet. There are over 3,500 species of mosquitoes on Earth, and they are found on every continent except Antarctica.

However, only a handful carry the pathogens that cause disease, and it is these species that have been studied most intensely.

Research on prevention

Such is the scale of fatalities caused by mosquitoes that even the combined deaths due to nuclear bombs, natural calamities, man-animal conflicts, and the latest, climate change, are less than that.

With the tried-and-tested prevention methods failing to curb mosquito-related illnesses, researchers are coming up with new ways of source-prevention. Recently, entomologists at the University of Maryland, USA, bioengineered a deadly fungus that spreads sexually in Anopheles (malaria-spreading) mosquitoes.

The naturally occurring fungus called Metarhizium produces insect-specific neurotoxins, potent enough to kill female mosquitoes – the ones that spread disease by laying as many as 250 of them in each clutch. By dusting male mosquitoes with modified fungal spores, the team essentially created a sexually transmitted infection for mosquitoes. Despite being lethal to mosquitoes, the transgenic Metarhizium fungus is harmless to humans.

Such approaches are necessary primarily because of the mosquito’s remarkable adaptability, as scientists have discovered that mosquitoes and mosquito-borne parasites have developed resistance to chemical treatments and anti-malarial drugs.

There are three types of mosquitoes in India that cause outbreaks of malaria, dengue, chikungunya, and yellow fever. Anopheles acts as host for the parasite Plasmodium, which causes malaria. Aedes Aegypti is the one that causes dengue by spreading the DENV virus. It also spreads other viruses causing Zika, chikungunya and yellow fever. Culex is known to transmit several diseases, including elephantiasis. It also plays a role in transmitting viral diseases like the West Nile virus.

Malaria, dengue

Once an epidemic, malaria had wiped out millions from across the world. Some research said that it has existed from 5,000 years ago. Some others claimed that malaria has been a menace to humans from the time our ancestors discovered fire. It’s a historical fact that during the Vietnam War, nearly 24,000 American soldiers got malaria. Thanks to the invention of drugs such as Chloroquine and Artemisinin, malaria-induced fatalities have been reduced over the years.

Anopheles lays eggs on clean water surfaces such as ponds and wells. So, those residing near clean waterbodies are at risk of being infected, especially during the rainy days.

Researchers have found evidence to claim that dengue has existed since the 8th Century, especially in China. Initially, people believed that the deaths were due to a water poison, till scientific analysis revealed that dengue was caused by the virus DENV. Aedes Aegypti, which bites during the few hours after dawn and a few hours before dusk, lays its eggs over the surface of water stagnant in small containers like discarded tyres, plastic cups, and coconut shells.

According to the data of the National Centre for Vector Borne Diseases Control, there were around 1.6 lakh dengue cases in India from 2019 to March 2025. And, in 2021, there were 346 fatalities. The data also reveals that over these years, Tamil Nadu has performed well in controlling dengue, as fatalities stood at 46 in the last 5 years.

City’s strategy

S Selvakumar, chief vector control officer, Greater Chennai Corporation, said that the monitoring and prevention of dengue activities have been going on throughout the year. “Using drones, we’re spraying chemicals over all the major canals to wipe out the larvae. Door-to-door fogging is also taking place simultaneously,” he added.

The GCC has 2,200 workers visiting homes within the city limits each day to ensure “a non-conducive environment for mosquito breeding. All these cumulative efforts keep the numbers under check”.

A study by former corporation commissioner J Radhakrishnan, Selvakumar, and three others on the retrospective aspect of dengue in Chennai was submitted before the WHO in 2020. They had collected the monthly incidence of dengue in Chennai since 1999 and identified a pattern using the long-term mean.

The study found that the intermittent rain during the southwest monsoon plays a vital role in the rise of mosquito breeding. Hence, dengue cases peak in the monsoon months of September, November, and December. Since Aedes Aegypti is a daytime biter, the duration of daylight also significantly impacts dengue infection.

“In Chennai, dengue outbreaks have occurred in 11 out of the last 20 years,” stated the study.

The Lanka connection

In 2016, the WHO awarded the Sri Lankan health department for achieving three consecutive years of zero cases of malaria. However, that changed in 2018 when the number of malaria cases increased, leaving health officials perplexed about the reason.

A team of health officials from Sri Lanka reached Chennai. After conducting analysis with medical samples and data, they found that the Anopheles mosquitoes laid its eggs on the fishing boats in Rameswaram. When these were seized by the Sri Lankan Navy and taken into custody, the boats acted as carriers of those eggs.

DENGUE

· 2019-2025 (March): Nearly 1.6 lakh dengue cases in India

· 346 fatalities in 2021

· TN performed well in controlling dengue with 46 fatalities in 5 years

(Source: National Center for Vector Borne Diseases Control)

MALARIA

Around 2.2 billion cases of malaria and 12.7 million related-fatalities averted since 2000, but the disease remains a serious global health threat

In 2023, around 263 million cases and 5.97 lakh deaths worldwide, which is around 11 million more cases when compared to 2022, and nearly the same number of deaths

(Source: WHO’s World Malaria Report, December 2024)

LESS THREAT

· China was certified malaria-free in 2021

· Malaysia has had no cases of human malaria for six consecutive years

· Cambodia, Lao People’s Democratic Republic, Republic of Korea, Vanuatu, and Viet Nam had fewer than 10,000 cases in 2023

THREE TYPES OF MOSQUITOES IN INDIA

The following transmit pathogens and cause outbreaks of malaria, dengue, chikungunya and yellow fever

· Anopheles: Acts as a host for the parasite Plasmodium which causes malaria

· Aedes Aegypti: Reason for the widespread of dengue cases. Caused by DENV virus, and also spreads other viruses causing zika, chikungunya and yellow fever

· Culex: Known to transmit several diseases, including elephantiasis. Also play a role in transmitting viral diseases like West Nile virus