Mosquitoes are packing a bite – in more geographies across the country than ever before.
In 2022, Ladakh reported its first two cases of dengue. With that, the vector-borne infection had breached its last bastion in India – from eight states in 2001, dengue touched every state and Union Territory that year.
This year too, with cases surging across the country – 31,464 cases by the end of July and counting – Minister of State of Health and Family Welfare S P Singh Baghel, in response to a query during the Monsoon Session of Parliament last month, said cases had risen from 16,517 in 1996 to 2,33,251 in 2022, an increase of 1,312%.
As reasons for the rising dengue cases, the minister cited shortage of entomologists (scientists who study insects), increased travel, failure to control the vector population, and less public participation in preventing the spread of the infection.
But the response missed one more factor, hidden in plain sight: climate change.
Dr Akshay Dhariwal, former director of the National Centre for Disease Control (NCDC) and National Vector Borne Disease Control Programme (NVBDCP), told The Indian Express that climate change does influence the spread of vector-borne diseases such as dengue.
“It is an established fact that the breeding and density of different arthropods (a zoological term used for insects like mosquitoes), which transmit vector-borne diseases, depend on climatic conditions. So, there is definitely a strong association between the spread of vector-borne diseases and climate change,” he added.
Several studies have shown that climate change has helped mosquitoes to thrive and multiply even in areas where they weren’t present before. This has led to an increasing number of outbreaks of infections such as Zika virus, chikungunya, malaria and dengue, around the world. These diseases kill more than one million people and infect up to 700 million each year – almost one in ten people — according to the World Mosquito Program. And if the planet continues to get warmer at the present rate, these numbers are set to increase multifold.
Vector-borne diseases kill more than one million people and infect up to 700 million each year – almost one in ten people. (Express photo)
The climate link
Mosquitoes, like all arthropods, are cold-blooded creatures. As a result, they can’t regulate their body temperature, which corresponds to the surrounding environment. Therefore, climate change and its impact on weather have an enormous influence on mosquitoes.
Temperature is probably the biggest driver of mosquito activity and most of their species thrive in warmer climes. Thus, with global warming, say experts, the geographical spread of where mosquitoes can breed and survive has increased, ultimately causing several infections.
Take Himachal Pradesh, for instance. Until 2011, the state reported zero dengue cases, but in the following seven years, the number jumped to more than 4,600 cases annually. The increase corresponds to a rise in temperatures in the larger region — mercury levels rose by about 1.6 degree Celsius in the northwest Himalayan area, according to the Himachal Pradesh State Disaster Management Plan 2012.
In ‘Risk of Dengue Epidemics in the Northern Himalayan State of India: Are We Prepared Enough?’, a correspondence by ICMR-National Institute of Malaria Research’s Gaurav Kumar and Shweta Pasi to the Journal of Microbiology and Infectious Diseases, the authors have pointed to many factors that may have facilitated the spread of dengue in Himachal Pradesh. However, climate change was held as the foremost reason.
“Factors that have been found to underlie the expansion of dengue include climate change, urbanisation, globalisation with increased transport, and lack of preparedness in terms of vector-control facilities. Among these, climate change appears to be primarily responsible for seeding the disease in Himachal Pradesh,” they wrote.
As reasons for the rising dengue cases, Minister of State of Health and Family Welfare S P Singh Baghel cited shortage of entomologists, increased travel, failure to control the vector population. (Express Photo)
A 2022 study, ‘Distribution Expansion of Dengue Vectors and Climate Change in India’, done by ICMR’s Ramesh C Dhiman and Syed Shah Areeb Hussain, and published in the journal AGU, noted that dengue vectors, Aedes aegypti and Aedes albopictus, are at risk of expansion in some parts of Thar Desert and upper Himalayas due to climate change.
Higher temperatures have also extended the length of the season when mosquitoes are active, allowing a longer time period for the transmission of vector-borne diseases. India may witness longer mosquito seasons in the following years, say experts.
But mosquitoes aren’t susceptible to very warm and dry conditions. Their activity usually starts to slow down beyond 32 degree Celsius and so does the transmission of vector-borne diseases. For instance, malaria is most likely to spread at 25 degree Celsius while the risk of Zika is highest at 29 degrees Celsius, a 2019 report by Stanford Earth Matters magazine said. Anything beyond that and mosquitoes are known to flounder.
“The good news: higher global temperatures will decrease the chance of most vector-borne diseases spreading in places that are currently relatively warm. The bad news: warming will increase the chance that all diseases spread in places that are currently relatively cold,” it added.
Apart from warmer temperatures, there are other ways through which mosquitoes multiply. Their population has also been fueled by climate change-induced extreme events. Untimely rainfall, storms, flooding, and rising sea levels create shallow, stagnant pools of water in which the bugs thrive. On the other hand, droughts lead people to collect and save water in containers that can provide breeding places for mosquitoes.
More ‘blood meals’
Climate change doesn’t just help mosquitoes proliferate or survive for longer; it also impacts their reproductive capacity. A 2009 study, ‘Local and Global Effects of Climate on Dengue Transmission in Puerto Rico’, published in the journal PLOS, showed that an increase in temperatures stimulates egg hatching and accelerates growth of the larvae, thereby reducing the time for maturity. This in turn spikes the rate of transmission of diseases.
In warmer climates, mosquitoes begin to bite more frequently. Only female mosquitoes are known to bite humans in order to get a “blood meal” — they extract proteins from our blood to produce their eggs. Higher temperatures result in a faster rate of blood meal digestion, making mosquitoes hungrier and leading to more bites.
Elevated humidity plays a role in changing their behaviour too. Research suggests that mosquito activity peaks on warm and humid days. The combination of humidity of 42% or higher and temperatures between 10-35 degree Celsius is ideal, a 2020 study by the US National Institute of Health noted. High humidity is associated with increased feeding activity, survival, and egg development, another research, ‘Climate and Dengue Transmission: Evidence and Implications’, published in 2013 indicated.
A 2015 file photo of Youth Congress leaders and supporters during a symbolic protest (mosquito net with a photo of then Mayor Sovan Chatterjee) in front of Kolkata Corporation head office regarding recent death caused by dengue in city. (Express photo)
Tackling the problem
The website of the National Centre for Disease Control lists the following measures to control mosquito-borne diseases: use mosquito nets or mosquito repellents while sleeping even during day time; remove water from coolers and other small containers at least once a week; use aerosol during daytime to prevent mosquito bites; ensure water doesn’t lie stagnant in drains, garbage and coolers for more than a week, and so on.
Speaking to The Indian Express, ICMR’s Dhiman highlighted the importance of raising awareness about the impact of climate change on vector-borne infections. He said people need to know “how to prevent the propagation of mosquitoes, how to prevent mosquito bites and seek health facilities if taken ill, and take complete treatment as prescribed by authorised medical practitioners”.
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But these actions may not be enough to fend off diseases. Experts have noticed a behavioural change in mosquitoes following the use of indoor residual spray (IRS) — a core vector-control intervention that can rapidly reduce malaria transmission — and long-lasting insecticidal nets.
Dhariwal explained: “In Tripura, because of the distribution of long-lasting insecticidal nets and use of IRS, we observed that mosquitoes changed their biting habits. Instead of coming and biting at night (when the nets and spray were usually used), they began to bite in late evenings when more individuals were awake.”
A better bet to control vector-borne diseases, therefore, could be to tackle climate change.