As the sun began to set on Delhi, 45-year-old Rani hiked up her salwar pants, squatted next to the iron pan just outside her home, and lit a match. The plastic grocery bags were the first items to catch fire. Soon the cow-dung cakes ignited, their chocolate-brown edges glowing in the dusk. Rani coughed as smoke rose from the pan.
All around, Rani’s neighbors performed a similar drill. Some substituted egg trays for cow dung, or omitted the plastic bags, but no matter the kindling, the goal was the same: to repel mosquitoes by means of smoke and other toxic fumes. Indians have long employed this do-it-yourself approach to insect control, but over the past couple of years, as the city’s mosquito population has exploded, the burning has become a nightly ritual in low-income housing developments across this city of more than 30 million people.
According to a recent survey conducted by the South Delhi Municipal Corporation, Delhi’s mosquito density was almost nine times higher than normal this past March and April, a 50 percent increase over the previous year. Yet local authorities did not mount a vigorous response because the insects belonged to the Culex genus, which is not known to transmit the well-known diseases—malaria, dengue, chikungunya—that are at the forefront of India’s public health initiatives.
When it comes to malaria, in particular, India has achieved success in reducing disease. But even as malaria deaths are on the decline, the sheer number of mosquitoes, particularly in urban areas, has shot up. This is partly due to climate change, said Ramesh C Dhiman, an expert in malaria epidemiology who spent three decades as a government researcher at the Indian Council of Medical Research before becoming an independent consultant. Mosquito populations are on the rise in other countries, too, fueled not just by climate change, but by increased urbanization and the decay of residual DDT in the environment.
A spokesperson for Delhi’s municipal government, Amit Kumar, told Undark that the local government has taken a number of actions to combat the problem, including spraying insecticides on public drains and other water bodies, which serve as breeding grounds for mosquitoes.
These measures were temporary and did not address the severity of the issue, said a Delhi public health official who asked not to be named for fear of retribution from his employer.
The mosquitoes in Rani’s neighborhood are so insufferable that children and adults struggle to sleep through the night. While it’s not yet much of a problem in Delhi, residents could also face some risk of diseases that are transmitted by Culex mosquitoes, including West Nile and Japanese encephalitis. According to experts, this risk may increase as mosquitoes evolve in response to changing climatic conditions. For the moment, low-cost do-it-yourself remedies like smoke and insecticides offer some measure of relief. But researchers note that these approaches pose a risk to human health and fail to address the underlying problems that allowed the mosquitoes to flourish in the first place.
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Delhi's surge of Culex mosquitoes comes at a time when public health officials are declaring notable victories against other kinds of mosquitoes, including the Anopheles genus that transmits malaria. While those gains have saved lives, the situation, mosquito experts say, is complicated: The very changes that have reduced Anopheles’ numbers may be allowing other species to thrive. And amid a changing climate, mosquitoes have found new niches to exploit, especially in urban areas.
Over the past few decades, malaria’s global footprints have diminished, thanks in part to interventions such as mosquito nets and insecticides used to target Anopheles. In India, such interventions have been implemented with the help of a government agency called the National Center for Vector Borne Diseases Control. The program’s efforts helped dramatically reduce malaria deaths in recent years.
A retired government official who worked in northeast India at the ICMR for nearly three decades, Vas Dev, said deforestation likely contributed to declining malaria rates in India, but it came at a cost. Increased urbanization creates more habitat for mosquitoes that prefer urban and suburban landscapes, including Culex and Aedes, the mosquito genus that transmits dengue, Zika, and chikungunya. Since 1970, dengue has spread dramatically in poor countries, killing thousands of people each year, mostly children.
Scientists are working to better understand how changing landscapes and climate will affect mosquito populations in the future. In Delhi, climate change has already extended the breeding season by bringing higher temperatures to months that were formerly too cool for reproduction. Untimely rains have also fueled the mosquito population by increasing humidity levels and contributing to standing water in the environment. As a result, said Dhiman, areas that might have once experienced a one-month mosquito season are now experiencing seasons that stretch for six to eight months.
The insects are known to adapt quickly to changes in their local environment. Anopheles mosquitoes provide an interesting example, said Karthikeyan Chandrasegaran, a postdoctoral researcher at Virginia Tech who has expertise in evolutionary ecology and mosquito biology. The malaria-transmitting insect is known to bite between dusk and dawn, so public health organizations working in sub-Saharan Africa invested in bed nets for the local residents there. Initially, these interventions proved effective, but within less than a decade, cases spiked. It turned out the mosquitoes were feeding in the early morning—after people had gotten out of bed. Mosquitoes can also evolve resistance against commonly used insecticides.
City-dwellers are likely to experience the brunt of any problems, said Chandrasegaran. Poor waste management, lack of sanitation, and irrigation all create opportunities for the insects to thrive. Some cities, like Delhi, are also contending with water shortages, a situation that has led residents to hoard scarce supplies in buckets that can become breeding sites. These conditions are less acute in rural areas, which also harbor greater numbers of mosquito predators, including certain fish and frogs.
But rural areas have challenges, too, including limited health care infrastructure and poor awareness of vector-borne diseases. “So, you’ll have to probably tailor your solution differently to urban areas, tailor your solution differently to suburban areas, rural areas, forested areas,” said Chandrasegaran. “If you do not identify the pain points exactly, you are going to spend a lot of time and effort and money trying to implement one scheme across the entire country, which is going to waste a lot of things.”
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Rani, who like many Indians goes by one name, sat outside with her children on a high cot not far from the iron pan and its steady smoke. They chatted about the day, and one of Rani’s daughters, Meenakshi, mentioned how her teacher had asked the class to participate in a mindfulness activity. The children were to keep their eyes closed and their bodies calm. Unlike her wiggly classmates, Meenakshi excelled at the task. In reality, she told her mother, she had fallen asleep.
Rani took this news in stride. The previous night, the mosquitoes made it hard to sleep, she explained. Many children skipped school because they were exhausted in the morning—a common occurrence that keeps low-income children out of classrooms. Adults struggle to sleep during mosquito season, too. One woman told Undark that her blood pressure rises when the mosquitoes get really dense. Other residents reported sleeping on busses, rickshaws, and trains while commuting to and from work.
Some families leave their pans burning all night, but when Rani is ready for bed, she douses hers with water so she won’t feel suffocated by smoke as she tries to sleep. Rani and her children do use mosquito netting, but they rarely spend the whole night behind its protective shield. Sometimes the children need to get up to use the toilet or get a drink of water, she said, or they get too hot inside. And even a small opening in the netting allows the mosquitoes to enter.
Research indicates that mosquito nets can protect the individual user while also reducing disease transmission within the wider community. Despite this, many individuals who own nets do not use them consistently. A small study conducted in homes in Asia and Africa found that the nets decrease airflow, and researchers have hypothesized that this could explain the spotty uptake. In homes like Rani’s, which lack regular electricity for fans or air conditioning, the reduced airflow can make it even harder to sleep at night.
But the DIY remedies that have become popular across various parts of India bring their own set of problems. Palak Balyan, a scientist in New Delhi who works for the U.S.-based nonprofit Health Effects Institute, said that burning of any kind of material produces the tiny particles known as PM2.5, a type of air pollution that is responsible for millions of premature deaths each year. Research suggests that emissions of PM2.5 have shortened the average Delhi resident’s lifespan by up to 10 years. While the biggest source of this pollution in Delhi is transportation, experts worry that DIY mosquito control is worsening the problem.
In addition to burning cow dung and plastic, Delhi residents also use coils, liquids, and incense sticks to repel insects with odor and fumes. The repellants’ effects on human health have not been well-documented, but the available research suggests that caution may be warranted. One study found that burning a coil releases the same amount of PM2.5 as burning 75 to 137 cigarettes. Another study found heavy metals like zinc, cadmium, and lead in popular coil brands. “Carcinogenic risk is there for 350 people per million population,” said the study’s lead author, S.N. Tripathy, a professor at the Indian Institute of Technology Kanpur.
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On its website, the National Center for Vector Borne Diseases Control lists the use of these mosquito repellents as one of several strategies for vector control. But the Delhi public health official characterized the repellants as a short-term strategy of dubious effectiveness. In India, they are part of a 50-billion-rupee business—over half a billion dollars—but they are not a solution. For one thing, the repellants don’t even kill the mosquitoes; they merely prompt the insects to go elsewhere. The mosquitoes, the official said, “just move from one place to another, but they do not die.”
The Delhi public health official and other experts interviewed by Undark said they were unaware of the extent of the outdoor burning in Rani’s neighborhood and beyond. The city’s low-income neighborhoods tend to be isolated, overlooked by the city, and looked down upon by other Delhi residents.
Several researchers said that municipalities need to step up and address mosquitoes so the burden doesn’t fall on individuals. This means better insect surveillance, as well as improvements to sanitation and drainage systems. In Rani’s neighborhood, for example, the homes do not have indoor plumbing, so wastewater flows directly into the streets, creating a breeding habitat for mosquitoes. The city’s biggest drain, which carries sewage into a local river, passes about 10 feet from Rani’s one-room house.
Housing quality is important, too. Mosquitoes love the dark, humid, and unventilated spaces so often inhabited by India’s poorest residents, said Dhiman. Rani’s house has just one window, often open so that air can circulate. Even so, moisture lingers on the mud floors and cement walls. A small light bulb hangs from a wire in the ceiling, providing minimal lighting.
Outside that house, as the evening wears on, Meenakshi turns to her homework. She’s still sitting on the cot, her hands kept busy, turning book pages, fanning the air to scatter smoke. She swats mosquitoes, scratches the bites. Rani is thinking of buying a topical repellant, but the ointment is expensive, and who knows if it will work? Perhaps tonight Rani will leave the pan burning, just to see if it helps her fall asleep.