The city's silent, invisible war against mosquitoes

BOMBAY: Premdas Rathod, 41, an employee of the municipal insecticide department, walks through the congested streets of Madanpura. He barely notices passers-by. His gaze is fixed on something higher: the roofs and window plants that shelter his enemy, the mosquito.

He sees a sagging blue tarp covering the roof of a store. With a flick of his wrist, Rathod cuts the plastic with his long, sickle-shaped tool. The water flows downward: a potential mosquito breeding site has been eliminated. He then climbs onto the roof to the water tank. As expected, the reservoir is teeming with larvae of Aedes aegypti, the mosquito that carries dengue fever.

Rathod and his colleagues in the insecticide department are the invisible soldiers in the fight against mosquito-borne diseases. Every morning, they set out to find and eliminate mosquito breeding sites in slums, construction sites and even high-rise buildings.

Last year, the department identified more than 131,000 breeding sites for Aedes mosquitoes, the highest number in eighty years, and an equal number of breeding sites for malaria-carrying Anopheles mosquitoes. The record detection rates reflect the BMC’s increasing vector control measures in response to the rising number of diseases. In 2023, there were more than 7,000 cases of malaria and more than 5,500 cases of dengue, the latter resulting in nearly 15 deaths.

Mumbai’s war on mosquitoes goes back more than a century, with the creation of the Special Malaria Department in 1912. Back then, says Chetan Chaubal, head of the insecticide department, malaria cases were concentrated in the southern part of the island, where there were thousands of wells and factory areas with ponds that supplied water to the factories. Those areas are still hotspots, Chaubal says, partly because some factories have become too dilapidated for his team to access safely.

Today, malaria is not the only threat, of course. A few decades ago, the ministry considered Aedes aegypti simply a “nuisance mosquito,” Chaubal says. Today, it is feared as a vector for dengue and chikungunya. Globally, dengue is on the rise due to urbanization and climate change. In tropical areas, warming temperatures may help Aedes aegypti thrive at higher altitudes and during traditionally cooler seasons.

In Mumbai this year, malaria cases have been concentrated in the island districts of G South (Parel, Sewri), G North (Dharavi, Dadar, Mahim), E (Mazgaon, Nagpada, Byculla) and C (Bhuleshwar, Pydhonie, Marine Lines). In contrast, dengue poses a greater threat in the more populated suburbs. Aedes breeding sites have been identified mainly in K West (Andheri W, Vile Parle W, Juhu), R North (Dahisar, Mandapeshwar) and R South (Kandivli, Charkop, Poisar).

Anopheles, vectors of malaria, and Aedes, vectors of dengue, prefer clean water. But aedes breed in containers, laying their eggs on the side of anything that can collect water, from tires to bottle caps. Their eggs also survive drying out. And while Anopheles thrive in outdoor environments, Aedes are “peri-domestic” insects. This makes them harder to detect because inspections inside homes are limited, Chaubal says.

Chaubal’s 2,000-person team uses a four-step protocol: source reduction, minor engineering, biological control and, as a last resort, chemical control.

Rathod and his colleagues mainly deal with source reduction by removing used tires, turning over water containers, checking flower pot plates and draining water from tarpaulins. Small engineering operations involve measures such as repairing leaks and ensuring water tanks are “mosquito-proof” with tightly sealed lids to prevent entry of a mosquito.

It only takes a 1/16-inch gap for a female Anopheles mosquito to enter and lay her eggs, Chaubal explains.

Biological control is used for water sources that cannot be completely closed off, such as open wells, fountains and sewers. It involves releasing guppy fish which love to eat mosquito larvae.

When all else fails, Chaubal explains, the department resorts to chemical control. This involves using larvicides as well as “adulticides”, the latter being administered by nebulization. “The timing of the intervention is crucial,” explains an insecticide manager. “Since mosquitoes go from larva to nymph stage within 48 hours and do not feed during this phase, chemicals are most effective in the larval stage. »

The surge in construction has given rise to mosquito breeding grounds. Construction sites are now a major priority for the municipal corporation, says Dr Sudhakar Shinde, additional municipal commissioner, not only to further control the pests but also to monitor diseases among migrant workers. On the first floor of a building under construction in central Mumbai, officials show us how they spray insecticide on the walls of temporary quarters for workers. A construction worker at the site told ToI that officials periodically visit their site to check for fogging.

Inspections are often followed by complaints to the police and then legal action. The current fine for hosting a breeding site is between Rs 1,000 for individual houses and Rs 10,000 for construction sites. For comparison, Singapore’s strict dengue control program imposes fines of up to S$5,000. (Last year, a Singaporean woman was fined S$1,200 after authorities discovered mosquitoes breeding in her bathroom toilet bowl.)

Unlike Singapore, where authorities routinely enter high-end homes in outbreak-hit neighborhoods, officers in Mumbai warn housing societies before the monsoon and only enter buildings when a case is reported there. It can be difficult for field workers to get through security checks at complexes, Chaubal says. (Also in the early 1900s, a British malaria official complained of opposition from the “educated and wealthy propertied classes.”)

Vector control is not the only weapon against the disease for the BMC. Health officer Dr Daksha Shah says they are trying to control the parasite load by getting people to complete their 14-day antimalarial treatment and conducting follow-up tests.