Explained: What are culex or common house mosquitoes that have resurfaced in Delhi, and how harmful are they?

What are the most vulnerable areas?

In the New Delhi Municipal Corporation (NDMC) area, the main hotspots for breeding of such mosquitoes are at the Kushak drain and at its border areas at Bapa Nagar and Bharti Nagar. East Delhi is most affected by Yamuna flood plain, South by Barapullah drain and North by Najafgarh drain. Unlike Aedes aegypti mosquitoes, which spread dengue and chikungunya and breed in clean water, culex mosquitoes breed in unclean stagnant water. The number of mosquitoes in areas along these drains is more.

How are they harmful?

Culex mosquitoes are known carriers of Japanese encephalitis, a potentially life-threatening but rare viral disease that causes “acute inflammation” of the brain. They breed in dirty, stagnant water.

How is the weather contributing to the increase in culex mosquitoes?

Medical health officer with East MCD Dr Som Shekhar said that as the water recedes in the summers, there are depressions in the flood plains where water still remains and doesn’t dry even in hot weather as the water table is high. Such areas are ideal breeding grounds for culex. Dr Arun Yadav, a senior doctor at Hindu Rao Hospital, said temperature for mosquito breeding is ideal at present: “The ideal condition for mosquitoes to breed is when the temperature is between 10 degrees Celsius and 40 degrees Celsius. This is the time to intensify the anti-mosquito drive.”

What are the civic bodies doing?

A senior official from the public health department of the EDMC said, “We have been putting mosquito larvicidal oil-coated blocks in major drains, which create a layer on the surface. Long-acting insecticides are also being put, which paralyses mosquitoes. South and North corporations are also planning similar exercises.

“Culex mosquitoes tend to breed in dirty water, especially in storm water and other drains, during March-April each year. To combat this, this year, we came up with a new method — we put barrels containing anti-larval medicines inside the drains and did underground fogging. This does not affect people. Sprays and medicines are also being put inside the drains for the last three days as a preventive measure,” said Dr R N Singh, senior chief medical officer of NDMC, who is heading the drive.

Then why are they still rising, what is hitting the MCDs anti-mosquito operations?

With the workforce for controlling the spread of mosquitoes engaged in tasks such as tax collection and house surveys, inspecting open defecation and weather conditions being favourable, a sudden rise in the number of mosquitoes has been observed in the city over the past few weeks. Debanad Sharma, president, Anti-Malaria Ekta Karamchari Union and a Domestic Breeding Checker (DBC) in South MCD, said that for past two months they have been engaged to monitor open defecation in the area, a house tax survey, and DDA’s door-to-door exercise in unauthorised colonies to encourage people to register their houses. Sources in the corporation said over 1,000 workers have been diverted for other duties. Data also shows that the number of house visits by DBCs has declined — from 44 lakh in 2018, 45 lakh in 2019, 19 lakh in 2020 and 21 lakh this year (all figures from January 1 till the second week of March). The figures in 2020 dipped massively because of the pandemic. Most of the workforce was diverted to disinfect areas and perform other Covid-related duties last year.

There are around 3,500 DBCs employed in the three MCDs– North, South and East– whose efforts have contributed to the control of vector-borne diseases in the capital over the past few years.

On a usual day, around 3,500 DBCs employed in the three MCDs visit 60-70 houses to check if coolers, overhead tanks and flower pots have stagnant water that could be breeding grounds for mosquitoes.