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South Delhi’s mosquito density up by 50% in a year: Survey

Updated on Apr 08, 2022 06:38 AM IST

However, the survey report shows that the detected mosquitoes were mostly of the Culex genus which do not cause dengue, malaria or chikungunya

A municipal worker fumigates a residential area to check the spread of mosquito-borne diseases in New Delhi. (Representative image/AP)

A six-day survey carried out by municipal entomologists has detected mosquito levels almost nine times higher than normal and around a 50% increase from last year in south Delhi areas. However, most of the mosquitoes detected were of the Culex genus which do not spread dengue, malaria or chikungunya.

The assessment exercise carried out between March 28 and April 2 by the public health department of the South Delhi Municipal Corporation (SDMC) has concluded that the Najafgarh zone has the highest mosquito density of 9.8 man per hour units, followed by the West zone at 9.4 units, Central zone at 9.2 units and South zone at 7.8 units. A ‘man per hour unit’, which is a measure of mosquito density in an area, is defined as the number of mosquitoes trapped in an hour in a particular area by a trained official.

Last year too, an early rise in temperature had led to a spike in mosquito density, with the corresponding period recording 6.1 units in SDMC areas. The 2021 report had stated that the West zone had the highest density at 7.2 units, followed by Central zone (6.6 units), South zone (5.8 units) and Najafgarh zone (4.8 units).

The report shows that the detected mosquitoes were mostly of the Culex genus which do not cause dengue, malaria or chikungunya.

The sampling exercise was carried out in areas like Airforce Colony in Tughlakabad, Pushp Vihar, Kapashera, and Paschimpuri, among other areas, said a senior public health official. “Under normal circumstances, the mosquito density should be between 0 and 1 man per hour but the disease occurrence depends on the level of viral or parasite presence within the mosquito population,” said the official. HT has seen a copy of the report.

The report shows that the detected mosquitoes were mostly of the Culex genus which do not cause dengue, malaria or chikungunya. “Culex mosquitoes are mostly harmless and cause skin irritation. Their high presence indicates a lot of stagnation and floating material in drains and water bodies as Culex breeds in dirty stagnated water. We have not been able to detect high presence of the disease-causing anopheles genus which causes malaria and aedes aegypti genus that causes dengue and chikungunya,” said a senior entomologist.

Delhi has so far reported 61 dengue, five malaria and eight chikungunya cases till April 2 this year.

Another SDMC official attributed the spurt in mosquito density to the sudden rise in temperature, adding that clogged drains and stagnant water has also increased mosquito breeding spots. “Mosquitoes become active and breeding levels rise rapidly above 35°C. This year, we have witnessed an unusually early summer,” the official said.

An entomologist from the civic body said that though the ideal mosquito density is zero, in dense urban areas like Delhi, the level should at least be brought down to 1-2 man per hour units.

Last year, the department had carried out a year-long mosquito breeding sites analysis which shows that peri-domestic containers -- such as money plant, vase, flower pots, bird pots, tin, tyre, fountain water etc -- account for largest chunk of mosquito breeding sites (38%).

Domestic water storage containers (drums, bucket, jerry cans etc), which are usually used in water-stressed regions, come second at 33% of overall cases.

Desert coolers (23%?), which find significant usage in summers, and overhead tanks (6%) come at third and fourth spot respectively.

“Three indices were used in the mosquito capturing exercises -- House index (HI), Container Index (CI) and Breteau Index (BI). CI goes up during the monsoon season and all three indices see year-long variation. With summer starting early, breeding sites in desert coolers are expected to see a spike in coming days,” official said.

HI is the percentage of houses found positive for larvae and the BI is the number of positive containers per 100 houses. CI is number of containers found with larvae among each 100 containers tested.

The three indices are used to judge the vulnerability of an area.

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