Aedes aegypti, the mosquitoes that transmit dengue, begin breeding in underground tanks, water containers and coolers months before monsoon begins, found researchers who tested water samples from multiple sources in urban south and west Delhi and rural Haryana.
The study, which tested fresh water samples from multiple neighbourhoods in four sites — two each in Delhi and Haryana — from May to October, found 98% of the mosquito larvae were of Aedes aegypti.
“The presence mosquito larvae before the monsoon season suggests that measures to control mosquito-borne illnesses such as dengue and chikungunya must start earlier in the year,” said Dr Sujatha Sunil, International Centre for Genetic Engineering and Biotechnology (ICGEB), who is one of the authors of the study that was published in the Acta Tropica journal.
“Before the rains, mosquito breeding is restricted to water containers and tanks, but as soon rains start, the mosquitoes move out to the fields and other open spaces to breed, increasing human exposure to infection,” said Dr Sarala Subbarao, a co-author and consultant in the vector-borne disease forum of Indian Council of Medical Research.
For the study, fresh water samples were collected from south and west Delhi, and Bahadurgarh and Bhadana in Haryana in the beginning of every month between May and October 2012. “Monsoons arrived late and rain was sparing all through July in 2012, yet mosquito breeding was happening in early May,” Dr Sunil said.
In South Delhi, plastic containers of 10-30 litres capacity used for water storage constituted 57% of the containers examined. The maximum breeding, however, was found in underground water tanks.
“These tanks are huge and are used to store water, which is then pumped to overhead tanks. They are not emptied out and cleaned regularly, which provides favourable conditions for breeding,” said Dr Subbarao.
“Mosquitoes need only a thin film of water to breed and can even breed in almost dry containers,” added Dr Sunil.
In West Delhi, where the water supply is more regular, coolers constituted the major source of water storage; 45% of total containers examined. It was also the major source of breeding.
The researchers also found breeding in peri-urban areas of Bahadurgarh and Bhadana too. “Aedes aegypti is largely considered to be an urban vector but we found it in the rural areas as well, mostly in cement water containers for the cattle,” said Dr Subbarao.