An unprecedented mosquito menace in winter has forced the mayor to order a crackdown on the dengue and malaria-carriers.

Bikash Ranjan Bhattacharyya last week asked chief municipal health officer Deb Dwaipayan Chattopadhyay to take steps to prevent the breeding of mosquitoes. The civic health department subsequently decided to deploy over 500 field-workers to spray bio-larvicide in the city from Monday.

“We are getting complaints about the number of mosquitoes from across the city. Complaints have been pouring in from Gariahat, Ballygunge, Tollygunge, Cossipore, Sealdah, Picnic Garden, Kasba, Behala and Jadavpur,” said a civic health official.

Municipal commissioner Alapan Bandyopadhyay said: “It is strange that the number of mosquitoes is increasing though the number of cases of malaria and dengue is much less than the number detected last year. I think this is an effect of global warming.”

On the commissioner’s direction, the civic vector control department conducted a three-day survey in different parts of the city. The study revealed an unusually large concentration of mosquitoes in several city pockets in the last week of November. About 85 per cent of the mosquitoes are of the Culex variety, which does not spread either dengue or malaria.

According to a table on mosquitoes prepared by the Calcutta Municipal Corporation experts, mosquito eggs are in hibernation from the first week of November. They do not hatch till February.

Civic entomologist Debasish Biswas said hibernation depends on temperature. This year, the temperature has seldom fallen below 22° Celsius. For mosquito eggs to go into hibernation, the temperature needs to be around 18° Celsius or below. The average relative humidity is also on the high side this winter, added Biswas.

The survey found that areas close to canals like Tolly’s Nullah and those with open surface drains were the worst affected by mosquitoes. The most vulnerable pockets are Ballygunge, Kalighat, Chetla, Tollygunge, Dhakuria, Prince Anwar Shah Road, Golf Green, Lake Gardens and the eastern fringes of the city, where large housing projects are coming up and surface drains are common.

So far, 12 people in the city have contracted classical dengue. The number of people diagnosed with malignant malaria this year is 1,000. The corresponding figures last year were 394 and 3,000, respectively, said Chattopadhyay.

In 2006, there were two deaths in the city — one owing to dengue, the other owing to malignant malaria. This year, none of the patients has died.