Bleaching powder won't do, crush eggs to tame dengue: Expert

As West Bengal is grappling with the outbreak of dengue, an expert has observed that spraying of bleaching powder or insecticides will be ineffective as mosquito eggs cannot be crushed by it.

The senior entomologist is of the view elimination of mosquito eggs is the only way to tame dengue. "Spreading bleaching powder or spraying insecticide will not help in controlling dengue because bleaching powder does not crush the eggs laid by dengue mosquitoes.

Effect of these insecticides lasts half-an-hour or at the most one hour. It kills only mosquitoes, a scientist of the Zoological Survey of India, Dr Devi Shankar Suman told PTI.

"The primary source of dengue mosquito is larval habitat which they lay around. So the most important part is to finish off the eggs," he said. The West Bengal government has ordered civic bodies to spread bleaching powder in areas where dengue cases have been reported.

So far, there have been 13 deaths in state-run hospitals due to dengue while over 20,000 cases reported from parts of the city and its surrounding suburbs.

Describing eggs of dengue mosquitoes as the "strongest stage in the life-cycle of mosquitoes carrying the virus", Suman said that the biological characteristics of this mosquito were very different and unless and until they were understood, it would be difficult to destroy it.

"To smash the eggs, you have to spray insecticide within 24 hours because after that the eggs create a protective layer around them and not let any insecticide penetrate it. So you need a 1,000-time more stronger dose of insecticide to quash them. Egg is the strongest stage in the life-cycle of dengue mosquitoes," he added.

The entomologist has been conducting a research work on mosquitoes and has collected samples from several areas like New Alipore, Maheshtala, Garia and Botanical Gardens and other places in the city. He disagreed with doctors' claims that dengue this season had changed character making it more difficult to deal with.

Elaborating on the character of the mosquitoes, he said that dengue mosquitoes lived in the periphery of houses and it could breed in one-litre of water as well as in water contained in a bottle cap.

"Locating the breeding place is more important before spraying insecticides here and there. Recent research shows that there is no successful method to penetrate in those bottle caps or cans we use daily. Only people can eliminate the vector. If it is not repetitive, it will not be affective. We do not have the tools and techniques to smash the eggs," he said.

Dengue mosquitoes lay eggs on moist soil which can last up to a year until the ground is flooded again, he said. According to the ZSI scientist, dengue mosquitoes can fly up to 200 metre and people should be very cautious if any dengue case is reported from a particular locality.

"It bites mostly during daytime when humans are active but not at night. We must remember that dengue mosquitoes do not lay eggs in dirty water but in water which is kept in a place for over a period of time," he said. "Once dengue mosquitos enter a locality, it is almost impossible to drive them away from that area. Once they lay eggs, those will remain intact for the next six months or even a year," he said.