KOLKATA: Preventing dengue attacks is becoming increasingly difficult, thanks to the Aedes Aegypti mosquito's ability to adapt to circumstances. A recent study by the Kolkata Municipal Corporation's (KMC) entomology department has revealed that the deadly mosquito — associated with both dengue and yellow fever — has changed its life-cycle in a major way, leading to severe concern among experts trying to combat the disease. While some believe this has happened due to mutation in the insects' genes, experts like Amiya Kumar Hati, former director of the School of Tropical Medicine in Kolkata, say that it has simply changed its behaviour to match the environment.

"Till recently, we knew that the Aedes Aegypti bred in stagnant water tanks inside bathrooms, under refrigerators (where water collects after decompression) or in small pots in the house where sunlight doesn't reach. But the recent study has revealed that this variety of mosquito is now breeding in open spaces along with the malaria-causing Anopheles. This may have happened due to rapid urbanization and changes in the environment," said Debasis Biswas, chief entomologist, KMC.

It seems that the Aedes Aegypti has turned 'photophilic' from 'photophobic', making it harder to trace. A photophobic organism prefers darkness while a photophilic one needs light. As a result, the dengue-causing mosquito is now breeding in water accumulated in old tyres, open water tanks and rooftops. These are the places where people never searched for its larvae till now.

In the 1930s, when malaria was endemic in Kolkata, the city was much greener and the concentration of Aedes Aegypti was much lesser compared to what it is today. The Aedes Albopictus variety — which breeds in trees — was much more common. The dengue vector started to increase in numbers as urbanization spread. Today, this variety is also breeding in water that accumulates on the streets after the rains, which used to be the domain for the Anopheles earlier.

"All this while, the KMC was trying to spread awareness among residents not to store water in their homes. Now, it seems that this was of little help. The Aedes Aegypti clearly got more aggressive and this led to mutation of its genes. The Anopheles is less harmful as it is a 'nervous' feeder. It continues to consume blood from a single person till its stomach is full. Normally, it doesn't bite more than a single person during one feeding. The Aedes Aegypti targets many people and consumes a small portion of blood from each," Biswas explained.

As a result, the number of cases of malaria has shown a clear decline while those of dengue have actually gone up. Between January and July, 2011, there were 726 cases of falciparum malaria in the KMC area. In 2012, in the same period, this figure dropped to 105. On the contrary, the number of dengue cases has risen from 60 in 2011 to 480 in 2012. Of the 480 cases, 180 people have been detected with the deadly haemorrhagic variety.

"A few days ago, we found Aedes Aegypti larvae in a bamboo pole used as a fence outside a school in Rajabazar. Earlier, we couldn't have imagined that this mosquito could have bred in only half a cm of water. We are certainly concerned," another expert said.

While MMiC (health) Atin Ghosh pointed out that malaria and dengue cases are on the rise worldwide, Hati said that even the Aedes Albopictus has made a comeback and is responsible for the rise in the number of cases.