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.