Sept 21, 2005
Dengue: Fever Pitch
Mozzies go for upgrades too
The enemy adapts to changing conditions - and fast. Lee Hui Chieh
and Judith Tan find out more.
The dengue-carrying Aedes mosquito bites only during the day,
breeds in clean water and can fly a distance of just 50m to
100m.
Not necessarily so, says Singapore's National Environment Agency
(NEA). Research by the agency's Environmental Health Institute has
shown that the Aedes mosquito can bite at night under artificial
light, can breed in water contaminated by repellent and can fly as
far as 740m.
When push comes to shove, the enemy will adapt. And scientists have
found mosquitoes to be quick at adapting to a changing
environment.
There are two known dengue-spreading Aedes mosquitoes here: the
Aedes aegypti and the Aedes albopictus.
Both have black and white stripes on their legs but the aegypti has
two lyre-shaped silvery markings on its body, while the albopictus
has one central line running down its body.
The aegypti is seen as a bigger menace than the albopictus as it
tends to be found more often indoors than outdoors and as it tends
to take multiple bites, often from different people, instead of
just one bite like the albopictus.
The Aedes mosquito is usually thought of as being active during
daylight hours, with the peak biting periods being dawn and
dusk.
But studies have shown that it bites under artificial lighting too,
so people at home and in offices at night may not be as safe from
dengue as they think.
Another study has demonstrated how the Aedes mosquito can adapt to
poor breeding conditions.
It usually lays eggs in clean stagnant water, avoiding dirty water.
However, the study found that mosquitoes bred in water contaminated
with repellent grew up to not exhibit any preference - dirty water
no longer bothered them.
This means that if the mosquito has no choice, it could lay eggs in
dirty water, breeding a new generation that will not have any
problems doing so.
In another study published last year, the institute's medical
entomologist and researcher, Dr Christina Liew, found that the
Aedes mosquito can fly as far as 740m and as high as a 21-storey
building, or 60m high.
She released female mosquitoes that had been fed blood with a
special marker and tracked where they laid their eggs. The
mosquitoes' mouths had been sealed shut with super glue to ensure
they died after laying their eggs.
In a rural area, where there was just open land, the marked eggs
were found as far as 740m away from the place where the mosquitoes
had been released. In an urban area with buildings, they were found
568m away.
In an unused apartment block with no working lift, they were found
quite evenly distributed over the building's 21 floors.
It had long been believed that the Aedes mosquito operated over
much shorter distances and at lower heights.
'We have removed a lot of their breeding sites so they are now
forced to fly further to find suitable sites,' explained Dr
Liew.
'It's possible for them to adapt to conditions that were previously
thought to be unfavourable, and that's why although vector control
is so good, mosquitoes continue to breed.'
The current breed of mosquitoes is a result of selection and
adaptation, said Associate Professor Rudolf Meier from the
biological sciences department at the National University of
Singapore.
'Every time we generate a new environment, some genetic strains of
the mosquito will have a breeding advantage over others that are
not as good at using the resources in the environment. The
mosquitoes with the better means of exploiting the resources will
replace those that are less capable,' he said.
'Is this through learning? No. Mosquitoes have little chance to
learn. They never meet their parents and live for only a few weeks.
But there is plenty of evidence that sophisticated behaviour can
have a genetic basis.'
He added that when a population of mosquitoes is treated with
insecticides, a small fraction of them will be naturally resistant
to the chemical.
'Given that most of the 'normal' mosquitoes would have been killed
by the insecticide, the few resistant survivors would have all the
breeding sites in the environment at their disposal. They very
rapidly rebuild the population, which now consists of mosquitoes
that have genes conferring resistance to the insecticides and
treating this population with the same insecticide again will be
largely ineffective,' he explained.
He suggests that the only way around the problem is to resort to
fogging and spraying only during a dengue outbreak. This will
quickly reduce the size of the mosquito population to prevent
infection.
'Fogging during an outbreak in the affected area is a good idea as
long as it still kills any of the adult mosquitoes. The chance of
getting infected is largely proportional to the number of
mosquitoes in the area; that is, even a relatively small reduction
in numbers can make a difference.
'Fogging in areas without an outbreak is more problematic because
the fogging starts a selection process for resistance. Once the
population has a high proportion of resistant mosquitoes, it will
be difficult to reduce the number of adults quickly when an
outbreak occurs,' he said.
Regular fogging, he said, will kill not only the mosquitoes but
also their natural enemies, making the mosquito problem more
serious. 'This leaves a stronger and more lethal mosquito
population that would be increasingly difficult to deal
with.'
The NEA is looking into how to enhance measures to deal with new
information about the mosquito's behaviour.
So, what does all this mean for the individual?
'Don't be complacent about where the mosquito can be found; it
could be in your home,' Dr Liew said.
The war continues.
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