For
every tropical and sub tropical country of the world, mosquito menace
perhaps is the major health hazard. India is no exception. As far as
I remember, my home town, Pune, always had armies of mosquitoes on
rampage, almost every night except may be few nights of the summer,
when temperatures really soared. I remember in my child hood, we used
to sleep inside mosquito- nets hung over our beds. The operation of
getting inside our beds was in no way a simple thing. These mosquito
nets used to have a slit on one of the sides and the sleeper had to
dart in quickly through this slit or a few reconnoitering mosquito
patrols were likely to sneak in and would later have feast of
sleeper's blood throughout the night. There was also a practice of
burning 'Dhoop' incense in the evenings, however the invading armies
of mosquitoes would not be much disturbed in their mission, by such
trivial measures taken up by their hopeless targets or humans.
Some
of my neighbours had put fine wire mesh grills on each and every
window of the house making the entire house a giant mosquito net.
This was really effective and I liked the arrangement. However my
parents never liked it as they felt that the fine mesh grills block
the breeze in the night. The mosquito nets, which we used had another
problem. They were very effective dust catchers too and within few
nights of use, collected enough house dust so that sleeping in one of
them was an open invitation for allergies. We had no choice but to
use mosquito-nets as nothing else would really work.
Then
sometimes in 1970 came the mosquito coils, which are really a
mosquito-repelling incense, usually shaped into a spiral, and
typically made from a dried paste of pyrethrum powder. The coil is
usually held at the center of the spiral, suspending it in the air,
or wedged by two pieces of fireproof nettings to allow continuous
smoldering. Burning usually begins at the outer end of the spiral and
progresses slowly toward the centre of the spiral, producing a
mosquito-repellent smoke. The smoke if inhaled occasionally was
tolerable but on daily basis used to be purely bad for the throat.
During
WWII United States army developed a wonder mosquito repellent
chemical popularly known as DEET or N,N-Diethyl-meta-toluamide. It is
a slightly yellow oil and the most common active ingredient in insect
repellents. Heater type mosquito repellents, which vaporize the DEET
in air, first appeared in the market in 1970's in India and are
perhaps the most effective method to get rid of mosquitoes.
Even
though DEET is being used for last so many years as mosquito
repellent, scientists were not able to find how or why mosquitoes
smell this chemical and what causes them to turn away in disgust. A
US scientist of Indian origins, Dr. Anandasankar Ray and his team at
the University of California at Riverside in the U.S. have been able
to identify for the first time, the molecule on an insect’s
smell-sensing nerve cells (a common fruit fly,) that acts as a
receptor for DEET, They have published recently a paper in the well
known scientific publication 'Nature', which highlights their work.
To do
this, Dr. Ray and his team used a recently developed technique on a
common fruit fly, Drosophila melanogaster. Strains of this fly were
genetically engineered so that its sensory neurons produced a green
fluorescent protein when stimulated by an odour. The neurons that
glowed green when exposed to DEET turned out to be in a pit-like
structure in the antenna of the fly. They were able to establish
that a protein known as Ir40a was the DEET receptor that activated
those neurons. Dr. Ray says in this paper and I quote:
“Novel
repellents that are safe and affordable can be used to limit
insect-human contact in disease-endemic areas of the world and to
provide an important line of defence against deadly vector-borne
diseases,”
Having
established how mosquito repellents really work, the next step was to
look for better DEET substitutes. DEET itself has many disadvantages.
Its cost is high s high cost and it dissolves plastics and synthetic
fabrics. Many believe that its prolonged use could be harmful to
humans. To analyze other substitutes Dr. ray and his team created a
computer programme that could examine structural information about
various chemicals and quickly identify those that might be most
effective in stimulating the Ir40a-bearing nerve cells. The programme
was used to screen close to half a million compounds, including over
3,000 naturally occurring ones.
Ten
natural compounds identified by the computer programme, were tested
in fruit flies. Eight of them activated Ir40a neurons and displayed
good repellence. Four of those molecules were also shown to be
capable of repelling female Aedes aegypti mosquitoes, which can
spread the viruses that cause dengue and chikungunya. Three
of those four natural compounds had already been cleared as food
additives, greatly simplifying the process of getting regulatory
approval for their use as insect repellents.
I
have used DEET based mosquito repellents for years without any side
effect. Some people say that it is a carcinogenic material. Though I
have not found any supporting information as yet. I do not use it any
more because, when I renovated my house, I fitted fine wire mesh
grills on all windows in my house. According to me that works as the
best solution to get rid of mosquito menace. However it may not be
possible for everyone to have their residences modified. They can
certainly use DEET, till Dr. ray comes out with a better repellent.
12
November 2013
nice post
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