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Source reduction involves preventing development of
mosquito larvae. The female anopheles mosquito lays eggs in collections of clean water.
Each female anopheles mosquito lays millions of eggs in its lifetime of 4-8 weeks. The
eggs hatch into larvae, which then develop into pupae and adults in a span of 7-10 days.
The best method of mosquito control is preventing the development of the eggs into adult
mosquitoes. These anti larval measures are not only simple and cost effective, but also
environment friendly.
Anti
larval measures are the mainstay in malaria control and include the following:
a. Preventing egg laying:
The easiest, cheapest and most environment-friendly meathod to control malaria is by
preventing the mosquito from laying eggs. This is done by avoiding or eliminating the
clean water collections. Most such collections are artificial, temporary and man made.
It is common habit to throw
the unutilized utensils, buckets, bottles, tyres, tender coconut shells etc. into the
open. During the rains, water gets collected in these containers and provides ample
breeding locations for the female anopheles mosquito.
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Buckets thrown in the open |
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Tyres thrown in the open |
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Tender coconut shells thrown in the open |
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Open tank |
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In
the cities, the other sites for mosquito breeding are the water tanks. Shortage of water
supply in large cities makes it necessary to have these tanks in virtually every building.
Overhead tanks, sump tanks, storage tanks, ornamental tanks etc. are often left uncovered
and this provides scope for mosquito breeding. |
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Open overhead tank |
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Open tank |
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Also,
it is common to find puddles of water everywhere during the rainy season. This is the
reason why malarial transmission is at its peak during the monsoon. |
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Puddle |
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Puddle |
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There
is abundant scope for water collection in and around the construction sites: water stored
in tanks; the layer of water on the surface of the cement concrete (used for 'curing' the
concrete and left as such for 3 weeks); puddles of water in and around the place of
construction - all these provide scope for mosquito breeding. To add to the problem,
construction workers tend to harbour the malarial parasite, due to frequent infections
owing to their poor standards of living. Thus, construction sites not only provide for
mosquito breeding but also supply the parasites. This is the reason why malaria tends to
be more common in cities where construction activities are in full swing. |
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Concrete Curing |
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Open tank |
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Tiled roof |
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Flat terrace |
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The
older houses have tiled roofs that are sloping. This helps easy drainage of water during
rains, thus minimising water logging. In the recent years, most new constructions have
concrete roofs and terraces that tend to be flat and non-sloping. These roofs/terraces may
not have proper drains for water-flow. As a result, water tends to collect on these
rooftops during the rains and this provides ample scope for mosquito breeding. |
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In
addition, there are the natural collections of water like the wells, lakes, ponds, paddy
fields, marshlands etc. where mosquito breeding occurs in abundance.
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Unused well |
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Therefore,
unless these breeding sites (most of which are man-made and temporary) are taken care of,
it is impossible to control mosquito breeding and hence malaria. And it is impossible to
achieve this without the participation of the general public. Education of the people is
thus very important for any meaningful action. The following measures are called for to
minimize mosquito breeding and these measures require only a trifle of human efforts:
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Do not throw utensils, vessels, buckets,
tyres, bottles, tender coconut shells etc. in the open. They should be either destroyed or
buried or at least kept inverted so that water cannot collect in them. All such things
should be cleared during the rainy season.
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All tanks should be kept tightly closed. A
black plastic sheet can be used for the purpose. Also, all tanks should be emptied,
cleaned and allowed to dry for at least half an hour, once every week.
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Terraces and roofs should ideally have a
slope, particularly in places where monsoon tends to be heavy. All such roofs/terraces
should have adequate drainage for water. Any collection of water on these surfaces should
be cleared at least once a week.
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At construction sites, all the care should
be taken to avoid collection of water at one place for more than a week. The layer of
water on the surface of the concrete, used for concrete curing, should be cleared at least
once a week and allowed to dry for half an hour. All other puddles should be cleared
regularly. Collections of water in the toilets and closets under construction should also
be cleared. All tanks should be kept snugly closed. All labourers should be frequently
checked for parasitemia and adequately treated. They should also be provided with mosquito
nets.
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All unused wells and tanks should be
closed or destroyed. Engine oil or kerosene has been used as a larvicidal on these
collections. Another method to prevent egg laying on unused wells is by adding EPS
polyesterene beads onto the surface of water. These beads are non-toxic, cheap and long
lasting. They coat the water surface and prevent the mosquito from laying eggs.
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Wells that are being used and ornamental
tanks can be treated with biological larvicides that do not harm the quality of drinking
water. Also, these wells should be covered with either mosquito-proof nets or with plastic
sheets.
How engineers can help in malaria control?
Public Health Engineering has lot to do with malaria control, especially by means of
Source Reduction.
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b. Use of Larvicides: If
the above mentioned measures are not adequate or difficult to achieve, then measures
should be taken to destroy the larvae developing in the breeding sites. This can be done
by either larvicidal chemicals or by biological larvicides like fish and bacteria.
i. Chemicals:
Themiphos and Fenthion are the two commonly used larvicidal agents. Themiphos is used on
potable water collections and Fenthion, being more toxic, is used on non-potable water
collections. Oils may be applied to the water surface, suffocating the larvae and pupae.
Most oils in use today are rapidly biodegraded. Insect growth regulators such as
methroprene is specific to mosquitoes and can be applied in the same way as chemical
insecticides.
ii. Biological
larvicides: One of the safest and interesting methods in mosquito control is the use
of biological agents that eat or destroy the larvae.
Guppy and Gambusia fish are
known for their larvivorous habits. These fish eat the developing larvae and thereby
effectively control the mosquito population. These fish can be introduced into all
collections of potable water like wells, tanks, ponds and lakes. They can also be put into
the paddy fields.
Bacteria are
also effective larvicides. Bacillus sphaericus and Bacillus thuringiensis var
israelensis are the examples. However, they need to be re-introduced every 15 days
and their culture may need expertise.
Mermitid Nematod (Romanomermis
culicivorax), Notonectid (Bug), Ambylospora (Protozoa), Coelomomyces (Fungus),
Nuclear Polyhedrosis (Virus), and Cyclopoid copepods (Crustacean) are the other biological
larvicides found to be effective.
A 'saline solution'
from Kochi:
The Kochi Corporation in
Kerala tried out a novel and cost effective method of reducing the mosquito population at
the larvae stage itself. It has conducted experiments suggested by the retired National
Institute of Oceanography (NIO) scientist, Dr. U.K. Gopalan, where the salinity of water
in canals and stagnant pools is increased by adding sea water. The experiment was
successful and mosquito larvae were found morbid in the canal portions where salinity was
increased. When the salinity level reaches 30 parts per thousand or PPT (the normal
percentage of salt in the sea), mosquito larvae cannot survive beyond 3 hours. Even at
lower concentrations of 15 PPT, they are dead in 12 hours. And when the concentration is
upped to 60 PPT, the larvae perish within the hour.
http://www.cochingateway.com/mkingdom.htm
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