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Source reduction involves
preventing development of mosquito larvae. The female mosquitoes
need a blood meal from a vertebrate host to nourish their eggs.
About 50-200
eggs are laid per oviposition on the surface of stagnant water and these eggs develop
into adult mosquitoes in a span of about 5-14 days, passing through the stages of larvae and pupae. High humidity and ambient temperature between 20-30ºC provide ideal conditions
for breeding of Anopheline mosquitoes. Common sites of breeding for
Anopheles mosquitoes include rainwater pools and puddles, borrow pits, river bed pools, irrigation channels, seepages, rice fields, wells, pond margins, sluggish streams with sandy margins, hoof prints, tyre
tracks etc. Water stagnation due to construction of dams, reforestation, shrimp farming, fish ponds etc.,
have also been identified as possible sites of Anopheles breeding.
An. stephensi is a well adapted urban vector, being a container breeder, making use of man-made sites such as building-construction sites, wells, garden ponds, cisterns,
overhead tanks, ground level cement tanks, water coolers, tyres, barrels and tins, intra-domestic containers etc.
Anopheles breeding sites increase with rainfall
and resultant water stagnation, ; however, some larvae and pupae
may be washed away by heavy rainfall.
The best method of mosquito
control is preventing the development of the eggs into adult
mosquitoes, by reducing the sources of breeding. These anti larval measures are not only simple and cost
effective, but also environment friendly.
a. Preventing egg laying:
The easiest, cheapest and most environment-friendly method to control malaria is by
preventing the mosquito from laying eggs. This is done by avoiding or eliminating the
clean water collections. As mentioned, most such collections are artificial, temporary and man made.
It is a common habit to throw
the unutilized utensils, buckets, bottles, tyres etc., into the
open. During the rains, water gets collected in these containers and provides ample
breeding locations for the female anopheles mosquito.
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. 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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Open overhead tank |
Open tank |
Unused well |
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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.
Concrete Curing
(left) and Open tank and stagnant water on roof top |
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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. In
addition, there are the natural collections of water like the wells, lakes, ponds, paddy
fields, marshlands etc. where mosquito breeding occurs in abundance.
Tiled roof(left) and Flat terrace |
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 or
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.
Eco-friendly
larvivorous fish such as the top water minnow or mosquito fish (Gambusia affinis) or the common guppy (Poecilia
reticulate) can be
effectively used to control the mosquito population. These fish can be
introduced into all collections of potable water like wells, tanks,
ponds and lakes, particularly in rural and peri-urban areas and in freshwater bodies in rural areas.
Bacteria such as Bacillus sphaericus and Bacillus thuringiensis var
israelensis are also effective larvicides. 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]
Further reading:
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CDC. Malaria: Anopheles Mosquitoes. Available at
http://www.cdc.gov/malaria/about/biology/mosquitoes/index.html
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White
NJ. Malaria. In Cook GC, Zumla AI. (Ed). Manson's Tropical Diseases.
22nd Edition. Saunders Elsevier. 2009. pp 1201-1300
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Dash AP, Adak T, Raghavendra K, Singh OP. The biology and control of
malaria vectors in India. Current Science. 2007;92(11):1571-78. Full
text at
http://www.ias.ac.in/currsci/jun102007/1571.pdf
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Operational Manual for Implementation of Malaria Programme. Government
of India, Directorate of National Vector Borne Disease Control Programme;
Directorate General of Health Services, Ministry of Health and Family
Welfare. 2009. Available at
http://nvbdcp.gov.in/Doc/Malaria-Operational-Manual-2009.pdf
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Singh
N, Mehra RK, Sharma VP. Malaria and the Narmada-river development in
India: a case study of the Bargi dam. Annals of Tropical Medicine and
Parasitology 1999;93:477-88.
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Lindsay S, Kirby M, Baris E, Bos R. Environmental management for malaria
control in the east Asia and Pacific (EAP) region. The International
Bank for Reconstruction and Development / The World Bank. Washington.
2004. Available at
http://www.who.int/water_sanitation_health/publications/whowbmalariacontrol.pdf
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Maheu-Girouxa M, Casapía M, Soto-Calle VE et al. Risk of malaria
transmission from fish ponds in the Peruvian Amazon. Acta Tropica.
2010;115(1-2):112-118
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Kumar
A, Thavaselvam D. Breeding habitats and their contribution to
Anopheles stephensi in Panaji. Indian Journal of Malariology.
1992;29:35-40
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CDC. Malaria: Larval Control and Other Vector Control Interventions.
Available at
http://www.cdc.gov/malaria/malaria_worldwide/reduction/vector_control.html
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Kolsky P. Engineers and urban malaria: part of the solution, or part of
the problem? Environment and Urbanization. 1999;11(1):159-163. Full Text
at
http://eau.sagepub.com/cgi/reprint/11/1/159.pdf
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NVBDCP. Guidelines on the use of larvivorous fish for vector control.
Available at
http://nvbdcp.gov.in/Doc/Guidelines-larvivorous-fish.pdf
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