| C ontrol of adult mosquitoes is not an easy task,
considering their varied habits, their ability to fly all over and to
hide in nooks and corners. Whereas vectors such as
An. gambiae and An. funestus are highly anthropophilic (prefer human blood meal), the Indian anopheline vectors
such as An. culicifacies, An. fluviatilis, An. minimus, An. philippinensis, An. dirus,
and An. stephensi are
essentially zoophilic (preferring blood meal from animals such as
cattle) and feed on human beings when high densities build up. During the day, these mosquitoes
rest in human dwellings and cattle sheds and enter the human dwellings between 5pm-10pm.
They start biting
indoors soon after, with peak biting at midnight, between 11 pm and 4 am. Adult female anopheles mosquitoes survive
for 1-2 weeks or more depending on the ambient conditions and have a flight range of 0.5-3 kms.
Control of adult mosquitoes
involve the following measures:
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Preventing entry of adult mosquitoes into
human dwellings
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Mosquito nets (regular and insecticide
treated)
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Personal protection measures [See]
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Protective clothing
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Mosquito repellents
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Adult insecticides
The
Global Malaria Action Plan enlists insecticidal nets (LLINs),
indoor residual spraying (IRS) with long-lasting chemical
insecticides, and other vector (mosquito) controls such as
larviciding and environmental management as the key tools of the
global malaria control strategy.
Preventing entry of adult mosquitoes into
human dwellings: Measures to make the human dwellings inaccessible to the vector mosquitoes so as to reduce man-mosquito contacts are important in controlling malaria transmission.
Mosquitoes do not fly more than about 2-4km from their breeding habitats and
therefore positioning houses 1.5 to 2 km from large breeding sites will reduce
the risk of transmission substantially. Villages at higher elevations and exposed to the wind
tend to have fewer mosquitoes compared to sites situated in the lowlands that
are less windy and have many small
water bodies. As most mosquitoes fly close to the ground, raising buildings off the ground or on silts can help in preventing mosquito entry.
Sitting on raised platforms or keeping the feet off the ground also help in minimizing mosquito bites.
Keeping the windows and doors
closed during evenings and early morning hours can prevent the mosquitoes from
gaining entry into households (it is important to close the doors of the
toilets, which always open to the exterior through windows or vents). Modifying the house structure and mosquito-proofing of the houses were used by Manson, Ross, Celli and others
to protect people from malaria in Italy, Greece, Panama and the USA and there is ample evidence that house screening contributed to the elimination of malaria from many parts of
the world. Homes with ceilings or closed eaves also protect from mosquitoes and malaria; a study using experimental huts in Gambia demonstrated that installing a ceiling
made of netting reduced transmission by 80%. As Anopheles mosquitoes tend to hide in the dark corners and amidst the
clothes and other linen left hanging in the rooms, such hiding places should be avoided
by keeping all the clothes and linen
inside wardrobes and cupboards.
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Mosquito nets: Mosquito nets act as physical barriers by blocking the vector mosquitoes.
Application of pyrethroid insecticides adds a chemical barrier to the physical
one, further reducing human–vector contact and increasing the protective
efficacy of the mosquito nets. Pyrethroid insecticides have a long residual action and low mammalian toxicity
and provide prolonged protection by their excito-repellent effect. As mosquitoes are positively attracted by the odour of the sleeper inside the net, these
insecticide treated nets (ITNs)
acts like a baited trap and the mosquitoes that come into contact with the ITN are,
most often, killed. As the ITNs shorten the mean
mosquito life span, very few mosquitoes can survive long enough for the sporogonic cycle to be completed, thus reducing the transmission.
As the ITNs also inhibit mosquito feeding, the reproductive potential of highly anthropophilic vectors is also reduced. Due to these multiple
effects, the ITNs have been shown to avert around 50% of malaria cases and provide at least double the protection than that provided by untreated nets.
The community-wide use of ITNs has been reported top reduce the vector population significantly and when used by a majority of the target population (around 60%),
to provide protection for all people in the community, including those who do
not themselves sleep under nets.ITNs have been found to be the most
cost-effective interventions against malaria, and long-lasting insecticidal
nets LLINs were found to be significantly cheaper to use than conventionally treated nets. ITNs/LLINs are particularly useful for
high-risk populations that cannot be reached by residual spraying, for people in forest-fringe areas who are at risk of infection from forest stay, and for
pregnant women who are highly vulnerable to malaria.
Under NVBDCP, ITNs/LLINs are provided free to the target population. |
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Currently, most mosquito nets are made of polyester and rarely last longer than 2–3 years under field situations. Conventional ITNs, treated with
pyrethroids such as alpha-cypermethrin, cyfluthrin, deltamethrin, lambda-cyhalothrin or permethrin, need to be re-treated after three washes, or at
least once a year to ensure continued insecticidal effect. Long-lasting insecticidal nets [LLINs] are factory-treated mosquito nets, made
with netting material that has the insecticide incorporated within or bound around the fibres and the insecticide is progressively released so that
the net retains the efficacy after repeated washings. The LLINs are expected
to retain their effective biological activity without re-treatment for at least
20 standard washes and for three years of recommended use under field
conditions. Permethrin (high density polyethylene monofilament
yarn blended with 2% permethrin), Deltamethrin (multifilament polyester netting treated with deltamethrin 55mg/m2), and alpha cypermethrin
(multifilament polyester netting treated with alpha cypermethrin 200mg/m2) are used in LLINs.
Zooprophylaxis intends to control vector-borne infections by diverting vectors from humans to domestic animals such as cattle that act as dead-end or decoy hosts.
Although this method has been suggested by WHO as one of the measures to control anopheline vectors, some of which are indeed zoophilic,
studies on its efficacy have yielded varying results.
Indoor Residual Spraying:
IRS is an integral component of the Global Malaria Action Plan and currently
DDT, pyrethroids (Deltamethrin 2.5% WP, Cyfluthrin 10% WP, Alphacypermethrin 5%
WP and Lambdacyhalothrin 10% WP) or Malathion 25% are used in different parts of
the world for this purpose. All the interior walls and ceilings as well as the
underside of furniture, back of the doors and porches of permanent human
dwellings as well as Jhoom huts where people sleep during the plantation or
harvesting season are sprayed. For protection during the entire transmission
season, two rounds of DDT or synthetic pyrethroids or three rounds of Malathion
are used.
DDT has once again staged a comeback after nearly thirty years of being phased
out from the widespread use in indoor spraying to control malaria. A 1990 cost
comparison by the WHO found DDT to be considerably less expensive than other
insecticides, which cost 2 to 23 times more on the basis of cost per house per 6
months of control and this advantage remains even today. In September 2006, the
WHO once again recommended the use of DDT for indoor residual spraying, not only
in epidemic areas but also in areas with constant and high malaria transmission,
including throughout Africa with an assurance that DDT presents no health risk
when used properly. The tough campaign by public health officials and malaria
experts who had argued for years that DDT was a necessary public-health weapon
in poor tropical countries, signature campaign by hundreds of physicians
from all over the world urging resumption of DDT spraying and arguments of Amir
Attaran, of Harvard University’s Center for International Development, that
unlike agricultural uses which inject tons of DDT into the outdoors, the indoor
residual house-spraying with DDT at minimal (2g/m2) quantities was an
inexpensive and highly effective practice against malaria, all helped in making
this decision.
See
The United Nations’ Scientific Fraud against DDT:
Roberts D et al [Full
Text] Space sprays: These
insecticides instantly kill the mosquitoes, but lack any residual effects. They are
therefore sprayed into the air. By killing adult mosquitoes, not only bites are prevented,
but breeding is also prevented, resulting in net reduction in the mosquito population.
Space sprays must be repeated often, at least once every week. Pyrethroids are commonly
used for this purpose.
Space spraying involves the
application of small droplets of insecticide into the air, but recent studies have
demonstrated that the method has little effect on the mosquito population. Moreover, when
space spraying is conducted in a community, it creates a false sense of security among
residents, which has a detrimental effect on community-based source reduction programmes.
(In fact, in Mangalore, the ward level committees formed in the year 1995-96, lost steam
and became defunct after fogging operations were introduced in late 1996!) Although it is
highly visible and conveys the message that the government is doing something about the
disease, this can be only a poor justification for using space sprays. (Often, members of
the City Corporation order fogging in their constituencies to 'satisfy' their voters!).
Space spraying operations
should be carried out at the right time, at the right place, and according to the
prescribed instructions with maximum coverage, so that the fog penetration effect is
complete enough to achieve the desired results. Fogging should be primarily reserved for
emergency situations: halting epidemics or rapidly reducing adult mosquito populations. It
must be timed to coincide with the peak adult activity, because resting mosquitoes are
often found in areas that are difficult for the insecticide to reach (e.g., under leaves,
in small crevices). Generally, there are two forms of space-sprays, namely thermal
fogs and cold fogs and both can be dispensed by
vehicle-mounted or hand-operated machines.
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Thermal
fogs
Thermal fogs are produced
when an insecticide formulation condenses after being vaporized at a high temperature.
These formulations can be oil-based or water-based; the oil (diesel)-based formulations
produce dense clouds of white smoke, whereas water-based formulations produce a colorless
fine mist.
Ultra-low volume
(ULV), aerosols (cold fogs) and mists
ULV involves the application
of a small quantity (<4.6 litres/ha) of concentrated liquid insecticides. Aerosols,
mists and fogs may be applied by portable machines, vehicle-mounted generators or aircraft
equipment.
House-to-house
application using portable equipment
Portable spray units can be used when the area to be treated is not very
large or in areas where vehicle-mounted equipment cannot be used effectively. This
equipment is meant for restricted outdoor use and for enclosed spaces (buildings) of not
less than 14m3. Congested low-income housing areas, multistoried buildings,
godowns and warehouses, covered drains, sewer tanks and residential or commercial premises
are some examples. |
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Vehicle-mounted
fogging
Vehicle-mounted fogging can
be used in urban or suburban areas with a good road system. One machine can cover up to
1500-2000 houses (or approximately 80 ha) per day. An educational effort may be required
to persuade the residents to cooperate by opening doors and windows. The best time for
application is in the early morning (6am-8.30am) or evening (5pm-7.30pm).
Insecticide
formulations for space sprays
Organophosphate
insecticides
- Malathion
- Undiluted technical grade malathion
(active ingredient 95%+) for ULV spraying (0.5 liters per hectare for vehicle-mounted
operations)
- One part technical grade diluted with 24
parts of diesel for thermal fogging respectively
- Fenitrothion
- Pirimiphos methyl
Pyrethroids
- Permethrin
- Deltamethrin
- Lambda-cyhalothin
Low dosages of pyrethroid
insecticides are usually more effective indoors than outdoors.
Novel Genetic
Methods: Sterile male release has been successfully applied in several
small-scale areas. However, the need for large numbers of mosquitoes for release makes
this approach impractical for most areas. Genetic modification of malaria vectors aims to
develop mosquitoes that are refractory to the parasite. This approach is still several
years from application in field settings.
Further Reading:
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The Global Malaria Action Plan For a malaria free world. Roll Back
Malaria Partnership, WHO. Geneva. 2008. Available at
http://www.rollbackmalaria.org/gmap/gmap.pdf
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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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Charlwood JD, Pinto J, Ferrara PR et al. Raised houses reduce
mosquito bites. Malaria Journal 2003;2:45. doi:10.1186/1475-2875-2-45
Full text at http://www.malariajournal.com/content/2/1/45
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Lindsay SW, Emerson P, Charlwood JD. Reducing malaria by
mosquito-proofing homes. Trends in Parasitology 2002;18:510-514.75.
548. Lindsay SW, Jawara M, Paine K, Pinder M et al. Changes in house
design reduce exposure to malaria mosquitoes. Tropical Medicine and
International Health. 2003;8(6):512–517.
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Ogoma SB, Kannady K, Sikulu M et al. Window screening, ceilings and
closed eaves as sustainable ways to control malaria in Dar es Salaam,
Tanzania. Malaria Journal 2009;8:221. doi:10.1186/1475-2875-8-221. Full
Text at http://www.malariajournal.com/content/8/1/221
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WHO. Malaria vector control and personal protection: report of a
WHO study group. (WHO technical report series: no. 936) World Health
Organization. Geneva. 2006 Available at
http://malaria.who.int/docs/WHO-TRS-936s.pdf
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WHO. Insecticide-Treated Mosquito Nets: a WHO Position Statement.
WHO. Geneva. 2007. Available at
http://www.who.int/malaria/publications/atoz/itnspospaperfinal.pdf
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WHO. Long-lasting insecticidal nets for malaria prevention: A
manual for malaria programme managers. World Health Organization.
Geneva. 2007. Available at
http://www.who.int/malaria/publications/LLINmanual.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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Kaburiae JC, Githutob JN, Muthamic L. Effects of long-lasting
insecticidal nets and zooprophylaxis on mosquito feeding behaviour and
density in Mwea, central Kenya. J Vector Borne Dis. 2009;46:184–190.Full
text at http://www.mrcindia.org/journal/issues/463184.pdf
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Saul A. Zooprophylaxis or zoopotentiation: the outcome of
introducing animals on vector transmission is highly dependent on the
mosquito mortality while searching. Malaria Journal. 2003;2:32.
doi:10.1186/1475-2875-2-32. Full text at
http://www.malariajournal.com/content/2/1/32
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Bøgh C, Clarke SE, Walraven GE, Lindsay SW. Zooprophylaxis,
artefact or reality? A paired-cohort study of the effect of passive
zooprophylaxis on malaria in The Gambia. Trans R Soc Trop Med Hyg.
2002;96(6):593-6.
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WHO gives indoor use of DDT a clean bill of health for controlling
malaria. Available at
http://www.who.int/mediacentre/news/releases/2006/pr50/en/index.html
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Walker K. Cost-comparison of DDT and alternative insecticides for
malaria control. Medical and Veterinary Entomology.2000;14(4):345–354
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Attaran A, Maharaj R. Ethical debate: Doctoring malaria, badly: the
global campaign to ban DDT. BMJ 2000;321:1403-1405. Full text at
http://www.bmj.com/cgi/content/full/321/7273/1403
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http://www.cdc.gov/malaria/control_prevention/vector_control.htm
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http://w3.whosea.org/en/Section10/Section332/Section554_2569.htm
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