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General Information | Insecticide-Treated Bed Nets | Indoor Residual Spraying | Insecticide Resistance | Source Reduction (Larval Control) | Other Methods | Other Sources of Information
General
Information
Vector
control aims to decrease contacts between humans and vectors of human
disease. Control of mosquitoes may prevent malaria as well as several
other mosquito-borne diseases.
Elimination of malaria in an area does not require the elimination of
all Anopheles
mosquitoes capable of transmitting the disease. In
North America and Europe, Anopheles mosquitoes capable of transmitting
malaria are still present, but the parasite has been eliminated. Socio-economic
improvements (e.g., houses with screened windows, air conditioning) combined
with vector reduction efforts and effective treatment have led to the
elimination of malaria without the complete elimination of the vectors.
Vector
control for the prevention of malaria includes:
Insecticide-Treated Bed Nets
Insecticide-treated bed nets (ITNs) are a form of personal protection that has repeatedly been shown to reduce severe disease and mortality due to malaria in endemic regions. In community-wide trials in several African settings, ITNs have been shown to reduce all-cause mortality by about 20%.
Insecticide-treated bed nets (ITNs) are now a major intervention for malaria control.
Untreated bed nets form a protective barrier around persons using them. However, mosquitoes can feed on people through the nets, and nets with even a few small holes provide little, if any, protection. The application of a residual insecticide greatly enhances the protective efficacy of bed nets. The insecticides used for treatment kill mosquitoes and other insects. The insecticides also have repellent properties that reduce the number of mosquitoes that enter the house and attempt to feed. In addition, if high community coverage is achieved, the numbers and longevity of mosquitoes will be reduced. When this happens, all members of the community are protected, regardless of bed net ownership. To achieve such effects, high community coverage is required, as for indoor residual spray.
There are several types of nets available. Nets may vary by size, material, and/or treatment. Most nets are made of polyester but nets are also available in cotton, polyethylene, or polypropylene.
Currently, only pyrethroid insecticides are approved for use on ITNs. These insecticides have very low mammalian toxicity but are highly toxic to insects and have a rapid knock-down effect, even at very low doses. Pyrethroids have a high residual effect: they do not rapidly break down unless washed or exposed to sunlight.
Previously, nets had to be retreated at intervals of 6-12 months, more frequently if the nets were washed. Nets were retreated by simply dipping them in a mixture of water and insecticide and allowing them to dry in a shady place. The need for frequent retreatment was a major barrier to full implementation of ITNs in endemic countries. The additional cost of the insecticide and the lack of understanding of its importance resulted in very low retreatment rates in most African countries.
Long-Lasting Insecticide-treated Nets (LLINs)
More recently, several companies have developed long-lasting insecticide-treated nets (LLINs) that retain lethal concentrations of insecticide for at least 3 years. The WHO Pesticide Evaluation Scheme has recommended five of these LLINs for use in the prevention of malaria. CDC is currently testing these and other LLINs in Atlanta and Kenya.
The WHO Pesticide Evaluation Scheme recommends these LLINs:
Travelers to malaria-risk areas may also use ITNs as one of several precautions against malaria.
For information on ordering insecticide-treated bed nets:
Insecticide-Treated
Bed Nets (ITNs): Commodity or Public Health Intervention?
Insecticide-treated
bed nets (ITNs) are now an important method for controlling malaria.
Their protective effect will be strongest if they are used by a
high proportion of the population at risk. How to achieve this high
coverage is currently the object of a debate.
Should
ITNs be sold as a commodity, using social marketing to stimulate
their sale? Sale of ITNs might increase their value to the users
and might ensure their sustainability (ITNs would continue to be
available after donor agencies have left).
Should
ITNs be provided free of charge to the groups most at risk? Like
vaccines, ITNs are a public health intervention that decreases death
and disease. Thus, like vaccines, ITNs might deserve to be given
free to those who need them most.
The
debate has not been settled. But in Africa, where 90% of malaria
deaths occur, many of those suffering most from malaria - the rural
poor - cannot afford even the modest cost (US$ 5) of an ITN. Rural
areas where ITNs are sold through social marketing have not achieved
the desired coverage (i.e., 60% of children under 5 and 60% of pregnant
women using ITNs). Initial trials of free distribution of ITNs during
immunization campaigns or at antenatal clinics have been very successful.
Coverage of children under 5 years of age has increased from less
than 10% to over 80% in one week, in trials during immunization
campaigns in Ghana and Zambia. Upcoming trials will determine how
successful this approach is on a larger scale and over a longer
term. If ITNs are adopted as a public health intervention (like
vaccines), their full scale deployment will require sustained political
and financial commitment from donors and from governments of malaria-endemic
countries.
Many
countries have adopted a combination of approaches, where nets are
sold through the private market and also distributed free or heavily
subsidized to groups at risk of severe malaria (pregnant women and
children under 5). Balancing these strategies so that they complement
each other may prove to be the best way to rapidly increase coverage
among vulnerable populations while ensuring that this very effective
malaria control intervention is sustained over the long term. |
Indoor Residual Spraying
Many
malaria vectors are endophilic, resting inside houses after taking a blood
meal. These mosquitoes are particularly susceptible to control through
indoor residual spraying (IRS). As its name implies, IRS involves coating
the walls and other surfaces of a house with a residual insecticide. For
several months, the insecticide will kill mosquitoes and other insects
that come in contact with these surfaces. IRS does not directly prevent
people from being bitten by mosquitoes. Rather, it usually kills mosquitoes
after they have fed, if they come to rest on the sprayed surface. IRS
thus prevents transmission of infection to other persons. To be effective,
IRS must be applied to a very high proportion of households in an area
(usually >70%).
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Indoor
residual spraying |
IRS
with DDT and dieldrin was the primary malaria control method used during
the Global Malaria Eradication Campaign (1955-1969). The campaign did
not achieve its stated objective but it did eliminate malaria from several
areas and sharply reduced the burden of malaria disease in others.
Resistance
to DDT and dieldrin and concern over their environmental impact led to
the introduction of other, more expensive insecticides. As the eradication
campaign wore on, the responsibility for maintaining it was shifted to
endemic countries that were not able to shoulder the financial burden.
The campaign collapsed and in many areas, malaria soon returned to pre-campaign
levels.
As a result of the cost of IRS, the negative publicity due to the failure of the Malaria Eradication Campaign, and environmental concerns about residual insecticides, IRS programs were largely disbanded other than in a few countries with resources to continue them. However, the recent success of IRS in reducing malaria cases in South Africa by more than 80% has revived interest in this malaria prevention tool. It has also reignited the debate over whether or not DDT should have a place in malaria control. With support from the Global Fund to fight AIDS, Tuberculosis and Malaria as well as the President’s Malaria Initiative, several countries have initiated IRS programs—many using DDT in their arsenal of insecticides—for the control of malaria.
Insecticide Resistance
Insecticide-based control measures (e.g. indoor spraying with insecticides, ITNs) are the principal way to kill mosquitoes that bite indoors. However, after prolonged exposure to an insecticide over several generations, mosquitoes, like other insects, may develop resistance, a capacity to survive contact with an insecticide. Since mosquitoes can have many generations per year, high levels of resistance can arise very quickly. Resistance of mosquitoes to some insecticides has been documented with just within a few years after the insecticides were introduced. There are over 125 mosquito species with documented resistance to one or more insecticides. The development of resistance to insecticides used for indoor residual spraying was a major impediment during the Global Malaria Eradication Campaign. Judicious use of insecticides for mosquito control can limit the development and spread of resistance. However, use of insecticides in agriculture has often been implicated as contributing to resistance in mosquito populations. It is possible to detect developing resistance in mosquitoes and control programs are well advised to conduct surveillance for this potential problem.
Related Sources:
Source Reduction (Larval Control)
Source
reduction is the method of choice for mosquito control when the mosquito
species targeted are concentrated in a small number of discrete habitats.
The larval habitats may be destroyed by filling depressions that collect
water, by draining swamps, or by ditching marshy areas to remove standing
water. Container-breeding mosquitoes are particularly susceptible to source
reduction as people can be educated to remove or cover standing water
in cans, cups, and rain barrels around houses. Mosquitoes that breed in
irrigation water can be controlled through careful water management.
For
some mosquito species, habitat elimination is not possible. For these
species, chemical insecticides can be applied directly to the larval habitats.
Other methods, which are less disruptive to the environment, are usually
preferred:
- Oils
may be applied to the water surface, suffocating the larvae and pupae.
Most oils in use today are rapidly biodegraded.
- Biological
control agents include toxins from the bacterium Bacillus thuringiensis
var. israelensis (Bti). These products can be applied in the
same way as chemical insecticides. They are very specific, affecting
only mosquitoes, black flies, and midges.
- Insect
growth regulators such as methroprene. Methoprene is specific to mosquitoes
and can be applied in the same way as chemical insecticides.
-
Mosquito fish (Gambusia affinis) are effective in controlling
mosquitoes in larger bodies of water.
- Other
potential biological control agents, such as fungi (e.g., Laegenidium
giganteum) or mermithid nematodes (e.g., Romanomermis culicivorax),
are less efficient for mosquito control and are not widely used.
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Source
reduction: The burrow pit resulting from the activity of this brickmaker
in Kisian (western Kenya) is a potential breeding site for Anopheles
larvae. Filling the burrow pit would remove a source of Anopheles
adults. |
Source
reduction is an ideal approach to mosquito control. Mosquito larvae are
concentrated in defined areas, and source reduction eliminates mosquitoes
before they reach the stage that is responsible for disease transmission.
Unfortunately, source reduction is not always feasible. The larval habitats
may be small, widely dispersed, and transient. Anopheles gambiae,
one of the primary vectors of malaria in Africa, breeds in numerous small
pools of water that form due to rainfall. The larvae develop within a
few days, escaping their aquatic environment before it dries out. It is
difficult, if not impossible, to predict when and where the breeding sites
will form, and to find and treat them before the adults emerge. Therefore,
larval mosquito control for the prevention of malaria in Africa has not
been attempted on a large scale.
Other Vector Control Methods
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Fogging
(area spraying) using a machine mounted on the back of a truck. |
Fogging
or area spraying is primarily reserved for emergency situations: halting
epidemics or rapidly reducing adult mosquito populations when they have
become severe pests. Fogging and area sprays must be properly timed to
coincide with the time of 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).
Personal
protection measures include the use of window screens, ITNs (see above),
and repellents (such as DEET) and wearing light-colored clothes, long
pants and long-sleeved shirts. Well-constructed houses with window screens
are effective for preventing biting by mosquitoes that bite indoors and
likely contributed much to the elimination of malaria from the United
States and Europe.
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.
Related Sources of Information
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Page last modified : January 10, 2008
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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