Methods of vector Control
Environmental management seeks to change the environment in order to prevent or minimize vector propagation and human contact with the vector-pathogen by destroying, altering, removing or recycling non-essential containers that provide larval habitats. Such actions should be the mainstay of dengue vector control. Three types of environmental management are defined:
- Environmental modification – long-lasting physical transformations to reduce vector larval habitats, such as installation of a reliable piped water supply to communities, including household connections.
- Environmental manipulation – temporary changes to vector habitats involving the management of “essential” containers, such as frequent emptying and cleaning by scrubbing of water-storage vessels, flower vases and desert room coolers; cleaning of gutters; sheltering stored tyres from rainfall; recycling or proper disposal of discarded containers and tyres; management or removal from the vicinity of homes of plants such as ornamental or wild bromeliads that collect water in the leaf axils.
- Changes to human habitation or behaviour – actions to reduce human – vector contact, such as installing mosquito screening on windows, doors and other entry points, and using mosquito nets while sleeping during daytime.
Improvement of water supply and water-storage systems
Improving water supplies is a fundamental method of controlling Aedes vectors, especially Ae. aegypti. Water piped to households is preferable to water drawn from wells, communal standpipes, rooftop catchments and other water-storage systems.
However, potable water must be supplied reliably so that water-storage containers that serve as larval habitats – such as drums, overhead or ground tanks and concrete jars – are not necessary. In urban areas the use of cost-recovery mechanisms such as the introduction of metered water may actually encourage household collection and storage of roof catchment rainwater that can be harvested at no cost, resulting in the continued use of storage containers.
Mosquito-proofing of water-storage containers
Water-storage containers can be designed to prevent access by mosquitoes for oviposition. Containers can be fitted with tight lids or, if rain-filled, tightly-fitted mesh screens can allow for rainwater to be harvested from roofs while keeping mosquitoes out. Removable covers should be replaced every time water is removed and should be well maintained to prevent damage that permits mosquitoes to get in and out.
Expanded polystyrene beads used on the surface of water provide a physical barrier that inhibits oviposition in storage containers from which water is drawn from below via a pipe and from which there is no risk of overflow. These beads can also be placed in septic tanks, which Ae. aegypti sometimes exploits.
Solid waste management
In the context of dengue vector control, “solid waste” refers mainly to non-biodegradable items of household, community and industrial waste. The benefits of reducing the amount of solid waste in urban environments extend beyond those of vector control, and applying many of the basic principles can contribute substantially to reducing the availability of Ae. aegypti larval habitats. Proper storage, collection and disposal of waste are essential for protecting public health. The basic rule of “reduce, reuse, recycle” is highly applicable. Efforts to reduce solid waste should be directed against discarded or non-essential containers, particularly if they have been identified in the community as important mosquito-producing containers.
Solid waste should be collected in plastic sacks and disposed of regularly. The frequency of collection is important: twice per week is recommended for housefly and rodent control in warm climates. Integration of Ae. aegypti control with waste management services is possible and should be encouraged.
A reliable and regular street cleansing system that removes discarded water-bearing containers and cleans drains to ensure they do not become stagnant and breed mosquitoes will both help to reduce larval habitats of Ae. aegypti and remove the origin of other urban pests.
During the planning and construction of buildings and other infrastructure, including urban renewal schemes, and through legislation and regulation, opportunities arise to modify or reduce potential larval habitats of urban disease vectors, including Ae. aegypti, Culex quinquefasciatus and An. stephensi. For example, under revised legislation in Singapore, roof gutters are not permitted on buildings in new developments because they are difficult to access and maintain. Moreover, property owners are required to remove existing gutters on their premises if they are unable to maintain them satisfactorily.
Chemical control: larvicides
Although chemicals are widely used to treat Ae. aegypti larval habitats, larviciding should be considered as complementary to environmental management and – except in emergencies – should be restricted to containers that cannot otherwise be eliminated or managed.
Larvicides may be impractical to apply in hard-to-reach natural sites such as leaf axils and tree holes, which are common habitats of Ae. albopictus, or in deep wells. The difficulty of accessing indoor larval habitats of Ae. aegypti (e.g. water-storage containers, plant vases, saucers) to apply larvicides is a major limitation in many urban contexts.
As Ae. aegypti often deposits eggs in water-storage containers, the larvicides should have low toxicity to other species and should not significantly change the taste, odour or colour of the water.
Productive larval habitats should be treated with chemicals only if environmental management methods or other non-chemical methods cannot be easily applied or are too costly. Perifocal treatment involves the use of hand-held or power-operated equipment to spray, for example, wettable powder or emulsifiable-concentrate formulations of insecticide on larval habitats and peripheral surfaces. This will destroy existing and subsequent larval infestations in containers of non-potable water, and will kill the adult mosquitoes that frequent these sites. Perifocal treatment can be used to treat containers, irrespective of whether they hold water or are dry at the time of application. The internal and external walls of containers are sprayed until they are covered by a film of insecticide, and spraying is also extended to cover any wall within 60 cm of the container. Perifocal treatment thus has both larviciding and residual adulticiding characteristics. This method is suitable only for collections of non-potable water (such as in large piles of tyres or discarded food and beverage containers).
The treatment cycle will depend on the species of mosquito, seasonality of transmission, patterns of rainfall, duration of efficacy of the larvicide and types of larval habitat. Two or three application rounds carried out annually in a timely manner with proper monitoring of efficacy may suffice, especially in areas where the main transmission season is short.
Extreme care must be taken when treating drinking-water to avoid dosages that are toxic for humans. Label instructions must always be followed when using insecticides.