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 behavior – 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.
The choice of approach should be effective, practicable and appropriate to local circumstances. Actual or potentially important container types that cannot be removed from the area should be dealt with summarizes the main actions used to control immature Aedes larval habitats.
Improvement of water supply and water-storage systems
Improving water supplies is a fundamental method of controlling Aedes vectors, especially . 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. Traditional water storage practices may also persist even when reliable supplies are available. The installation of reliable piped water supplies in houses should therefore be accompanied by a communication strategy that discourages traditional storage practices.
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 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 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 Aegypti control with waste management services is possible and should be encouraged.
It is also important to provide information on these activities to encourage and promote them. Globally, recycling is on the increase. This practice places value on many items previously classified as waste products, leading to growth in the recycling market and profit for both small and large-scale businesses as a consequence. But although recycling can contribute to significant economic improvements, the recycling market can potentially impact dengue vector populations. For there to be an impact, however, containers of importance must have value in the marketplace, be it real (e.g. plastics or tyres for recycling) or created (e.g. beverage container deposit laws), and advertising and promotion must be sustained.
Used tyres are common and sometimes highly productive larval habitats that may warrant special attention in urban areas. Discarded tyres should be collected, recycled or disposed of by proper incineration in waste transformation facilities (e.g. incinerators, energy-production plants, or lime kilns fitted with emission control devices). Regulation of the sale of new tyres mandating the payment of an additional deposit and return charge may also be an incentive for better management and disposal of old tyres. Tyres can be recycled in a variety of ways, including for use as shoe soles, flooring, industrial rubber gaskets or household hardware (e.g. buckets, rubbish bins). Industrially shredded tyres can be incorporated into road surfacing materials. Sanitary regulations may require that whole tyres are buried in a separate area of a landfill to avoid their rising upwards under compaction and disrupting soil cover.
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 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 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.
The International Programme on Chemical Safety (IPCS) has assessed the toxicity of the active ingredients methoprene, pyriproxyfen and temephos and those in Bacillus thuringiensis serovar israelensis (Bti) to determine their safety for use as mosquito larvicides in drinking-water at dosages that are effective against Aedes larvae. However, the safety of the active ingredients in the final formulation varies from product to product and requires further study, as does the possible microbiological contaminants in formulations of Bti. WHO’s Guidelines for drinking-water quality (3) provide authoritative guidance on the use of pesticides in drinking-water. Understandably, placing chemicals in domestic water, particularly drinking-water, is often viewed with suspicion and may be unacceptable in some communities.
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).
Insecticides lists the mosquito larvicides that are suitable for application to non-potable water containers. For treatment of drinking-water, temephos and methoprene can be applied at dosages of up to 1 mg of active ingredient (a.i.) per litre (1 ppm); pyriproxyfen can be applied at dosages up to 0.01 mg a.i. per litre (0.01 ppm) and Bti at1–5mg per litre
WHO-recommended compounds and formulations for control of mosquito larvae in container habitats.
. Application procedures
Hand-operated compression sprayers are suitable for applying liquid insecticides to larger larval habitats. Knapsack sprayers are also suitable, especially for delivering wettable powder formulations. A syringe or pipette can be used for treating indoor flower vases and ant traps. Granule and certain other solid formulations are applied directly by (protected) hand to confined larval habitats or by a convenient standard measure (e.g. a dessert spoon or teaspoon). When treating containers of drinking-water, sufficient insecticide should be added for the volume of the container even if the container is not full of water (e.g. 1 g of 1% temephos granules for 10 litres of container volume).
188.8.131.52. Treatment cycle
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.
3.2.3. Chemical control: adulticides
Methods of chemical control that target adult vectors are intended to impact on mosquito densities, longevity and other transmission parameters. Adulticides are applied either as residual surface treatments or as space treatments.
184.108.40.206. Residual treatment
Perifocal treatment, as described above, has both adulticiding and larviciding effects. Suitable insecticides can be applied with hand-operated compression sprayers. Power sprayers can be used to treat large accumulations of discarded containers (e.g. tyre dumps) rapidly. Care must be taken not to treat containers used to store potable water Space sprays and their application
Space spraying is recommended for control only in emergency situations to suppress an ongoing epidemic or to prevent an incipient one. The objective of space spraying is the massive, rapid destruction of the adult vector population. However, there has been considerable controversy about the efficacy of aerosol insecticide applications during epidemics of dengue and yellow fever. Any control method that reduces the number of infective adult mosquitoes, even for a short time, should reduce virus transmission during that time, but it remains unclear whether the transient impact of space treatments is epidemiologically significant in the long run. There is no well-documented example of the effectiveness of this approach in interrupting an epidemic. Nevertheless, if space spraying is used early in an epidemic and on a sufficiently large scale, the intensity of transmission may be reduced, which would give time for the application of other vector control measures that provide longer-term control, including larviciding and community-based source reduction. Thus, if disease surveillance is sensitive enough to detect cases in the early stages of an epidemic, and if the resources are available, emergency space spraying can be initiated at the same time as source reduction measures and larviciding are intensified.
Not only insecticide susceptibility but also droplet size, application rate and indoor penetration of the insecticide are all crucial to the efficacy of this method for controlling Ae. aegypti. Indoor penetration of an insecticide depends on the structure of the building, whether doors and windows are left open during spraying and, when applied from vehicle-mounted equipment, residential block configuration, the route of the spray vehicle and meteorological conditions. Where indoor penetration of droplets is likely to be poor, indoor application with portable equipment will be more effective against Ae. aegypti. However, rates of coverage are much lower and accessibility may be difficult, particularly in large cities.
Vector populations can be suppressed over large areas by the use of space sprays released from low-flying aircraft, especially where access with ground equipment is difficult and extensive areas must be treated rapidly. Indoor penetration of insecticide droplets is again a critical factor for efficacy. In applying space sprays from the air, careful consideration must be given to meteorological conditions, especially wind speed at spray height and at ground level, and to the droplet size spectrum obtained at the flying speed of the aircraft. For all aerial spraying operations, clearance must be obtained from the civil aviation authority. For safety reasons, populated areas must usually be sprayed from twin-engined aircraft. Modern aircraft are fitted with global positioning systems so the exact position of the aircraft while the insecticide is being applied can be accurately recorded.
Since total coverage can rarely be achieved during ground applications, space spraying should focus on areas where people congregate (e.g. high-density housing, schools, hospitals) and where dengue cases have been reported or vectors are abundant. Selective space treatment up to 400 metres from houses in which dengue cases have been reported is commonly practised (and is sometimes also referred to as “perifocal spraying”). However, by the time a case is detected and a response mounted, the infection is likely to have spread to a wider area. Thorough planning is required to ensure that adequate resources (equipment, insecticide, human and financial resources) can be deployed in a timely manner to ensure proper coverage. Only if resources permit should area-wide treatment be considered.
Table 3.3 lists the insecticides that are suitable for space spraying as cold aerosols or thermal fogs. The choice of insecticide formulation for space spraying in and around dwellings should be based on its immediate environmental impact and the compliance of the community. Only insecticide products with high flash-points should be used for thermal fogging. Space-spraying formulations are usually oil-based, as the oil carrier inhibits evaporation of small fog droplets. Diesel fuel has been used as a carrier for thermal fogging agents, but it creates thick smoke, has a strong smell and creates oily deposits, which may lead the community to reject its use. Water-based formulations are also available, some of which contain substances that prevent rapid evaporation. Label instructions should always be followed when using insecticides.
Selected insecticides for cold aerosol or thermal fog application against mosquitoes.
Space sprays can be applied either as thermal fogs at 10–50 l/ha or as ultra-low-volume applications of undiluted or slightly diluted technical-grade insecticide in the form of a cold aerosol of droplets of controlled size (15–25 μm) at a rate of 0.5–2.0 l/ha. Portable or vehicle-mounted thermal or cold-fog generators can be used for ground application. If the target area exceeds 1000 ha or cannot be covered by ground equipment within 10 days, aerial cold fog application is sometimes used. However, several factors must first be considered – including safety, timeliness, cost, meteorological conditions, vector behaviour, biological effectiveness and availability of equipment, operational sites, and highly skilled air and ground crews. The difficulties of ensuring penetration of insecticide droplets into the resting sites of the target species are similar to those for aerosols dispensed from road vehicles. For ground applications, maps of the areas to be sprayed showing all passable roads are helpful in planning routes. The development of Geographic Information Systems (GIS) may also be helpful. A communication plan should be prepared to inform the population, encouraging them to open their doors and windows in order to improve the effectiveness of the spraying programme.
Application rates vary with the susceptibility of the target species and environmental considerations. Wind speed has a strong effect on droplet distribution and contact with insects. In most situations, a wind speed of 1–4 metres per second (approximately 3.6–15 km/h) is needed to drift droplets downwind from the line of travel. Furthermore, space sprays should be applied when there are temperature inversions – i.e. colder air closer to the ground – which occur early in the morning or in the evening when the ground temperature begins to fall. Space spray applications should correspond to the activity of the target species. Ae. aegypti and Ae. albopictus are active during the day, with peak flight activity in the morning and afternoon. For these species, spraying outdoors is therefore usually carried out in the early morning or late afternoon. Indoor treatments with portable cold or thermal fog generators are particularly effective against Ae. aegypti because its resting behaviour is mainly indoors. Indoor treatments are the only choice where there is no access for vehicles.
For application from vehicle-mounted equipment in areas with narrow roads and houses close to the roadside, the spray should be directed backwards from the vehicle. In areas with wide roads and buildings far from the roadside, the vehicle should be driven close to the side of the road and the spray should be directed at a right angle (downwind) to the road rather than directly behind the vehicle. More detailed information on operational guidelines for space spraying is available in the WHO manual on this subject (5).
Cold fog applications from large fixed-wing aircraft are made at approximately 240 km/h and 60 m above the ground, with swath spacing of 180 m. Smaller, fixed-wing aircraft are flown at slower speeds and usually lower altitudes (approximately 160 km/h, 30 m above the ground, with a swath width of 50–100 m). In emergencies, agricultural spraying aircraft can be used so long as they are fitted with rotary atomizers or other suitable nozzles calibrated for the insecticide, its formulation and the desired application rate.
- PERSONAL PROPHALATIC MEASURES
Use of mosquito repellent creams, liquids, coils, mats etc.
Wearing of full sleeve shirts and full pants with socks
Use of bednets for sleeping infants and young children during day time to prevent mosquito bite
- BIOLOGICAL CONTROL
Use of larvivorous fishes in ornamental tanks, fountains, etc.
Use of biocides
- CHEMICAL CONTROL
Use of chemical larvicides like abate in big breeding containers
Aerosol space spray during day time
- ENVIRONMENTAL MANAGEMENT & SOURCE REDUCTION METHODS
Detection & elimination of mosquito breeding sources
Management of roof tops, porticos and sunshades
Proper covering of stored water
Reliable water supply
Observation of weekly dry day
- HEALTH EDUCATION
Impart knowledge to common people regarding the disease and vector through various media sources like T.v., Radio, Cinema slides, etc.
- COMMUNITY PARTICIPATION
Sensitilizing and involving the community for detection of Aedes breeding places and their elimination
- ____ seeks to change the environment in order to prevent or minimize vector propagation
(a) environmental degradation
(b) environmental pollution
(c) environmental administration
(d) environmental management
- All of these will help to reduce mosquito propagation except
(a) installing mosquito screening on windows, doors and other entry points
(b) using mosquito nets
(c) leaving the environment dirty
(d) draining the gutters
- “Solid waste” refers mainly to all these except
(a) non-biodegradable items of household
(b) community waste
(d) industrial waste
- Used tyres are common and sometimes highly productive larval habitats
(c) nether true nor false
(d) all of the above
- _____ is used to control mosquitoes and other insects
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