WHAT IS FOGGING?

Fogging is a technique used for killing insects that involves using a fine pesticide spray (aerosol) which is directed by a blower. In some cases a hot vapour may be used to carry the spray and keep it airborne for longer. Fast acting pesticides like pyrethroids are typically used. This is widely used for sampling or studying insects in the canopy of tall of forests which cannot be effectively reached. The floor below the trees are lined with plastic sheets and the fog causes insects to fall in a rain. These are collected for later study in the laboratory.

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.

Larvicides in water-storage containers should have low toxicity to other species and should not significantly change the taste, odour or colour of the water. WHO’s Guidelines for drinking-water quality 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.

More information on safety, quality control, guidelines for testing, insecticide resistance and application of larvicides is available from WHOPES.

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.

Residual treatment

Perifocal treatment, as described above, has both adulticiding and larviciding effects. Suitable insecticides can be applied with hand-operated compression sprayers. Care must be taken not to treat containers used to store potable water.

Indoor residual spraying (IRS) is the application of long-acting chemical insecticides on the walls and roofs of all houses and domestic animal shelters in a given area, in order to kill the adult vector mosquitoes that land and rest on these surfaces.

More information on safety, quality control, guidelines for testing, insecticide resistance and application of residual treatments is available from WHOPES.

Space sprays

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.

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.

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.