Mosquito ‘egg sinks’ offer target for dengue control

[LIMA] Mosquitoes’ love of crowds when laying eggs could be used against them to help control dengue fever outbreaks, according to a study in Peru.

The WHO’s recommended control strategy for the dengue vector Aedes aegypti targets the mosquito’s immature stages, and assumes that eliminating most water containers, where they lay eggs, would cut mosquito numbers by a similar proportion.

But a study in Peru has found that mosquitoes choose to lay eggs at sites that are already heavily infested with them. If these are removed or treated with larvicides, the mosquitoes look for other suitable sites, reducing the effectiveness of control measures.

Dengue fever is a potentially fatal disease caused by a virus and transmitted by mosquitoes. It threatens two-fifths of the world’s population, mainly in the tropics and sub-tropics, and its incidence has been on the rise, according to the WHO.

The study was carried out in Iquitos, Peru, where a virulent dengue outbreak killed 14 people earlier this year (see Peru: a new dengue strain has caused an epidemic, in Spanish).

A team from the University of California, Davis, and Naval Medical Research Unit 6, in Lima, looked for factors that influence the selection of egg-laying sites, such as the density of larvae and pupae of the same species, container size or exposure to sun. They found that the key factor that determines where mosquitoes choose to lay eggs is the presence of lots of other larvae.

Instead of removing such heavily infested sites, they could be targeted with insect growth regulators (IGRs), such as pyriproxyfen, which do not kill the eggs or larvae but target later-stage pupae. This would create ‘egg-sinks’, which attract egg-laying mosquitoes but reduce the number of adults emerging from them, according to the study, published in PLoS Neglected Tropical Diseases last month (12 April).

Under this strategy, "there would be fewer adults", Jacklyn Wong, the lead author, from the University of California, Davis, told SciDev.Net.

Alternatively, the chemical attractants that females use to choose egg-laying sites could be used to lure them into traps, says the study.

Wong said these findings could also improve current control techniques. For example, growth regulators could be combined with a technique that uses mosquitoes to carry insecticide to their own breeding site.

"IGR can be transferred from one site to another by adult A. aegypti, and could potentially turn those attractive containers into IGR dissemination stations," Wong said. "This would also have the advantage that females could transfer the IGR to subsequent containers they visit."

Palmira Ventosilla, a biologist at the Alexander von Humboldt Tropical Medicine Institute, in Lima, said these novel findings should be considered for future dengue control programmes.

But she warned that results may differ in other tropical countries, and that different locations may require different control strategies, depending on findings from local studies.

Link to full paper in PLoS Neglected Tropical Diseases  [342kB]