The use of genetically engineered mosquitoes to control dengue and chikungunya-causing Aedes aegypti mosquito population in the wild got a shot in the arm in a study undertaken in Juazeiro, northeast Brazil. Researchers successfully reduced the A. aegypti population in the wild by as much as 95 per cent, thus effectively preventing dengue and chikungunya epidemic disease. The study was carried out in 2011-12.

The results, published recently in the journal PLOS Neglected Tropical Diseases, are in line with an earlier field trial at Grand Cayman.

Dengue resurfaced in Brazil in 1981, and it is estimated that 16 million dengue infections occur every year. Over 390 million dengue cases are reported annually across the world.

Sustained release of genetically modified sterile male mosquitoes (OX513A) in the city every week for one year by Oxitec, a British company founded and part-owned by the University of Oxford, helped cut the transmission rate drastically.

The transgenic mosquitoes compete with the naturally occurring A. aegypti male mosquitoes to mate with the females. With sustained release, the number of transgenic mosquitoes outnumbered and suppressed the naturally occurring wild males.

The offspring from the mating of transgenic male A. aegypti mosquitoes with female mosquitoes die before adulthood (at the larval or pupal stage) as a consequence of transgenic modification. As a result, the number of female mosquitoes that can cause dengue falls dramatically. Since the male mosquitoes do not bite humans, the release of transgenic males will not increase the risk of dengue.

With no specific drugs or vaccines for dengue, the only way to reduce the incidence is by reducing transmission. While resistance against pesticides is eliminated with transgenic mosquitoes, the suppression of disease transmission depends on the sustained release of GM mosquitoes. According to the authors, the mating competitiveness between male transgenic mosquitoes and wild mosquitoes declined “substantially” with time.

However, the suppression of wild A. aegypti mosquitoes depends on the arrival of pre-mated female mosquitoes from outside the treated area. According to studies, the dispersal distance of A. aegypti mosquitoes is no more than 100 metres. “Effects of immigration would therefore likely be limited to a relatively small boundary zone in a larger programme, or not even that if the whole of an isolated population were treated,” the authors write.

Also, the persistence of viable eggs laid prior to start of treatment may delay the wild mosquito suppression as the eggs may hatch over a “period of months after deposition, depending on environmental conditions.” In the study, a gradual reduction in target population was seen, thus offsetting the harm of residual viable eggs.

Following approval of the Oxitec mosquito by the national biosafety group (CTNBio) for release throughout Brazil, the city of Piracicaba has started the world’s first municipal project of genetically engineered mosquito control.

Published in The Hindu 0n July 18, 2015