Earlier this year, a remarkable advance was made in the way technology, and specifically genetic modification, is being used to fight vector-borne diseases. A small British company, Oxitec, started commercial production of an ingenuous method for reducing mosquito populations in the wild, using the mosquitoes’ own mating success as the weapon. These genetically modified mosquitoes are now being released in Brazil, in an effort to reduce the transmission of dengue virus.
Focusing specifically on Aedes aegypti, a species which is a widespread vector for dengue, Oxitec’s researchers were able to modify male mosquitoes – which do not bite humans, and feed only on nectar – to contain a gene that causes the mosquito’s offspring to die before reaching sexual maturity. The males are still able to mate with female mosquitoes, and seem not to suffer any disadvantage in terms of competing with non-altered males. Gradually, as the females that mate with the genetically altered males fail to produce offspring, the population of mosquitoes will decline, and in turn reduce the rate of transmission of diseases to humans.
Aware of the need to keep a close eye on this process, the scientists also included a fluorescent “marker” with the added gene, to facilitate tracking the progress of the new gene in the mosquito population. Likewise, environmental impact assessments were performed at trial sites prior to the release of the modified mosquitoes, to ensure there wouldn’t be unintentional effects on the ecosystem. In contrast to genetically modified crops or other organisms that are altered to make them more durable or successful in the wild, these mosquitoes are specifically designed to be less reproductively successful, mitigating the concern that they will spread outside the target area or have negative impacts on native organisms.
The method was trialed at field sites in Malaysia, the Cayman Islands and Brazil over the past several years , and the effects were impressive, with over 80% reduction in mosquito population during the trial period. In Brazil, results were even more staggering – the mosquito population decreased by over 95% , leading Brazil to become the first country in the world to license the commercial release of the modified mosquitoes, which will be produced by a new factory Oxitec is building there.
Of course, Oxitec’s trial did not focus on malaria mosquitoes, but rather those that transmit dengue. However, appreciating the enormous potential that genetic modification may have on vector control, other research efforts have investigated similar strategies for Anopheles mosquitoes, which transmit malaria. A group from Imperial College in London, for example, published promising results in 2014, which entailed a different way to reduce mosquito populations . Led by Andrea Crisanti, this group injected male mosquitoes with a faulty X chromosome, which is not transmitted to offspring, resulting in a much larger proportion of male mosquitoes among offspring than females. Offspring are still fully fertile, allowing for greater spread of the faulty chromosome, to the point where natural populations are displaced entirely by the modified males, leading to a collapse in the population and much reduced transmission. While this method has not yet been field-tested, laboratory results are promising,
With insecticide resistance, resistance to first line anti-malarials, and other challenges such as enduring poverty and climate change continuing to present obstacles to eliminating vector-borne diseases such as malaria, perhaps genetic modification can continue to be a good news story in 2015.
- Oxitec, http://www.oxitec.com/faqs/have-oxitecs-mosquitoes-been-tested-in-the-open-field-before/
- Oxitec press release, http://www.oxitec.com/press-release-oxitec-report-96-suppression-of-the-dengue-mosquito-in-brazilian-trials/
- Galizi et al (2014) A synthetic sex ratio distortion system for the control of the human malaria mosquito, Nature Communications, vol 5, article 3977. doi:10.1038/ncomms4977