The Copulation Trap

That’s what a UK-based firm is employing to control the mosquito population, and with good results, too. T.V. Jayan reports

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Sex could be the latest weapon in mankind’s unrelenting fight against mosquitoes.

Researchers at a fledgling UK firm are laying a “copulation trap” for mosquitoes which will eventually drive down the population of these pesky insects.

The technique that the five-year-old Oxitec or Oxford Insect Technologies — a spin-off from the University of Oxford — is developing is based on an interesting strategy: release a swarm of genetically engineered male mosquitoes whose mating with the female insects in the wild will yield progeny that are doomed to die as larvae or pupae. Sustained release of such healthy but reproductively-compromised mosquitoes will eventually lead to a crash in their population.

The Oxitec technology, which bagged the Technology Pioneers 2008 award at the World Economic Forum, recently completed its first ever confined field study under the supervision of Malaysia’s Institute of Medical Research. “The study has yielded encouraging results,” says Seshadri S. Vasan, Oxitec’s head of public health. Vasan, who is also a visiting fellow at Oxford, is scheduled to release the results of the Malaysian study at an international conference in Puri, Orissa, later this month.

For the study, the scientists used Aedes aegypti mosquitoes, the main vector for dengue and chikungunya. Dengue is the fastest growing vector-borne disease in the world, affecting 55 per cent of the global population with an estimated 100 million cases in over 100 countries, including India. Chikungunya, too, has of late become a major problem, at least in India. In 2006, there were 14 lakh chikungunya cases in the country. Last year, too, there were a large number of chikungunya cases, reported mainly from Kerala.

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“Conventional Aedes control methods (such as fogging and larvicide for mosquito breeding site reduction) have not been able to prevent outbreaks as is evident from the experience of a highly committed and organised city like Singapore. This is because of the low entomological threshold for transmission — as few as two or three adult female Aedes aegypti mosquitoes emerging every day in a locality of 100 people could be a sufficient threshold for a dengue threat,” remarks Vasan.

Mosquitoes of both sexes may be a nuisance, but it is the female that has the ability to bite and hence can transmit a disease-causing germ. That’s because only the female mosquitoes possess a proboscis that is capable of piercing the skin. As a result, an effective strategy for controlling a mosquito-borne disease is to control the female population of the vector.

The Sterile Insect Technique (SIT) which involves releasing millions of sterile male insects over a wide area to mate with the native female ones, has been around for decades. A good example is the new world screwworm, a pest of livestock. The United States eradicated the screwworm using SIT.

However, conventional sterilisation programmes using radiation or chemical treatment to foster sterility — which were tried on mosquitoes as well — did not work. Irradiation rendered these mosquitoes so sickly and unattractive that their female counterparts shunned them for the wild ones!

On the other hand, mosquitoes subjected to Oxitec’s proprietary technique — called RIDL-SIT — remained healthy and attractive enough to woo the native female insects. This was proved in independent case studies in Oxford and France and in contained semi-field trials in Malaysia. For instance, the results of the latest study in Malaysia showed that “up to 50 per cent of the wild type female mosquitoes chose to mate with Oxitec’s RIDL male mosquitoes,” says Vasan.

“The Oxitec technology appears to be based on good genetics. However, prior to using it in vector control programmes, one would like to see the degree of lethality induced by these genetically modified mosquitoes when mated with wild mosquitoes isolated from the target region,” says L.S. Shashidhara, a geneticist with the Centre for Cellular and Molecular Biology, Hyderabad.

Oxitec has received regulatory and import permits for confined evaluation in the US, France and Malaysia, while it is still holding discussions with regulators of other endemic countries such as India.

Sources: The Telegraph (Kolkata, India)

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