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GM insect company Oxitec today published results of its trials of genetically modified (GM) mosquitoes in the Cayman Islands in 2010. The results were originally submitted to the journal Science in January 2011 but have now finally been published in Nature Biotechnology. The paper shows that Oxitec has no clear baseline for claims made in the press that it achieved an 80% reduction in the target population of mosquitoes, and that to achieve the claimed effect it significantly increased the number of adult GM mosquitoes it expected to release and also released additional GM pupae at locations spaced at 70 to 90m apart across the release site.

“This poor quality paper pours cold water on the idea that Oxitec’s GM mosquitoes will be an effective way to tackle dengue” said Dr Helen Wallace, Director of GeneWatch UK. “Staff would be better employed using the well-established public health approach of removing mosquito breeding sites rather than in placing GM mosquito pupae at intervals across a site. Removing the flower pots and water containers where mosquitoes breed has the added benefit of reducing both mosquito species that spread dengue, not just one of them. It is hard to see how Oxitec can justify commercial releases of its GM mosquitoes based on such poor data.”

During the experiments, Oxitec increased releases of GM male mosquitoes from the expected 3,150 males per hectare per week to about 14,000. When local residents complained about the nuisance caused by the very large number of mosquitoes, Oxitec halved the number of adults released and deployed GM pupae at the site. Oxitec reports several different estimates of the reduction in the wild population of mosquitoes, ranging from 60 per cent to 85 per cent depending what comparisons are made. Because there is no baseline data on mosquito populations at the site there is considerable uncertainty in the results. At different times, Oxitec moved mosquito traps from one location to another and changed the size of the release site, adding to difficulties in interpreting the results.

This paper (published as Correspondence to the journal, two years after Oxitec press released its results) will increase the growing doubts about the efficacy of Oxitec’s approach and whether it will be a cost-effective way to tackle dengue fever.