GM mosquito trial results in Cayman prove ineffective in tackling dengue

Activist Post

GM insect company Oxitec today published results of its trials of genetically modified (GM) mosquitoes in the Cayman Islands in 2010 (1). The results were originally submitted to the journal Science in January 2011 but have now finally been published in Nature Biotechnology (1). 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.

GM dengue mosquito – Wiki image

“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 (2). 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.

Releasing such large numbers of mosquitoes adds to concerns that the small percentage of surviving GM mosquito offspring and accidental releases of GM female mosquitoes could pose unnecessary risks (3). Oxitec’s failure to publish and consult on a risk assessment and obtain informed consent from Cayman Islands’ residents before going ahead with its experiments remains unacceptable. Oxitec’s summary of the necessary regulatory requirements is misleading because it fails to mention that it should have sent a risk assessment which meets European standards to the EU and UK authorities before exporting GM mosquito eggs to the Cayman Islands (4). Informed consent to such experiments is not possible unless public information on the risks has been made available.

Progress on dengue vaccines and other approaches means that it is likely that more effective public health approaches can be combined with vaccination in the future. Impacts of vaccination on immunity and cross-immunity must be considered as part of the regulatory assessment for new vaccines, but these important issues have been ignored by Oxitec when it promotes its GM mosquitoes in dengue-endemic countries (3). Poorly effective approaches to reducing mosquito populations can actually increase the risk of the more severe form of dengue, due to the loss of cross-immunity to different serotypes of the dengue virus. Other risks include a possible increase in the numbers of the second mosquito species that transmits dengue (the Asian Tiger mosquito); and increases in the number of surviving GM mosquitoes over time.

“Plans to scale-up releases of GM mosquitoes in dengue-endemic Brazil should be halted whilst the implications of these poor results are thoroughly considered” said Dr Wallace. “Authorities in other places where releases have been planned, such as Florida and Panama, should also stop and think again”.

For further information contact:

Dr Helen Wallace: 01298-24300 (office); 07903-311584 (mobile).

Notes for Editors:

(1) Harris et al. (2012) Successful suppression of a field mosquito population by sustained release of engineered male mosquitoes. Correspondence to Nature Biotechnology.

(2) “The only consistent project-related criticism from the community related to nuisance from the large numbers of males in each individual release in the first part of Period 3. In response we promptly reduced these numbers and moved the release points further from habitations. We also partly substituted with pupal releases, from which adults emerge over a period of time. Following these changes, no further adverse comments were received”. Supplementary material to (1).

(3) Reeves RG et al. (2012) Scientific Standards and the Regulation of Genetically Modified Insects.

Lehane MJ, ed. PLoS Neglected Tropical Diseases, 6(1), p.e1502.

(4) Oxitec’s Genetically Modified Mosquitoes: Ongoing Concerns. GeneWatch UK briefing. August 2012. On:

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