A recent study published in Scientific Reports studied the aftermath of an insect population control experiment in Brazil. The study’s conclusions have attracted significant attention from news outlets, but also significant criticism.
If asked to list the deadliest animals in the world, few would include mosquitoes to their top answers. In reality though, the mosquito Aedes and Anopheles species can transmit malaria, yellow fever, and dengue. In 2017, there were 219 million malaria cases and more than 400,000 people died according to the WHO. The best way to control the disease spreading is mosquito population control.
Aedes aegypti is a remarkably resilient insect that can adapt to different environments. It is an invasive species in many countries and very difficult to control. Insect repellents and bite avoidance (e.g. long sleeves, mosquito nets) don’t offer adequate protection. Organic insecticides – such as DDT – were successful in eradicating malaria from many regions globally however their extensive use causes environmental and health hazards. Ideally, a population control strategy would quickly reduce the insect population, and target only the specific disease-carrying species.
Biological control holds a lot of promise. One method that has been tested in the field is the release of a large population of radiation-sterilized male mosquitoes (that don’t bite) that mate normally with females but don’t give viable offspring. However the actual sterilization is laborious and damaging to the insects.
An improved control method makes use of synthetic biology. Oxitec, a UK-based company, generated an Aedes aegypti strain called OX513A. These mosquitoes contain a lethal gene that will kill the larvae unless an antidote is administered. This means that OX513A can grow in the lab but its offspring won’t survive in the environment. The transgenic variant also contains a fluorescent protein for easy identification.
Oxitec, in collaboration with the local authorities, performed a population control experiment in Jacobina, Brazil. This work showed a significant drop in the wild Aedes aegypti population, while the transgenic mosquitoes were absent from the environment a few months post-release. Interestingly, the mosquitoes managed to reestablish themselves 4-5 months after the GM mosquito release. Consequently, a successful approach would need a continuous release of insects with the suicide gene.
A few weeks ago, a study on the same topic was published in Scientific Reports. Benjamin Evans and his collaborators assessed the post-release mosquito population for genetic alterations. As the Oxitec mosquito offspring have a 3-5% survival rate, the researchers wanted to see if the OX513A genes passed on to the local insects. Indeed, they could detect SNPs that didn’t exist beforehand and are associated with the Oxitec strain. However the two heterologous genes – the larvae killing one and the fluorescent protein – could not be detected in the environment.
This work fills a gap in the implementation of a population control technique. “Studies like the one by Evans and collaborators are absolutely necessary and should follow every implementation of a new technology. They will help redesign our tools in order to become both safer to the environment and more effective to their targets,” says Professor Kostas Mathiopoulos from the University of Thessaly, Greece. However, the way the study findings are presented sparked some strong reactions.
“The title of the paper itself leads the casual reader to think that the transgene was introduced and spread into the population – this is NOT the case,” says Professor Jason Rasgon from the Pennsylvania State University. He adds that “the paper had, in its discussion, a statement that the mosquito population would now be fitter due to ‘hybrid vigor’. No data were presented to support this statement”. Oxitec has reacted to the publication by published a rebuttal on its website. “We have no issue with the study’s methodology, just with the conclusions,” notes Nathan Rose, Oxitec’s head of regulatory affairs. Scientific Reports has issued a notice that “the conclusions of this paper are subject to criticisms that are being considered by editors.” Professor Jeffrey Powell, corresponding author of this study did not respond to an email request for comment. But on a darker note, Dr Margareth Capurro, associate professor at the University of São Paulo, and the other Brazilian co-authors asked the editors for the retraction of the paper, as reported by the Question of Science magazine.
“Failed GM mosquito control experiment may have strengthened wild bugs,” reads one of the news articles covering the study. Unfortunately, this title does not reflect the study findings and creates unnecessary fear-mongering. The data and the scope of any study have to be reported in the right context without overhyping. In this case, the authors of the work failed to do so. And the fact that the local scientists seem to have been excluded from the final version of the article is shameful at the least.
Last year, a UN treaty agreed to limit the implementation of gene drives – a more aggressive use of genetics that makes a lethal gene persist in a population and in theory could eradicate a whole species. But the UN did not ask for a total ban, due to concerns from several African countries. Professor Mathiopoulos is of the same opinion: “gene drive are new and, apparently, very powerful tools for insect (and not only) control. In areas where thousands of children die every year of malaria and other diseases, it would be criminal to NOT use it.”
It’s easy to be critical of genetic population control tools, using the words “unnatural” and “irresponsible” out of context and in a romanticized way. As people keep dying of mosquito-borne diseases and as the insects extend their geographical spreading due to human activities, we need to take action. Of course, we need rigorous assessment of control strategies – ideally by independent research bodies. But we have little need for improper science and practices that may cost lives.