Genetically modified male mosquitoes who sire offspring that die early have finally been released in Brazil to suppress dengue. And according to results published in PLOS Neglected Tropical Diseases last week, these “sterile” skeeters have reduced the dengue mosquito population in one Brazilian suburb by 95%.
Dengue (also known as “bone breaker”) infects as many as 390 million people a year. It’s the second most significant mosquito-borne disease after malaria, but unlike malaria, dengue is increasing in both incidence and severity. There are no specific drugs nor licensed vaccines to treat this infectious disease, at least not yet. Its primary vector is Aedes aegypti, and researchers around the world have been developing a so-called self-limiting gene: When a transgenic male mates with a wild female, the transgene is passed to embryos through sperm, leading to larvae death before they’re old enough to start transmitting diseases.
In the last few years, a self-limiting strain called OX513A reached the field evaluation stage after extensive lab tests. Releases of OX513A males in the Cayman Islands in 2010 led to 80% suppression of a target population, while isolated field demonstrations in Brazil have achieved similarly successful results after six months.
Then, beginning in 2011, a team led by Andrew McKemey of Oxitec conducted a sustained series of OX513A field releases in Itaberaba, a suburb of Juazeiro in the semi-arid northeast region of Brazil. Because piped water services are irregular in this dense suburb, the 1,810 residents depend on stored water – ideal habitats for Aedes aegypti. Throughout the study, local mosquito control was deployed as normal: Public health agents continued to destroy breeding sites and treat homes with larvicide.
Just over a year later, the local Aedes aegypti population was reduced by 95% based on adult trap data. According to a separate measure based on ovitraps (which mimic breeding sites), the population was reduced by 81%. "For context, with currently available methods including pesticides – the best you can get is about 50% reduction – not enough to prevent the epidemic spread of disease," Oxitec CEO Hadyn Parry tells IFLScience. Sustained release of OX513A males, the researchers say, may therefore be an effective and widely useful method for suppressing the dengue vector and preventing epidemics.
“In theory, if you have fewer mosquitoes you have less transmission, but in reality, this is something we still need to investigate,” study co-author Margareth Capurro from the University of São Paulo tells New Scientist. “You can have lots of mosquitoes with only a few infected, or very few with all of them infected,” she adds. “If this happens, you suppress the population but don't affect dengue transmission.” But plans are in place to conduct analyses of correlations between mosquito numbers and dengue cases, and if the studies are conducted on a sufficiently large enough scale, then the scientists should be able to deduce whether statistically significant reductions in dengue have occurred, Parry tells IFLScience.
This past April, six million of these modified mosquitoes, known locally as “friendly Aedes aegypti,” were released in the Brazilian city of Piracicaba, which is facing one of the worst infestations. “The next step is to scale up to even larger studies and run mosquito control projects on an operational basis,” McKemey says in a statement. The company is waiting for permission from the U.S. Food and Drug Administration to test in the Florida Keys. Although this area has been dengue-free since 2010, there are fears that it could return, which is why there is interest in testing out Oxitec's technology, Parry says.