[CURITIBA, BRAZIL] Genetically modified (GM) mosquitoes released into the environment, during a trial in the Cayman Islands, reduced the population of dengue-carrying Aedes aegypti mosquitoes by 80 per cent, according to a study.
The findings, published this month (10 September) in Nature Biotechnology, were followed by another successful trial of GM mosquitoes in Juazeiro region, Brazil, where a controlled release of GM A. aegypti reduced the mosquito population in the region by 90 per cent, according to a press release by the Brazilian science ministry issued last week (20 September).
The GM mosquitoes for the Cayman Islands trial were developed by the UK-based biotech company Oxitec. Around 3.3 million sterile male mosquitoes, which do not bite people and so do not spread dengue, were released in a site in Grand Cayman, over a period of 23 weeks in 2010.
The sterile male mosquitoes mate with wild, non-GM females, and pass a gene, inserted into GM mosquitoes by scientists, which causes their offspring to die before reaching adulthood, thus preventing them from reproducing and leading to a reduction in mosquito populations.
By the trial's end, scientists found an 80 per cent reduction in the number of mosquitoes in the 16 hectare trial area, compared to untreated areas.
"It is an extremely encouraging result and the first demonstration that you can suppress a target population through the release of engineered male mosquitoes," Luke Alphey, Oxitec's chief scientific officer, told SciDev.Net.
However, environmental and animal welfare group GeneWatch remains sceptical about the effectiveness of using this method to tackle dengue.
"To achieve that level of suppression, they had to increase significantly the number of GM mosquitoes they expected to release... That's not a very efficient method for reducing the [naturally occurring] population," Helen Wallace, executive director of GeneWatch UK, told SciDev.Net.
Alphey said that Oxitec calculated a ratio of GM to wild mosquitoes needed to supress numbers, but given that it is difficult to determine the total number of mosquitoes in an area, Oxitec had to increase the number of released GM mosquitoes.
GeneWatch also raised concerns that the possibility of a small percentage of GM mosquitoes surviving could pose risks to the public health, as they may end up being more effective at transmitting other diseases, such as yellow fever.
But Constância Ayres, a researcher from the department of entomology at Fiocruz Pernambuco, a Brazilian research institute, believes the method is safe.
"It is not a transgene [a transferred gene] that will remain in nature. It sterilises offspring, so it has no role in the epidemiology of the disease. The gene will not persist in the environment," Ayres told SciDev.Net.
However, Ayres is not confident the approach is cost-effective.
"I believe this is a promising tool, but we are a long way from using it in nationwide control programmes. Production [of GM mosquitoes] is still a limiting factor. I don't think it's feasible [to carry out programmes] in big cities."
In order to scale up GM mosquito production and roll out, Oxitec has built a new factory in Brazil. Oxitec is still waiting for the Brazilian ministry of health to use their ideas in national control programmes, and further trials — particularly in large urban centres — are required.
"This tool could be added to a well-established control programme — for instance when you need to momentarily suspend the use of insecticides, you would have another option," said Ayres.
Alphey emphasised that the use of these GM insects in the wild has been approved by regulatory bodies in the three countries — Brazil, the Cayman Islands and Malaysia — where it has been tested.
"The persistence of genetic material in the environment is a major issue for regulatory agencies, but different regulatory authorities have independently concluded that there is no significant risk from this," he said.
The WHO estimates that 50 to 100 million dengue infections occur each year, causing 500,000 cases of dengue haemorrhagic fever and 22,000 deaths, mostly among children.
Nature Biotechnology doi:10.1038/nbt.2350 (2012)