Locked in a secure lab near Melbourne is the newest addition in the fight against dengue: genetically engineered mosquitoes that are resistant to all strains of the potentially deadly virus.
While there have been attempts to genetically engineer mosquitoes to make them resistant to the pathogen in the past, they have been limited to only a couple of the four strains, said study co-author Prasad Paradkar of CSIRO's Australian Animal Health Laboratory.
"This the first time we have made a line of mosquito which is resistant to all four dengue types," Dr Paradkar said.
Dengue fever affects around 390 million people each year around the world, causing symptoms such as severe fever, headaches and muscle aches.
But exposure to one of the strains does not provide immunity to the other strains.
"What we have seen is the second infection actually causes a more severe form of the disease," Dr Paradkar explained.
A second infection can cause haemorrhage, shock and even death.
"So whatever strategy we use has to be effective against all four types of dengue."
The dengue virus is mainly spread by one species of mosquito known for the white bands on its legs — Aedes aegypti — as it jumps from human victim to victim.
How do you genetically engineer a mosquito?
In new research published today in the journal PLOS Pathogens, Dr Paradkar and colleagues detail how they genetically engineered the bandy-legged bloodsucking insect to stop it getting infected or transmitting the virus.
The international team, led by Anna Buchman and Omar Akbari of the University of California, San Diego, inserted proteins from the human immune system into the mosquitoes' DNA.
The antibody gene was activated as soon as the genetically engineered mozzies got a taste of infected blood.
"When the mosquito feeds on the infected person, this human protein becomes active in the mosquito and it neutralises the virus so the mosquitoes don't get infected," Dr Paradkar explained.
"Because they don't get infected they won't be able to transmit that to someone else."
Along with the antibody gene, the researchers also inserted a fluorescent protein gene that makes the mosquitoes' eyes glow so they could distinguish which mosquitoes had been genetically altered from those that hadn't.
"We found 100 per cent of the genome-engineered mosquitoes were resistant to dengue virus infection and were unable to transmit it."
The researchers said the technology could potentially be coupled with other powerful gene editing techniques such as CRISPR-Cas9 to cut and paste the antibody gene on chromosomes inherited from either parent and push it throughout all future generations.
This technique, known as a gene drive, has been shown to work in the lab.
"But no-one has really done it in the wild so we don't really know if it will actually work in the wild," Dr Paradkar said.
Creating GE mozzies is just the first step
These little blood suckers, with their cargo of dengue antibodies, are not likely to see the light of day any time soon.
"Before any sort of release, we have to do a number of experiments in the lab in regards to [the] safety and fitness of these mosquitoes."
Last year, controversial concerns were raised by some scientists about the potential for genetically engineered mosquitoes released in Brazil to create hybrid insects.
"There have been some releases of genetically engineered mosquitoes done by others and none of them have shown any major safety issues," Dr Paradkar said.
"Before we release there has to be a number of steps taken including community engagement and involvement and engagement with regulators."
Cameron Simmons, director of the Institute of Vector-Borne Disease at Monash University, said the research was an encouraging first step and showed the science around using genetic techniques was accelerating.
But the second step, to create a genetic drive mechanism, was "a much bigger challenge in both the scientific sense, but also more importantly from a community acceptability sense".
"The regulatory environment for releasing genetically engineered organisms is complex, communities need to be taken along that journey," Professor Simmons commented, citing instances in the US where trials of the technology have been met with community resistance.
"It's a long road to getting these second-generation, genetically modified organisms into the field at scale."
Wolbachia alternative being tested in the field
Genetic engineering is not the only technique being used in the war against dengue.
Professor Simmons and scientists involved in the World Mosquito Program (WMP) have been trialling mosquitoes laden with the Wolbachia bacterium in 12 countries since 2011, including sites in north Queensland.
Wolbachia makes mosquitoes resistant to infection by the four dengue strains as well as other mosquito-borne diseases such as Zika, chikungunya and yellow fever.
"North Queensland has been dengue-free for five years in all the locations where Wolbachia has been deployed," he said.
Once a mosquito is infected with the bacterium it continues to infect its offspring.
"It's also got its own clever gene drive if you like."
The WMP team is now awaiting the outcome of a major trial of the bacterium-laden mozzies in Yogyakarta, which, if successful, could inform WHO deployment of the technique.
"This is real world. North Queensland is not necessarily representative of South-East Asia or Latin America where dengue is problematic every year," Professor Simmons said.
Dr Paradkar agreed Wolbachia had been very successful in certain regions but he said it may not work in some areas.
"Dengue is such a big and complex problem that it's good to have different kinds of alternative strategies in controlling this disease."