An Australian City Beats Dengue Fever Using Special Mosquitoes
There has not been a case of the disease in Townsville for four years after the release of insects carrying a naturally occurring bacteria
Globally, dengue fever is on the rise. Just last year, cases of the mosquito-borne disease reached an all-time high in Australia, infecting around 2,000 people total. But the city of Townsville in the state of Queensland hasn’t seen a single case in the last four years, despite being in the perfect habitat for the disease.
That’s because, reports Sarah Boseley at The Guardian, the citizens of Townsville released millions of mosquitoes infected with a certain bacteria that prevents them from spreading the disease. If deployed globally, the technique could help protect the over 2 billion people living in the range of the disease.
It turns out, about 60 percent of insects on Earth carry around bacteria known as Wolbachia. Though the microbe does not normally colonize Aedes aegypti, the super-resilient and adaptable tropical mosquito species that transmits dengue fever, zika, chikungunya and other human diseases. But researchers have found a neat trick: If Wolbachia does take root in those mosquitoes, it disrupts the growth of the viruses and reduces their transmission.
In Townsville, population 187,000, researchers from Monash University and the nonprofit World Mosquito Program, attempted to do just that. Kelly Servick at Science reports that the program enlisted 7,000 families in the metro area to host a small tub of Wolbachia-infested Aedes aegypti eggs in their yard, where they fed them fish food and nursed them to maturity. Eventually, 4 million bacteria-filled mozzies—as they’re called in Australia—hatched and mixed with wild populations over 25 square miles, spreading the bacteria throughout the population during mating. The study, which has yet to be peer-reviewed, appears on Gates Open Research.
Monash medical entomologist and World Mosquito Program director Scott O'Neill tells Servick that, so far, the mosquito population in Townsville seems to be maintaining the Wolbachia. In the decade before the study, the city experienced dengue outbreaks every year. But since the release of the inoculated mosquitoes, there have been no cases reported.
That’s encouraging news, since researchers have had little success in beating mosquito-borne illnesses in recent decades. “We’re wanting to have a really major impact on disease. For dengue and Zika nothing’s working at the moment for control. There’s evidence of a growing disease burden and there was the big Zika pandemic that stripped through the Americas recently and the rest of the world,” O’Neill tells The Guardian’s Boseley. “Nothing we’ve got is slowing these diseases down – they are getting worse. I think we’ve got something here that’s going to have a significant impact and I think this study is the first indication that it’s looking very promising.”
The Townsville test is not the first trial of the Wolbachia method, but it is by far the largest test case so far. Currently, the program is doing research on the technique in 12 countries and has released the mosquitoes in six, including the favelas of Rio de Janeiro that bore the brunt of the Zika outbreak. The next major case study is in the city of Yogyakarta in Indonesia, a city of about 400,000 people, where researchers are conducting a randomized trial of the method. Those results should be ready in about 18 months.
According to a press release, the current cost of releasing the treated mosquitoes is about $15 per person. With time, the team hopes to get the cost down to about $1 per resident, which would allow the project to be rolled out in some of the poorer regions of the world where it’s needed most. Even more promising, O’Neill tells Boseley that lab tests suggest the same technology could be used to prevent malaria as well, though field trials of that are still a way’s away.
The other advantage of the Wolbachia method is that, since the bacteria is naturally occurring throughout most of the world, there is little danger of spreading a pathogen that might hurt other insect species. Other researchers are also looking at ways to stop the spread of dengue and other diseases by creating genetically modified mosquitoes resistant to carrying the virus. Another type of genetic modification causes the male offspring Aedes aegypti mosquitoes to die, eventually leading to a population crash. Wolbachia can be used to produce similar results—if only male mosquitoes are infected with the bacteria any eggs they fertilize with non-infected females will be sterile, also leading to a population crash, a technique recently tested in Florida.
The World Mosquito Program’s Wolbachia method, however, has several advantages. Genetic modification is controversial and could lead to public resistance to the release of the insects. And methods that kill off mosquito populations, though welcome by many humans, could have negative impacts on the environment. The World Mosquito Program says that their program, at least so far, is self-sustaining, has little impact on the environment and doesn’t raise red flags the way genetic modification projects can.