An international team working off the coast of Nha Trang is taking an alternative, potentially game-changing approach to combating mosquito-borne dengue. By Michael Tatarski. Photo by Richard Harper.
Though they are now the enemy of anybody living in the tropics, mosquitoes were limited to Africa up until 400 years ago. Through European expansion they were delivered to much of the globe, and now their range covers over 100 countries and can affect roughly 2.5 billion people. While everyone knows the annoyance of a mosquito bite, certain types of these insects carry dangerous, sometimes deadly diseases. One of the most virulent is dengue, a name many in Vietnam are familiar with.
According to the World Health Organisation, up to 100 million cases of dengue occur worldwide annually. Of these cases, 500,000 may develop into dengue haemorrhagic fever, which can result in up to 40,000 deaths. Currently there is no effective cure for the infection.
However, the people behind the Eliminate Dengue Program are working on a project that could change the way we think about fighting insect-borne diseases, while halting dengue in its tracks. Research behind this effort began in the 1990s and centred on a type of bacteria called Wolbachia. According to professor Scott O’Neill, head scientist of Eliminate Dengue and dean of science at Monash University outside Melbourne, Wolbachia is carried by 70 percent of insects. However, it is not present in the type of mosquito, known scientifically as Aedes aegypti, that hosts dengue.
Originally the research focused on shortening the lifespan of Aedes aegypti. According to a video put together by Eliminate Dengue, one scientist discovered a strain of Wolbachia that halved the lives of fruit flies, from 30 days to 15 days. Aedes aegypti has a similar lifespan, and researchers knew that it takes up to 10 days for a mosquito to be able to transmit dengue once it has already bitten a person. They decided to see if transferring Wolbachia from a fruit fly to Aedes aegypti would shorten its lifespan, thus reducing its ability to spread dengue. The results stunned scientists on the team.
The life-shortening theory worked, but it also turned out that Wolbachia acted as a vaccine.
“Simply having Wolbachia in the mosquito helped prevent the spread of dengue. We didn’t need any fancy tricks,” O’Neill told me.
Just as important is the fact that Wolbachia spreads naturally once insects infected with it mate with an insect that is not carrying the bacteria. This breakthrough altered the way researchers looked at a cure for dengue.
“We’ve been trying to create vaccines for humans for years unsuccessfully, so this is an alternative approach,” O’Neill says. “The key feature is that once you put it out there it’s self-sustaining … when you put it into a population it spreads by itself.”
The Eliminate Dengue team moved from laboratory tests to field trials around eight years ago, when the program received funding from the Bill and Melinda Gates Foundation. They worked on injecting Wolbachia into a few mosquitoes in a small population in northern Australia, and the results were positive. But one problem was that Australia doesn’t have enough dengue to provide conclusive measures, so the team looked elsewhere, and testing in Vietnam was their next step.
With cooperation from the National Institute of Hygiene and Epidemiology, the Ministry of Health, Institute Pasteur, the Khanh Hoa province health department, and locals, Eliminate Dengue selected Tri Nguyen Island, two kilometres off of Nha Trang, as its proving ground.
“We started off by trying to suppress the local mosquito population before we started releasing our own,” O’Neill says. “That way we wouldn’t add to the population once we started the program.”
The team is now figuring out which strains of Wolbachia will be the most efficient to deploy. The program is halfway through the release period on Tri Nguyen, and the rate of mosquitoes infected with Wolbachia has reached 60 percent.
“The test will be to see if the rate reaches around 80 percent, and then the question becomes will it stay that high,” O’Neill says.
Depending how things go on the island, the program could be extended to mainland Vietnam. There is also the possibility that Wolbachia could one day be used to suppress the most dangerous mosquito-borne illness — malaria. It has already been proven that the bacteria neutralises yellow fever and a few other diseases also carried by Aedes aegypti.
Another positive of this novel approach is its low human and environmental impact.
“We did a lot of work on this and we found that Wolbachia is too big to fit down the salivary ducts of the mosquito,” O’Neill says. Therefore, if you get bitten by an infected mosquito, the bacteria won’t be transmitted.
It is important to remember that Wolbachia occurs naturally as well. People get bitten by other bugs carrying the bacteria and no traces of it have ever been found in human blood, making this method non-invasive. O’Neill and his team are confident there will be no negative consequences from an environmental or human health perspective.
While questions remain regarding this method, particularly the issue of how long it may take for Aedes aegypti to develop resistance to Wolbachia, if any, Eliminate Dengue has made great progress. From an accidental discovery to full-field testing, the team has shown that we may finally have a leg up in the fight against insects and illnesses that have plagued humans for centuries.
Tips to Avoid Dengue
Professor Scott O’Neill offers some advice on how to reduce the chance of catching dengue, information that is especially important now that we are in the monsoon season, when mosquito populations explode: “The most important thing to appreciate is that Aedes aegypti is a day biting mosquito, so sleeping under a net or something like that won’t do anything. People usually get bitten around their house [this type of mosquito is highly domestic] so the key thing is to not get bitten during the day. Make sure there are no mosquitoes breeding in your house, so get rid of any containers of water in or around your house. The mosquitoes don’t fly very far so if you can control them around your own dwelling you can reduce the risk of dengue.”