It started with an arm covered with mosquitos.
But that wasn’t just your unhappy friend at a summer barbecue. Perran Ross’ arm was lunch for a swarm of mosquitoes infected with a bacterium called Wolbachia – part of a comprehensive and ambitious strategy to rid the world of dengue fever.
Those who have had dengue fever are unlikely to forget it. The dengue virus occurs between people via mosquitoes, with those infected suffering for days from headache, vomiting, muscle aches, rashes and a characteristic high fever.
A smaller proportion of cases will continue to develop dengue hemorrhagic fever or shock syndrome, causing bleeding under the skin and severe vomiting. The number of infections is increasing every year, and in 2019 the World Health Organization (WHO) recorded 4.2 million cases, albeit fortunately relatively few deaths.
However, in certain countries like Australia, dengue outbreaks are a thing of the past.
Time-lapse mosquito feeding pic.twitter.com/AJx5iy1gqr
– Perran Ross (@MosWhisperer) December 12, 2017
Although dengue fever has never been endemic in Australia, dengue outbreaks sometimes occur sporadically in northern Queensland when an infected traveler was bitten by a mosquito, which then bite someone else and passed the virus on.
However, in recent years the number of cases in Australia has decreased. So far this year, only two locally acquired positive cases have been identified.
“Far North Queensland is now essentially a dengue-free area for the first time in well over 100 years,” said Richard Gair, director of the Tropical Public Health Services in Cairns, back in April.
We have to thank Wolbachia for this.
Since 2011, researchers and members of the public have been releasing Wolbachia-infected mosquitoes into the wild in north Queensland.
Wolbachia was already a very common form of infectious bacteria – a study in the 1990s found that 17 percent of the insect species tested were infected, although there are likely significantly more.
The bacterium – which naturally blocks the transmission of dengue fever – is transmitted from generation to generation of mosquitoes and doesn’t seem to affect humans at all, making it a compelling choice for dengue eradication efforts.
There is only one problem, however: Wolbachia is not naturally found in the mosquito that carries dengue fever, Aedes aegypti, and there is no easy way to infect it in large quantities.
Instead, A. aegypti mosquito eggs must be infected individually with the bacteria under a microscope.
“We align mosquito eggs on a glass slide and then use the micromanipulator to pierce the egg with a very fine needle,” explains Ross, an entomologist at the University of Melbourne.
“We then suck the cells that contain Wolbachia out of one egg and inject them into another. If you’re lucky, it will survive and be passed on to the next generation.”
It is tedious work. A researcher can potentially inject a few hundred eggs a day, but it can take anywhere from 200 to 10,000 eggs to find a single female Wolbachia-infected mosquito that will pass the bacteria on to the next generation.
“It could be about six months of full-time work to get a single stable population,” Ross told ScienceAlert. “But really, given the value of a single line of Wolbachia-bearing mosquitoes, this is a small price to pay for.”
Once you have the mosquito line you can grow it in the lab. If you want the infected mosquitoes to have a good chance of breeding with enough wild mosquitoes in the area, you’ll need roughly one mosquito for every three to ten homes in a location. You can imagine how quickly these numbers add up.
“You have to raise hundreds of thousands of mosquitoes in the lab and then release them everywhere,” Ross told ScienceAlert. “These particular mosquitoes don’t really travel far alone.”
Ross works with these mosquitoes on a daily basis, monitoring the long-term effects and stability of Wolbachia on Australian mosquitoes. Part of this surveillance is feeding thousands of hungry mouths. Ross himself is the bait for this.
A photo of his arm covered in bites went viral in May after Ross fed 5,000 mosquitoes in a single day.
Record day of mosquito blood feeding today. ~ 5000 female mosquitoes fed and 16 ml of blood lost. pic.twitter.com/7OzeQ9rGl7
– Perran Ross (@MosWhisperer) May 7, 2020
“Sometimes it can sting a bit when they get you in the right place, but most of the time it’s just a slight irritation,” says Ross. “It absolutely itches later. As soon as I take my arm out, I have to resist the urge to scratch myself.”
Ross is likely to have a lot more mosquitos to feed on in the future. As it turns out, Wolbachia not only lowers the rates of dengue infection, it can also limit infections of other mosquito-borne diseases while reducing the lifespan of A. aegypti mosquitoes infected with it.
Because of this, Wolbachia-infected mosquitoes are also being released to other parts of the world, especially places where Zika, dengue, and chikungunya viruses pose serious health risks.
In 2019, scientists stated that they had completely eradicated mosquitoes on two Chinese islands by using a strain of Wolbachia in conjunction with a dose of radiation to sterilize the insects.
There is currently another release of Wolbachia-infected mosquitoes in Malaysia in the hopes of stopping the spread of Dengue, Zika and Chikungunya viruses.
“They released mosquitoes in Kuala Lumpur – dengue fever is endemic there,” explains Ross. “It’s reduced by 40 to 60 percent. That’s pretty extensive.”
Just last year, the Malaysian Ministry of Health expanded the program because of its success.
Until COVID-19 temporarily suspended the program, nonprofit groups like the World Mosquito Program were working to move more Wolbachia-infected mosquitoes to other virus-stricken areas around the world.
The World Mosquito Program is run by Monash University in Australia and has already released infected mosquitoes in 12 countries. However, it is a separate project that the University of Melbourne is not involved in, although both programs work with Wolbachia.
In relation to Ross’ research, the results suggest that Wolbachia appears to remain stable in a population. Even in places where COVID-19 affected the rollout of the program, the mosquitos that have already been released are likely to continue to be used.
Despite the ongoing challenges, Ross is optimistic about Wolbachia’s role in fighting dengue around the world.
“It gets expensive and requires a lot of community commitment and planning,” explains Ross.
“But I think it’s possible.”