It started with an arm covered in mosquitoes.
But this wasn’t just your unlucky friend at a summer barbecue. Perran Ross’s arm was lunch for a swarm of mosquitoes infected with a bacterium called Wolbachia – part of a sweeping and ambitious strategy to rid the world of dengue fever.
Those that have experienced dengue fever aren’t likely to forget it. The dengue virus passes between humans via mosquitoes, with those infected suffering headaches, vomiting, muscle pains, skin rash, and a characteristic high fever for days on end.
A smaller subsection of cases goes on to develop dengue haemorrhagic fever or shock syndrome – causing bleeding under the skin and severe vomiting. The number of infections are going up year on year, and in 2019 the World Health Organisation (WHO) recorded 4.2 million cases, although thankfully relatively few deaths.
In certain places like Australia, though, 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 was never endemic in Australia, in North Queensland dengue outbreaks would sometimes occur sporadically when an infected traveller would be bitten by a mosquito, which would then bite someone else, passing the virus on.
“Far North Queensland is now essentially a dengue-free area for the first time in well over 100 years,” said physician Richard Gair, Director of Tropical Public Health Services in Cairns, back in April.
We have Wolbachia to thank for that.
Since 2011, researchers and members of the public have been releasing mosquitoes infected with Wolbachia into the wild in North Queensland.
Wolbachia was already a very commonly distributed form of infectious bacteria – one study in the 1990s found 17 percent of insect species tested were infected, although it’s likely to be significantly more.
The bacteria – which naturally blocks the transmission of dengue fever – transfers from generation to generation of mosquito, and doesn’t seem to affect humans at all, making it a compelling choice for efforts to eradicate dengue.
There’s just one problem, though: Wolbachia is not naturally occurring in the mosquito that transmits dengue, Aedes aegypti, and there’s not an easy way to infect them in bulk.
Instead, A. aegypti mosquito eggs have to be individually infected with the bacteria under a microscope.
“We line up mosquito eggs on a glass slide, then use the micromanipulator to stab the egg with a very fine needle,” explains Ross, an entomologist from the University of Melbourne.
“We then suck out the cells containing the Wolbachia from one egg, and inject it into another. If you’re lucky, then it’ll survive and it’ll be passed on to the next generation.”
It’s painstaking work. A researcher might be able to inject a few hundred eggs a day, but it can take anywhere from about 200 to 10,000 eggs to find a single female Wolbachia-infected mosquito that will pass on the bacteria to the next generation.
“It might be something like six months of full-time work to get a single stable population,” Ross told ScienceAlert. “But really, it’s a small price to pay given the value of a single Wolbachia-carrying mosquito line.”
Once you’ve got the mosquito line, you can begin breeding them in the lab. If you want the infected mosquitoes to have a good chance of breeding with enough wild mosquitoes in the area, you need around one mosquito for every three-to-ten houses in a location. You can imagine how quickly those numbers add up.
“You have to rear hundreds of thousands of mosquitoes in the lab and then go around releasing them everywhere,” Ross told ScienceAlert. “These particular mosquitoes don’t really travel very far by themselves.”
Ross works with these mosquitoes on a daily basis, monitoring the long-term effects and stability of Wolbachia in Australian mosquitoes. Part of that monitoring is feeding thousands of hungry mouths. For this, Ross himself is the bait.
A photo of his arm covered with bites went viral back 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 little bit if they get you in the right spot, but mostly it’s just slight irritation,” Ross says. “It’s absolutely itchy later. As soon as I take my arm out, I have to resist the urge to scratch.”
Ross is likely to have many more mosquitoes to feed well into the future. As it turns out, Wolbachia not only lowers infection rates of dengue, but may also limit infections of other mosquito-borne diseases, while shortening the lifespans of the A. aegypti mosquitoes infected with it.
Because of this, Wolbachia-infected mosquitoes are also being released in other parts of the world, particularly in places where Zika, dengue, and chikungunya virus threaten to become serious health risks.
In 2019, scientists explained that they had wiped out mosquitoes all together on two Chinese islands, using a strain of Wolbachia in conjunction with a dose of radiation to sterilise the insects.
There’s another release of Wolbachia-infected mosquitoes currently underway in Malaysia, with hopes it’ll stop the spread of dengue, Zika, and chikungunya virus.
“They’ve released mosquitoes in Kuala Lumpur – dengue is endemic there,” Ross explains. “It’s reduced it by 40 to 60 percent. That’s pretty substantial.”
Just last year, Malaysia’s health ministry expanded the program because of its success.
Until COVID-19 put a temporary stop to the program, not-for-profit groups such as The World Mosquito Program were working on getting more Wolbachia-infected mosquitoes to other virus-affected areas around the world. The project has already released infected mosquitoes in 12 countries.
Ross’s research has shown that Wolbachia seems to remain stable in a population, so even in locations where COVID-19 has affected the rollout of the program, the mosquitoes that have already been released are likely staying put.
Despite the ongoing challenges, Ross is upbeat about Wolbachia‘s role in stopping dengue around the world.
“It is going to be expensive and it requires a lot of community engagement and planning,” Ross explains.
“But I think it’s possible.”
Source:ScienceAlert – Latest