For decades, preventing dengue fever in Honduras has meant teaching people to fear mosquitoes and avoid their bites. Now, Hondurans are being educated about a potentially more effective way to control the disease — and it contradicts everything they’ve learned.
Which explains why a dozen people cheered last month when Tegucigalpa resident Hector Enriquez held a glass jar filled with mosquitoes above his head, and then released the buzzing insects into the air. Enriquez, a 52-year-old mason, had volunteered to help publicize a plan to suppress dengue by releasing millions of special mosquitoes. Honduran capital.
The mosquitoes that Enriquez spread in his El Manchen neighborhood — an area rife with dengue — were bred by scientists to carry a bacteria called Wolbachia that disrupts transmission of the disease. When these mosquitoes breed, they pass the bacteria on to their offspring, reducing future outbreaks.
This emerging strategy to fight dengue was launched last decade by the nonprofit World Mosquito Program, and is being tested in more than a dozen countries. With more than half the world’s population at risk of dengue, the World Health Organization is paying close attention to the mosquito outbreak in Honduras and elsewhere, and is prepared to boost the strategy globally.
In Honduras, where 10,000 people become ill from dengue each year, Doctors Without Borders is partnering with a mosquito program to release approximately 9 million mosquitoes carrying the Wolbachia bacteria over the next six months.
“New methods are desperately needed,” said Scott O’Neill, founder of the mosquito program.
Dengue defies general prevention
Scientists have made great progress in recent decades in reducing the threat of mosquito-borne diseases, including malaria. But dengue is an exception: its infection rate keeps increasing.
Models estimate that about 400 million people in about 130 countries are infected with dengue each year. The mortality rate from dengue is low – an estimated 40,000 people die from it each year – but outbreaks can overwhelm health systems and force many people to miss work or school.
“When you come down with a case of dengue fever, it’s often very similar to the worst case of influenza you can imagine,” said mosquito researcher Connor McMeniman of Johns Hopkins University. There’s a reason it’s commonly known as “break-bone fever,” McMeniman said.
Traditional methods of preventing mosquito-borne diseases have not been as effective against dengue.
Aedes aegypti mosquitoes, which most commonly spread dengue, have been resistant to insecticides, with fleeting results even in the best case scenario. And because the dengue virus comes in four different forms, it is difficult to control it through vaccines.
Aedes aegypti mosquitoes are also a challenging enemy because they are most active during the day – that is, when they bite – so mosquito nets are not much help against them. Because these mosquitoes thrive in warm and wet environments and dense cities, climate change and urbanization are expected to make the fight against dengue even more difficult.
“We need better tools,” said Raman Velayudhan, a researcher with WHO’s Global Neglected Tropical Diseases Programme. “Wolbachia is definitely a long-term, sustainable solution.”
Velayudhan and other WHO experts plan to publish a recommendation early this month to promote further testing of the Wolbachia strategy in other parts of the world.
Scientists surprised by bacteria!
The Wolbachia strategy has been decades in the making.
The bacteria are naturally present in about 60% of insect species, not just the Aedes aegypti mosquito.
“We worked on this for years,” said O’Neill, 61, who, with the help of his students in Australia, finally figured out how to inject bacteria from fruit flies into Aedes aegypti mosquito embryos using microscopic glass needles. be transferred.
About 40 years ago, scientists aimed to use Wolbachia in a different way: to reduce mosquito populations. Because male mosquitoes carrying the bacteria can only mate with female mosquitoes that carry the bacteria, scientists will release infected male mosquitoes into the wild to breed with uninfected females, whose eggs will not hatch.
But along the way, O’Neill’s team made a surprising discovery: Mosquitoes carrying Wolbachia don’t spread dengue — or other related diseases, including yellow fever, Zika and chikungunya.
And as infected females pass Wolbachia to their offspring, they will eventually “replace” local mosquito populations that carry the virus-blocking bacteria.
The replacement strategy requires a major change in thinking about mosquito control, said Oliver Brady, an epidemiologist at the London School of Hygiene and Tropical Medicine.
“Everything in the past has been about killing mosquitoes or at least preventing mosquitoes from biting humans,” Brady said.
Since O’Neill’s lab first tested the replacement strategy in Australia in 2011, the World Mosquito Program has run trials affecting 11 million people in 14 countries, including Brazil, Mexico, Colombia, Fiji and Vietnam.
The results are promising. In 2019, a large-scale field trial in Indonesia showed a 76% drop in reported dengue cases after the release of Wolbachia-infected mosquitoes.
Still, questions remain whether the replacement strategy will be effective and cost-effective on a global scale, O’Neill said. The three-year Tegucigalpa trial will cost $900,000, or about the $10 per person that Doctors Without Borders hopes to cover.
Scientists aren’t yet sure how exactly Wolbachia prevents viral transmission. And it’s not clear whether the bacteria will work equally well against all strains of the virus, or whether some strains could become resistant over time, said mosquito researcher Bobby Reiner of the University of Washington.
“This is certainly not a one-time solution that is guaranteed to last forever,” Rainer said.
Special mosquitoes bred in Colombia
Many of the world’s mosquitoes infected with Wolbachia were born in a warehouse in Medellín, Colombia, where the World Mosquito Program runs a factory that breeds 30 million mosquitoes per week.
Edgard Boquin, one of the Honduras project leaders who works for Doctors Without Borders, said the factory imports dried mosquito eggs from different parts of the world to ensure that the specially bred mosquitoes will hatch. The local population will have similar traits, including resistance to pesticides.
Dry eggs are placed in water with powdered food. Once they hatch from the egg, they are allowed to reproduce with the “mother colony” – a lineage that carries Wolbachia and is composed of more females than males.
There is a constant buzz in the room where insects mate in cube-shaped cages made of mosquito netting. Caregivers ensure they receive the best diet: males receive sweetened water, while females “harvest” pouches of human blood kept at 97 °F (37 °C).
“We have perfect conditions,” said Marlene Salazar, the factory’s coordinator.
Once workers confirm that the new mosquitoes carry Wolbachia, their eggs are dried and packed into pill-like capsules so they can be sent to hatch sites.
Doctors sought help in Honduras
A Doctors Without Borders team in Honduras recently went door-to-door in a mountainous area of Tegucigalpa to enlist residents’ help in hatching mosquito eggs hatched in a Medellin factory.
In half a dozen houses, they received permission to hang glass jars filled with water and capsules filled with mosquito eggs on tree branches. After about 10 days, the mosquitoes will hatch and fly away.
That same day, a dozen young workers from Doctors Without Borders rode through northern Tegucigalpa on motorcycles carrying jars of dengue-fighting mosquitoes already hatched and, at designated locations, released thousands of them into the air.
Because community participation is key to the program’s success, doctors and volunteers have spent the past six months educating neighborhood leaders, including influential gang members, so they can obtain permission to operate in the areas under their control.
Some of the most common questions from the community were about whether Wolbachia would harm people or the environment. Activists explained that any bites from particular mosquitoes or their offspring are harmless.
Maria Fernanda Marin, a 19-year-old student, works for Doctors Without Borders in a facility where Wolbachia-infected mosquitoes are prepared for eventual release. She proudly shows the neighbors a picture of her severed hand to help her win their trust.
Lourdes Betancourt, 63, another volunteer on the Doctors Without Borders team, was skeptical of the new strategy at first. But Betancourt — who has been sick with dengue several times — now encourages her neighbors to let “good mosquitoes” breed in their yards.
“I tell people not to be afraid, it’s nothing bad, have faith,” Betancourt said. “They will bite you, but you won’t get dengue.”