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 goes against everything they have learned.
A dozen people cheered last month as Tegucigalpa resident Hector Enriquez held a glass jar filled with mosquitoes above his head, and then freed 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 in the Honduran capital.
The mosquitoes Enriquez unleashed in his El Manchen neighborhood — an area rife with dengue — were bred by scientists to carry bacteria called Wolbachia that interrupt transmission of the disease. When these mosquitoes reproduce, they pass the bacteria to their offspring, reducing future outbreaks.
Photo: AP
This emerging strategy for battling dengue was pioneered over the last decade by the nonprofit World Mosquito Program, and it is being tested in more than a dozen countries. With more than half the world’s population at risk of contracting dengue, the WHO is paying close attention to the mosquito releases in Honduras, and elsewhere, and it is poised to promote the strategy globally.
In Honduras, where 10,000 people are known to be sickened by dengue each year, Doctors Without Borders is partnering with the mosquito program over the next six months to release close to 9 million mosquitoes carrying the Wolbachia bacteria.
“There is a desperate need for new approaches,” said Scott O’Neill, founder of the mosquito program.
Scientists have made great strides in recent decades in reducing the threat of infectious diseases, including mosquito-borne viruses like malaria, but dengue is the exception: Its rate of infection keeps going up.
Models estimate that about 400 million people across in 130 countries are infected each year with dengue. Mortality rates from dengue are low — an estimated 40,000 people die each year from it — 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 akin to getting the worst case of influenza you can imagine,” said Conor McMeniman, a mosquito researcher at Johns Hopkins University.
It is commonly known as “breakbone fever” for a reason, McMeniman said.
Traditional methods of preventing mosquito-borne illnesses have not been nearly as effective against dengue.
The Aedes aegypti mosquitoes that most commonly spread dengue have been resistant to insecticides, which have fleeting results even in the best-case scenario. Because dengue virus comes in four different forms, it is harder to control through vaccines.
Aedes aegypti mosquitoes are also a challenging foe because they are most active during the day — meaning that is when they bite — so bed nets are not much help against them.
As these mosquitoes thrive in warm and wet environments, and in dense cities, climate change and urbanization are expected to make the fight against dengue even harder.
“We need better tools,” said Raman Velayudhan, a researcher from the WHO’s Global Neglected Tropical Diseases Program. “Wolbachia is definitely a long-term, sustainable solution.”
Velayudhan and other experts from the WHO plan to publish a recommendation as early as this month to promote further testing of the Wolbachia strategy in other parts of the world.
The Wolbachia strategy has been decades in the making.
The bacteria exist naturally in about 60 percent of insect species, just not in the Aedes aegypti mosquito.
“We worked for years on this,” said O’Neill, 61, who with help from his students in Australia eventually figured out how to transfer the bacteria from fruit flies into Aedes aegypti mosquito embryos by using microscopic glass needles.
About 40 years ago, scientists aimed to use Wolbachia in a different way: to drive down mosquito populations. Because male mosquitoes carrying the bacteria only produce offspring with females that also have it, scientists would release infected male mosquitoes into the wild to breed with uninfected females, whose eggs would not hatch.
However, along the way, O’Neill’s team made a surprising discovery: Mosquitoes carrying Wolbachia did not spread dengue — or other related diseases, including yellow fever, Zika and chikungunya.
Since infected females pass Wolbachia to their offspring, they would eventually “replace” a local mosquito population with one that carries the virus-blocking bacteria.
The replacement strategy has required a major shift 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 the very 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 across 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 percent drop in reported dengue cases after Wolbachia-infected mosquitoes were released.
Still, questions remain about whether the replacement strategy would be effective — and cost effective — on a global scale, O’Neill said.
The three-year Tegucigalpa trial would cost US$900,000, or roughly US$10 per person that Doctors Without Borders expects it to protect.
Scientists are not yet sure how Wolbachia actually blocks viral transmission.
It is not clear whether the bacteria would work equally well against all strains of the virus, or if some strains might become resistant over time, said Bobby Reiner, a mosquito researcher at the University of Washington. “It’s certainly not a one-and-done fix, forever guaranteed.”
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