[Above: A child cries as she is comforted by a relative inside a crowded ward filled with dengue patients at a hospital in Quezon City, Metro Manila]
HONG KONG - Injecting a bacteria into mosquitoes can block them from transmitting the dengue virus and help control the spread of a disease that kills 20,000 annually in more than 100 countries, scientists said.
In two papers published in the journal Nature on Thursday, researchers in Australia showed how female mosquitoes infected with the Wolbachia bacteria passed the bug easily to their offspring, making them all dengue-free.
They said such infected mosquitoes should be released into the wild, so that the spread of dengue to people may be reduced.
"The main feature we saw was their ability to reduce dengue transmission," said Professor Scott O'Neill, lead author and science faculty dean at Monash University. "It almost completely abolished dengue virus in the body of the mosquito."
In their experiment, O'Neill and colleagues injected the bacteria into more than 2,500 embryos of so-called Aedes aegypti mosquitoes that can spread dengue fever. After they hatched, they were treated to blood meals laced with the dengue virus, and none picked up the virus.
"The (Wolbachia) bacteria doesn't spread environmentally, it gets passed on from mother to children through the eggs," O'Neill told Reuters by telephone.
"When an infected male mates with an uninfected female, all her eggs die. That gives an indirect benefit to the females with Wolbachia because when they mate with infected males, their eggs hatch normally ... all their eggs have Wolbachia in them so Wolbachia gets more and more common with every generation."
O'Neill said there were two theories to explain why the Wolbachia was able to block the uptake of dengue.
One, that the Wolbachia boosts the mosquito's immune system and protects it from viruses like dengue. Two, the Wolbachia competes with dengue for food inside the mosquito, making it harder for the dengue virus to replicate.
INFECTING WILD POPULATIONS
More than 50 million people in over 100 countries fall sick and 20,000 die each year from dengue fever. There is no vaccine or specific treatment for the disease.
The only way of prevention is to control mosquito populations through eliminating breeding sites and insecticides.
O'Neill's team released nearly 299,000 infected mosquitoes in January at more than 370 sites in northeastern Australia, and the bacteria spread into the wild mosquito population successfully, with their offspring also infected over a three-month period.
The team is seeking approval to release such infected mosquitoes into dengue-endemic sites in Vietnam, Thailand, Indonesia and Brazil to see if it would reduce rates of dengue transmission in people.
"It is an alternative strategy for dengue control which could be low-cost and sustainable and suitable for deployment in large urban cities in developing world," O'Neill said.
But he added: "With any control (measure) over time, we might expect them to become less effective, like insecticides.
"We don't know how long that might take to occur. If it provides effective control for 20-30 years, that is still a very good step forward for dengue control."