Stopping transmission at the source
THERE HAVE been nearly 40,000 cases of dengue this year in the Philippines, and amid the ongoing distribution of traps for the virus-carrying mosquitoes and reminders to clean up areas where the insects might breed, work by Australian scientists offers a potential alternative method for countering the transmission of dengue fever.
In two papers published in the Aug. 25 issue of the journal Nature, researchers from the University of Melbourne and the University of Queensland first led an international team of scientists to develop a way of reducing the mosquito populations that transmit the dengue virus. They then conducted field tests in northeastern Australia, where dengue outbreaks occur, to see if their population control technique would work outside a laboratory.
Malaria researcher Jason Rasgon of the Johns Hopkins Bloomberg School of Public Health in the United States called the work “a groundbreaking first step” toward what will ultimately be “the elimination of the scourge of dengue fever” in a commentary on the studies that also appears in the same journal issue.
Bacteria-infected mosquitoes
Dengue affects some 50 million people in 100 countries, so in the first paper, researchers led by University of Melbourne geneticist and senior author Ary Hoffmann, tried to stop disease transmission by infecting the Aedes aegypti mosquito with bacteria that can suppress the virus. The Wolbachia bacteria used by the researchers can also shorten its host’s lifespan as well as suppress the virus. After two years of work, the team had developed whole populations of bacteria-infected mosquitoes who did not transmit the dengue virus.
Tests with uninfected mosquito populations showed that the bacteria also prevented infected male mosquitoes from successfully fathering offspring with uninfected females, while infected female mosquitoes could breed with either infected or uninfected males. The results told Hoffmann and his colleagues that if released into a public space, the bacteria-infected mosquitoes could be effective at reducing the transmission of dengue virus.
“Control options for dengue are very limited and currently focus on reducing population abundance of the major mosquito vector,” Hoffmann and his colleagues wrote in the first paper. “These strategies are failing to reduce dengue incidence in tropical communities and there is an urgent need for effective alternatives.”
Tested their theory
Earlier this year, with permission from the Australian government and community awareness campaigns, Hoffmann and his colleagues tested their theory by releasing these infected mosquitoes in Cairns, a coastal city in the state of Queensland that has had outbreaks of dengue fever during the wet season. The work is detailed in the second paper, on which Hoffman is first author.
Over two and a half months starting in January, the researchers released nearly 300,000 adult mosquitoes, all infected with the Wolbachia bacteria that reduced their ability to transmit dengue virus, at two locations with wild populations of mosquitoes. More than a month after the final batch of mosquitoes was released, the team found that in both areas, the mosquitoes infected with the bacteria made up over 90 percent of the population. The next step, the researchers said in their paper, is to see what impact altering the mosquito populations now has on future population figures, as well as the number of dengue cases.
“We have demonstrated that it is possible to produce Wolbachia-infected mosquito populations that can act as ‘nursery’ areas for future human-assisted collection and further dispersal of Wolbachia-infected mosquitoes, without the need to rear additional mosquitoes in an insectary,” the researchers concluded in their second paper. “This should provide a strategy for sustainable dengue control at low cost, with a relatively simple deployment system suitable for implementation in developing countries.”
E-mail the author at [email protected].
