Strains of a bacterium commonly found in fruit flies can prevent the Aedes aegypti mosquito from transmitting the virus that causes dengue fever, researchers have found. Their discovery could lead to a more effective way to control dengue worldwide.
North Carolina State University mathematical biologist Dr. Alun Lloyd is part of the Eliminate Dengue program, a research consortium that includes scientists from Australia and the United States. The program aims to stop the Aedes aegypti mosquito from transmitting dengue virus between humans by introducing a naturally occurring bacterium called Wolbachia – which is not harmful to humans – into the existing wild mosquito population.
"When mosquitoes carrying Wolbachia are introduced into the environment, they mate with wild mosquitoes, and pass Wolbachia to their offspring until all Aedes aegypti mosquitoes have Wolbachia. If mosquitoes don't become infected with dengue, they cannot transmit the virus to people," Lloyd explains.
The researchers infected female mosquitos with two different strains of Wolbachia bacteria – known as wMel and wMelPop-CLA – and did experiments to test the ability of the strains to spread throughout mosquito populations in controlled conditions. They found that both strains seemed to block the transmission of dengue virus, and that the wMel strain was able to infect almost the entire test population in just a few generations.
Lloyd contributed mathematical models that helped the researchers interpret the results of their experiments, which pointed to the wMel strain as the bacterium with the greater potential for suppressing the spread of dengue virus.
"This is a simple, non-chemical, non-harmful way to reduce the threat of dengue to humans," Lloyd says. "It could have a transformative effect on the health of literally millions of people worldwide."
The research is published in the Aug. 25 edition of Nature. Dr. Scott O'Neill from Monash University, Melbourne, Australia leads the program. Primary investigators in Australia and co-authors on the papers include Dr. Ary Hoffmann from the University of Melbourne, Dr. Scott Ritchie and Dr. Petrina Johnson from James Cook University and Dr. Thomas Walker from the University of Queensland. The Foundation for the National Institutes of Health through the Grand Challenges in Global Health Initiative of the Bill and Melinda Gates Foundation provided funding.
The Department of Mathematics is part of NC State's College of Physical and Mathematical Sciences.
Note to editors: An abstract of the paper follows.
"A non-virulent Wolbachia infection blocks dengue transmission and rapidly invades Aedes aegypti populations"
Authors: T. Walker, University of Queensland, Australia; P.H. Johnson, James Cook University, Cairns, Queensland, Australia; A. Lloyd, North Carolina State University, et al.
Published: Aug. 25, 2011 in Nature
Abstract: Dengue fever is the most important mosquito-borne viral disease of humans with more than 50 million cases estimated annually in over 100 countries worldwide. Disturbingly, the geographic range of dengue is currently expanding and the severity of outbreaks is increasing. Control options for dengue are very limited and currently focus on reducing population abundance of the major mosquito vector, Aedes aegypti. These strategies are failing to reduce dengue incidence in tropical communities and there is an urgent need for effective alternatives. It has been proposed that endosymbiotic bacterial Wolbachia infections of insects might be used in novel strategies for dengue control. For example, the wMelPop-CLA Wolbachia strain reduces the lifespan of adult Aedes aegypti mosquitoes in stably transinfected lines. This life-shortening phenotype was predicted to reduce the potential for dengue transmission. The recent discovery that several Wolbachia infections, including wMelPop-CLA, can also directly influence the susceptibility of insects to infection with a range of insect and human pathogens has dramatically changed the potential for Wolbachia infections to control human diseases. Here we describe the successful transinfection of Aedes aegypti with the avirulent wMel strain of Wolbachia, which induces the reproductive phenotype cytoplasmic incompatibility with minimal apparent fitness costs and high maternal transmission, providing optimal phenotypic effects for invasion. Under semi-field conditions, the wMel strain increased from an initial starting frequency of 0.65 to near fixation within a few generations; invading Aedes aegypti populations at an accelerated rate relative to trials with the wMelPop-CLA strain. We also show that wMel and wMelPop-CLA strains block transmission of dengue serotype 2 (DENV-2) in Aedes aegypti.
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