September 01, 2011

The Trojan Mosquito

Dengue fever is an important viral disease, affecting around 50 million people a year. It’s a vector-borne disease, meaning that people cannot directly infect each other. Rather, mosquitos, most commonly Aedes aegypti, transmit the infection. First, a female mosquito must first bite an infected person, then the dengue virus multiplies within the mosquito (a process taking 7-14 days), then the infected mosquito bites an uninfected person, and transmits dengue through its saliva. Aedes mosquitos live near humans, and tend to bite during the day, making bednet-based strategies much less effective at reducing transmission of dengue. Despite attempts to control Aedes mosquitoes, dengue is not being effectively controlled in tropical countries, and dengue’s range (and the severity of outbreaks) is increasing.

Recent research published in Nature suggests a promising new approach to controlling dengue. Briefly, the researchers have managed to infect mosquitos with a strain of Wolbachia (a type of bacteria) that stops dengue from reproducing in those mosquitos. Better still, the strain seems able to infect over 90% of wild mosquitos when infected mosquitos are released in field conditions.

How does this work? Well, Wolbachia pipientis has a neat mechanism for invading insect populations called cytoplasmic incompatibility. It is maternally transmitted (i.e. infected females pass it to their offspring), but if an infected male mates with an uninfected female, then the resulting embryos die. An infected female, however, can successfully reproduce with both infected and uninfected males. This means that infected females have a reproductive advantage over uninfected females, which helps Wolbachia to invade the mosquito population. Nonetheless, many Wolbachia strains are pathogenic in mosquitos, making the mosquitos shorter-lived or their eggs less viable. Walker and colleagues have developed a strain (derived from one found in Drosophila, the fruit fly beloved of lab biologists) that has relatively little deleterious effects whilst still blocking dengue transmission, and shown that it can invade caged populations of Aedes aegypti. In a related article published in the same edition of Nature, Hoffmann and colleagues take this approach further, and release infected mosquitos into two areas of north-eastern Australia. They were able to infect the vast majority of the wild population of A aegtypi with Wolbachia by 9 roughly-weekly releases of infected mosquitos.

This is exciting stuff, as it means that populations of mosquitos infected with a strain of Wolbachia that blocks transmission of dengue can be established reasonably straightforwardly, with obvious implications for the control of dengue. The next step, of course, is to show that this approach can actually reduce the incidence of disease in the field…

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