Infecting mosquitos with bacteria could block malaria

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Getting Wolbachia bacteria to grow in mosquitos may help human health.

Mosquito bites kill an estimated 1-2 million people every year. It is not the mosquitoes’ fault, though—it's the pathogens they transmit that are lethal, not the bites themselves. Nets and insecticides can help, but they can also be costly, logistically difficult to distribute, and not particularly green. So alternative strategies to prevent disease transmission are needed.

Wolbachia are bacteria that reside in insect cells and have a very complicated relationship with their hosts. They can render mosquitoes resistant to certain pathogens, and they can reduce mosquitoes' lifespans, which is significant because it is often the older mosquitoes that transmit the pathogens that make us sick. Wolbachia infect up to 76 percent of the 2-5 million insect species on Earth—but not, of course, the mosquito species that carry dengue fever or malaria. That would be far too convenient.

So researchers have been trying to infect disease-carrying mosquitoes with Wolbachia in the lab and then let these infected mosquitoes out into the wild to mate with and infect disease-carrying strains in order to reduce disease transmission. This has in fact already happened in northeastern Australia, where researchers spent four years maintaining Wolbachia in mosquito cells in the lab before letting infected mosquitoes loose in January 2011 to infect wild Aedes aegypti, the mosquitoes that transmit dengue fever. The trial is going so well that it is being repeated in Vietnam.

But the mosquitoes that transmit malaria in the Middle East and South Asia, Anopheles stephensi, have resisted attempts to infect them with Wolbachia for the past twenty years or so. Finally, researchers have succeeded in not only infecting A. stephensi with Wolbachia but in maintaining the infection over 34 generations.

In each generation, each infected female passed the bacteria on to 100 percent of her offspring. Infected mosquitoes had an almost four-fold reduction in Plasmodium falciparum, one of the parasites that causes malaria. When the researchers seeded cages of uninfected mosquitoes with only five percent infected females, 100 percent of the mosquitoes were infected within eight generations. However, they also had to put in twice as many infected males as there were uninfected ones to "suppress the effective mating" of uninfected females, which could prove problematic if this strategy were to be implemented on a large scale in the wild.

There is no evidence that Wolbachia is transferred to humans via mosquito bites or to mosquito predators like geckos and spiders. The incidence of mosquito-borne diseases is increasing due to the ease of air travel and the rapid urbanization of developing countries; infecting mosquitoes to make them resistant to pathogens might be a relatively cheap and green addition to the public health measures currently used to deal with them.

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