Bacteria-Infected Mosquitoes Are Resistant To Malaria, Study

Scientists report the establishment of a strain of malaria-resistant mosquitoes.

AsianScientist (May 14, 2013) – Scientists have established a stable and inheritable Wolbachia bacterial infection in malaria-transmitting mosquitoes to make the insects more resistant to the malaria parasite.

“Wolbachia-based malaria control strategy has been discussed for the last two decades. Our work is the first to demonstrate Wolbachia can be stably established in a key malaria vector, the mosquito species Anopheles stephensi, which opens the door to use Wolbachia for malaria control,” said study leader Dr. Zhiyong Xi, Assistant Professor at Michigan State University (MSU) and Director of the Sun Yat-sen University-MSU Joint Center of Vector Control for Tropical Diseases in China.

Members of the Wolbachia genus of bacteria naturally infect many insect species, including mosquitoes, wasps, and ants, but not the major mosquito vectors of malaria or dengue. Several research groups, including Dr. Xi’s, have already generated lines of dengue-transmitting Aedes mosquitoes that are stably infected with Wolbachia, and found that certain strains of the bacteria render the insects resistant to a wide variety of mosquito-borne human pathogens, including dengue virus, chikungunya virus, and filarial nematodes. Wolbachia-infected Aedes mosquitoes have already been released into the wild in Queensland, Australia in field trials aimed at interrupting dengue transmission.

Despite two decades of trying, however, no one had been able to stably infect malaria-transmitting Anopheles mosquitoes with Wolbachia. Only transient infections had been achieved, and these made mosquitoes more resistant to human malaria parasites – tantalizing evidence for the potential of Wolbachia as a malaria control agent, if only a stable and inheritable bacteria-mosquito association could be obtained.

In this study, published in the journal Science, MSU research associate Dr. Guowu Bian injected hundreds of mosquito embryos with Wolbachia. Only one of these developed into a female carrying a Wolbachia infection. This female was used to generate a mosquito line which, at the time of publication, has remained infected for 34 generations.

To be used effectively as a malaria control strategy, Wolbachia must be able to drive itself quickly through an insect population. To assess this, the researchers seeded cages of uninfected mosquitoes with a small percentage of Wolbachia-infected ones. They found that 100 percent of the mosquitoes became infected within eight generations, from a starting frequency of only five percent.

Next, the researchers infected the Wolbachia-carrying mosquitoes with Plasmodium falciparum, the most deadly human malaria parasite. Compared to mosquitoes that did not harbor the bacteria, Wolbachia-infected mosquitoes had significantly fewer parasites in their midguts, where the parasite develops, and in their salivary glands, where parasites are secreted into mosquito saliva during transmission.

The mechanisms behind the extraordinary ability of Wolbachia to inhibit pathogen development in insects are not fully understood. One hypothesis is that the bacteria activate the mosquito’s immune response, priming it to act more quickly and robustly against invading pathogens. In support of this, the researchers found higher levels of reactive oxygen species (ROS), important mosquito defense molecules that are effective killers of the malaria parasite, in Wolbachia-infected mosquitoes.

Anopheles stephensi, the mosquito that the researchers worked with, is the major malaria vector in South Asia and the Middle East. In sub-Saharan Africa, however, where the burden of the disease is by far the largest in the world, the parasite is transmitted by Anopheles gambiae, a mosquito species which still eludes attempts at stable Wolbachia infection.

To tackle this, Dr. Xi says, “We are moving to Anopheles gambiae considering the importance of this mosquito in transmitting malaria in Africa. We think the Wolbachia adapted in An. stephensi could provide a unique donor option for this work and may facilitate establishment of a stable Wolbachia in the other Anopheles mosquitoes as well.”

The article can be found at: Bian et al. (2013) Wolbachia invades Anopheles stephensi populations and induces refractoriness to Plasmodium infection.


Source: Michigan State University.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Shuzhen received a PhD degree from the Johns Hopkins Bloomberg School of Public Health, USA, where she studied the immune response of mosquito vectors to dengue virus.

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