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See Inside August 2011

Anatomy of a Mosquito-Borne Outbreak

Researchers untangle the genetics of how a crippling virus mutated and spread via mosquito from Africa to Asia



Alex Wild Corbis

Chikungunya is a scary-sounding virus with some scary symptoms: joint pain so excruciating that patients often can’t stand or even sit upright for months. The mosquito-borne virus got its start thousands of years ago in southeastern Africa, where it generally caused a slow but steady stream of cases. About 50 years ago a mild strain of the virus spread to Asia. Then, following a drought in Kenya in 2004, cases of chikungunya in Africa soared and spread eastward across the Indian Ocean, causing severe disease and affecting hundreds of thousands of people across Asia.

This new strain of chikungunya is apparently replacing the older, milder strains that previously circulated in Asia. But how? In a study published in the Proceedings of the National Academy of Sciences USA, researchers have figured out that as it traveled, the virus picked up a single mutation that allowed it to be transmitted much more efficiently by Asian mosquitoes. 

Scott Weaver, an infectious disease specialist at the University of Texas Medical Branch in Galveston, compared the preoutbreak African and Asian strains with the newer outbreak strain. He and his colleagues found two mutations that made it possible for the bug to hitch its fortunes to the Asian tiger mosquito (Aedes albopictus)—a ubiquitous insect that transmits the virus 100 times better than its previous host, the rarer A. aegypti. Some of the older African strains contained one of these variations, whereas the older Asian strains did not. That made it difficult for the strain that traveled to Asia 50 years ago to adapt to the Asian tiger mosquito. When the recent outbreak started in Africa, the virus needed only one mutation to adapt to the Asian tiger mosquito and become more virulent. “It’s an elegant and very convincing study,” says Peter Palese, a virologist at the Mount Sinai School of Medicine who was not involved in the research.

Understanding how genetic mutations increase or decrease a virus’s ability to be passed on by mosquitoes may one day help public health officials take action to prevent an outbreak before it starts, Weaver says. 

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