A genetic mutation in the chikungunya virus may have led to a massive outbreak of the deadly tropical disease on an island in the Indian Ocean in 2005 and 2006,according to the findings of research published Thursday.

The mutation made it easier for the virus to reproduce inside the mosquitoes that transmit it to humans, said the findings published in the online journal of PLoS.

Chikungunya kills about one in every 1,000 infected people. The outbreak at La Reunion, a French island 700 km east of Madagascar, infected at least a third of the 800,000 inhabitants there.

From analyzing various versions of viruses isolated from patients, researchers knew that chikungunya had undergone a mutation early in the epidemic. They suspected that the change had facilitated the spread, but this was mostly speculation. Now they have the proof.

The virus was spread mainly by the Asian tiger mosquito, A. albopictus. Scientists at the Pasteur Institute in Paris bred populations of the A. albopictus mosquitoes from La Reunion and nearby Mayotte and fed them a blood meal spiked with mutated and non-mutated versions of the virus.

The scientists then ground up the insect bodies at different time intervals after feeding and measured the amount of virus inside.

In mosquitoes fed with the mutated version, the virus occurred in quantities almost 100 times higher than in those without.

This mutated virus was also better able to pass the wall of a mosquito's midgut and make its way to the salivary glands, from where it could pass to a new victim with the insect's next bite.

Apparently, the mutation made the virus a much better fit for La Reunion's Asian tiger mosquito population and thus made the epidemic soar.
 
(Xinhua News Agency November 16, 2007)