The study in PLoS Medicine suggests that the chikungunya virus has mutated in a way that makes it better at infecting the mosquitoes that spread the virus to people.

Last year, the virus began to infect people living in the Comoros Islands in the Indian Ocean before spreading to the nearby islands of Madagascar, Mauritius, Mayotte, Réunion and the Seychelles.

The virus causes fever and joint pain but is not usually fatal. It has infected one-third of Réunion's population of 790,000 and, since December 2005, about 180,000 people in southern India.

The authors of the new study, led by Sylvain Brisse of France's Pasteur Institute, identified the entire genetic sequence of virus samples from six patients in Réunion and the Seychelles.

They also sequenced a viral gene called E1 from samples taken from an additional 127 patients in Madagascar, Mauritius, Mayotte, Réunion and the Seychelles.

The results show that the chikungunya outbreak began with a strain related to East-African forms of the virus that then developed into several distinct variants.

The strain now dominant differs genetically from those involved in earlier outbreaks and this, say the researchers, could explain why the virus has become more aggressive.

In particular, they say, two changes to the structure of E1 could make the virus more likely to enter mosquito cells and replicate after the insect has fed on the blood of an infected person.

The virus was first recorded in 1952 in Tanzania and gets its name from a Swahili word meaning 'that which bends up' — a reference to the stooped posture of infected people whose joints swell up.

Since then periodic outbreaks have hit parts of Africa and Asia. The virus is not usually fatal, but deaths have been reported during the current outbreak, raising concerns that the virus's genetic changes have also made it more deadly.

Experiments are now underway to test which of these features might be responsible for the apparent increase in the virus's ability to infect people and cause disease.

Link to full paper in PLoS Medicine