In 2006, an outbreak of chikungunya fever — an arthralgic disease caused by a mosquito-borne alphavirus — swept over a number of islands in the Indian Ocean (the Comoros, Mauritius, the Seychelles, Madagascar, Mayotte, and Reunion). In Reunion, which has a population of 770,000, there were 265,000 clinical cases (an incidence of 34%), and the disease was implicated in 237 deaths (about 1 per 1000 clinical cases); a recent report by Reunion health authorities indicated that the seroprevalence was 35%, with very few asymptomatic cases. The epidemic had started with outbreaks in Kenya in 2004 and the Comoros early in . . .

Chikungunya virus (CHIKV) emerged in Indian Ocean islands in 2005 and is causing an ongoing outbreak that involves >260,000 patients, including travelers returning home from these islands. We investigated cases in 4 patients returning from Mayotte and Reunion Islands with CHIKV infection and a nurse infected in metropolitan France after direct contact with the blood of a traveler. Four patients had tenosynovitis and pain at wrist pressure, and 1 had life-threatening manifestations. Four CHIKV strains were isolated, including 1 from the patient with the autochthonous case. The complete genomic sequence identified a new CHIKV variant emerging from the East/central African evolutionary lineage. Aedes albopictus, the implicated vector of CHIKV in Indian Ocean islands, has dispersed worldwide in recent decades. High viral loads in patients returning from Indian Ocean islands to countries where Ae. albopictus is prevalent may be a source of epidemics.

Since 2004, several million indigenous cases of Chikungunya virus disease occurred in Africa, the Indian Ocean, India, Asia and, recently, Europe. The virus, usually transmitted by Aedes aegypti mosquitoes, has now repeatedly been associated with a new vector, Ae. Albopictus. Analysis of full-length viral sequences reveals three independent events of virus exposure to Ae. Albopictus, each followed by the acquisition of a single adaptive mutation providing selective advantage for transmission by this mosquito. This disconcerting and current unique example of "evolutionary convergence" occurring in nature illustrates rapid pathogen adaptation to ecological perturbation, driven directly as a consequence of human activities.