Chikungunya fever, for which there is no known cure or vaccine, has infected more than 150,000 people across Mauritius, Seychelles and Reunion, hurting productivity and threatening the tourist industry.
Though the fever is not considered deadly, it can weaken the resistance of those it affects, leaving them more susceptible to death from other diseases. The fever is very painful and often forces sufferers into a stooped posture before they recover.
Leonard Tapsoba, the World Health Organisation's country director, said one case of chikungunya had been confirmed in eastern Madagascar where there was already an outbreak of dengue fever - a less-severe mosquito-borne illness.
"There is one case of chikungunya confirmed in samples that have been tested from Madagascar," he said. "The others were dengue. What is certain is that there is an outbreak of dengue fever."
Health officials are concerned about the impact that a chikungunya outbreak could have on one of the world's poorest countries, where three-quarters of its 17 million people live on less than a dollar a day.