Travelers to the Caribbean island of St. Martin are being warned that, for the first time, a mosquito-borne illness called chikungunya has been detected there. Chikungunya causes fever, headache and severe joint pains that last about a week, and sometimes longer. There is no vaccine, so the only prevention is to avoid mosquito bites, the Centers for Disease Control and Prevention said on Wednesday. So far, 10 cases have been confirmed in St. Martin, and health officials believe the virus is present in the island’s mosquito populations — the first time that it appears to have established itself anywhere in the Western Hemisphere. Previous outbreaks have occurred in Africa, Asia and parts of Europe.