Given a choice between dengue fever or another mosquito-borne disease called chikungunya fever, choose dengue every time. Neither has an available vaccine or treatment, but chikungunya (pronounced chik-un-GUHN-ya) is far more severe – it literally means “that which bends up” because patients are often stooped over from debilitating joint pain. If you’re a resident of the Caribbean island of St. Martin (or lucky enough to be traveling there for the holidays) you are now at risk of both. The island, roughly the size of Manhattan and located some 300 kilometers east of Puerto Rico, has the first confirmed outbreak of chikungunya in the Western Hemisphere. As of late last week there were 26 laboratory-confirmed cases of the disease on the island, with over 100 total cases suspected. Chikungunya has been found in both the French and Dutch parts of the island. It has also spread to the nearby island of Martinque, which has two laboratory-confirmed cases. And representatives of the U.S. Centers for Disease Control and Prevention (CDC) say that the virus could spread to other Caribbean islands and the surrounding mainland areas in the coming months or years. The outbreak in such a prime spot for tourism also heightens the risk of a future outbreak in the United States. Disease experts are not sure how the virus got to St. Martin. The patients in St. Martin had not recently left the island, so presumably the virus was locally acquired. The top theory is that a traveler contracted the disease in another region of the world and was then bitten by a local mosquito in St. Martin that went on to infect other people. Another, less likely option is that an infected mosquito traveled to St. Martin, perhaps as a stowaway on a ship or plane. “We know the area has the right mosquitoes to potentially transmit chikungunya, so you could question, ‘why not before now’ or ‘why not a year from now,’” says Erin Staples, an arboviral diseases expert at the CDC. “This just happened to be the right combination of factors,” such as ongoing transmission in other areas and an unlucky bite. All it takes to spread chikungunya is for a female mosquito to feed twice, first engorging herself on an infected person’s blood and then later biting someone else. But unlike dengue, which is already common in the region and is transmitted via the same Aedes aegypti and Aedes albopictus mosquitoes, locals do not have any immune protection for chikungunya since it’s new to the area—meaning it could spread quickly. Based on the geographic creep of the disease, the CDC and the Pan American Health Organization have expected the virus to crop up in the Americas for the past several years. In 2011 they issued guidelines for how to respond to chikungunya in the region. And with some 9 million U.S. residents traveling to the Caribbean each year, the CDC says that chikungunya could occur more frequently in the U.S. in the future. But it’s not a sure thing. Despite 109 laboratory-confirmed cases of the disease in the United States between 1995 and 2009—all imported cases from travelers and most of them occurring in recent years—none sparked an outbreak of the disease at home. The U.S. does host mosquitoes that can spread the disease (and has recently seen an uptick in dengue), but much of the country is too cold for mosquitoes to have much activity right now. The disease has been a problem in Africa and southern Asia for decades. Fatalities are rare but its symptoms include high fever and severe joint pain. And although the symptoms usually clear up in about a week, they can cause long-term joint pain in some people. The word “chikungunya,” for suffering that makes infected people double over, comes from the Makonde language of southeast Africa. Usually symptoms of chikungunya begin within two weeks of being bitten by an infected mosquito. Once a mosquito dines on infected blood, the virus needs to incubate for about 10 days inside the insect’s body before it can spread the disease to another human with its bite. Since no vaccines or therapies (except pain killers) exist for patients with chikungunya, public health experts instead look to limit mosquito populations in places close to humans. (The strategy is imperfect at best—mosquito control efforts in the Americas have not been very successful, as dengue continues to ravage many areas.) Mosquito nets, a common tool to help prevent malaria, are less useful against chikungunya-carrying mosquitoes, as these insects are aggressive daytime biters. Bed nets are not completely useless, though. One way to help combat the spread of the disease would be for infected patients to convalesce under nets during the day to avoid being bitten, says Staples. The CDC has also issued a travel health notice advising travelers to St. Martin to use insect repellant and wear long sleeves and pants. Making matters worse, it is possible for a person to acquire both dengue and chikungunya at the same time. Although there are documented cases of individuals with both, Staples says it remains unclear whether the same mosquito bite caused both or those individuals acquired separate mosquito bites. The good news is that scientists believe that once a person is exposed to chikungunya, the bite provides natural lifelong immunity against reinfection. 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