Until 2013, chikungunya was an Old World disease, writes Maggie Koerth-Baker. Not any more.

When the name of a virus translates as "to become contorted" (as in, with joint pain) you know it is not something you want to catch. Unfortunately, your chances of encountering chikungunya are increasing.

Chikungunya is a mosquito-borne illness that has no cure. On the plus side, it's unlikely to kill you. On the downside, if you catch it, treatment is about easing the discomfort of symptoms and waiting for it to pass. This is a virus that exists on a spectrum. Lots of people might have it and never realize they have contracted something serious. But mosquitoes bite those people, contract the virus, and then spread it to somebody else who could end up with debilitating pain lasting for weeks.

Until 2013, chikungunya was an Old World disease. Then, last December, experts identified the first cases in the Caribbean that were known to have been acquired there (as opposed to cases diagnosed in the Caribbean in people who had recently traveled to chikungunya-affected countries). You'll note that that is not very long ago. Since then, the Caribbean has seen more than 100,000 suspected cases (only a handful verified in a lab, so they could have been dengue, but there's reason to think not) and the disease is starting to make landfall in the US. Again, it's in the form of victims who acquired the virus in another place, this time the Caribbean, and are only diagnosed after returning home. But, as with the Caribbean, that's the first step in introducing the virus to our native mosquito populations.

What's more, all of this is just one part of a larger, relatively sudden, globalization of chikungunya. The virus has been known since the 1950s, but because it was largely non-lethal and largely confined to developing countries in Africa and Asia, the Western medical establishment didn't much care about it until 10 years ago. That's when chikungunya showed up on the French-controlled island of La Réunion, in the Indian Ocean, where it infected 40 percent of the population. Since then, it's exploded in parts of Asia where it hadn't been seen in decades (and other parts where it hadn't been seen at all), reached Australia and Taiwan, and made landfall in Italy and France. And all of that was before the outbreak in the Caribbean.

So what changed? The sudden spread of chikungunya seems to be related to two things. First, the virus itself mutated. The strain that's spreading around the world is different from the one that hung around sub-Saharan Africa. Specifically, it's much more efficient at replicating itself in the guts of mosquitoes. That seems to have increased both its ability to move into new places and its ability to be carried by different species of mosquito.

That last bit is important because it ties in with the other reason chikungunya is suddenly global. The mutated virus is following in the footsteps of an invasive mosquito that spread around the world years before. The Asian tiger mosquitoAedes albopictus — has a wider range than other species that can carry chikungunya. And that mutation in the chikungunya virus made it a much better vector for chikungunya than it was in the past. Bring the mutation and the invasive mosquito together, and you have a recipe for the global spread of a virus — including its spread into the United States.

While the current cases of chikungunya here are ones that people acquired while visiting the Caribbean, it's very likely that that will change. The Asian tiger mosquito has been in this country since 1985 and its range extends from Texas to the Carolinas, and as far north as New Jersey. The stage is already set. It's just going to take enough infected people coming back to the US and being bitten once they're here.

At the Superbug blog, Maryn McKenna has a good article about how quickly chikungunya is spreading in the Caribbean and the rapid increase in diagnosed cases here. It ends with a pleasant assurance from the Tennessee state health commissioner that it is only a matter of time before chikungunya is something you can acquire on US soil.

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