“An increasingly important factor is the mobility of people,” says Paul Reiter, a medical entomologist at the Pasteur Institute in Paris. “The biggest vector of chikungunya is the Boeing and the Airbus.”
“We have seen numerous individuals who have traveled to places where chikungunya transmission is going on who have been unfortunate enough to pick up the virus and then travel back to the United States,” says Staples. The CDC has already noted more than 100 cases of chikungunya since 2006, all brought back from overseas travel.
The virus hasn’t gotten into the local mosquito populations so far, but recent outbreaks around the world show how easily the virus, with its new mosquito host, can infiltrate a new country.
In August 2007, a man returned from India to the small Italian town of Castiglione di Cervia, 115 miles south of Venice along the Adriatic Sea. Shortly after his arrival, he visited the doctor with a high fever, headache, rash, and joint pain. By the time public health authorities realized that the man was ill with chikungunya, more than 100 other people in Castiglione had developed the virus. Part of what drove this outbreak, Fonseca says, was the Italian habit of long, leisurely outdoor meals. This tradition gave the Asian tiger mosquito, which had arrived in 1990, ample opportunity to bite people.
Ongoing outbreaks of chikungunya are occurring in India, Southeast Asia, New Caledonia and Brazzaville, the capital of the Congo Republic, where more than 1,000 cases had been reported in early June.
The best, and most disturbing, lesson for how a possible chikungunya outbreak could strike the United States is the West Nile virus, Staples says. Both viruses are transmitted by mosquitoes. Both were relatively obscure viruses from Africa that caused massive outbreaks when they reached other locations. And both cause serious and potentially deadly symptoms. West Nile virus first arrived in New York in 1999, and that one introduction was enough to permanently change the disease landscape of North America. West Nile virus has spread across the continental United States and is now endemic, meaning that the virus is transmitted within the population year after year.
The viruses have some important differences, however. West Nile infects birds as well as humans, and once the virus entered the bird population, halting its spread became impossible. Chikungunya would be confined to humans (it infects other primates in Africa but is not known to infect any North American animals) which gives epidemiologists an advantage in fighting the disease.
Researchers have begun to develop vaccines and treatments for chikungunya. One vaccine candidate is currently being tested, and researchers in France and Singapore have identified potential immunological treatments to help reduce the severity of infections. Epidemiologists at the CDC and the World Health Organization are working hard to make sure that chikungunya doesn’t spread any farther, but with no treatments yet and no ability to stop the Asian tiger mosquito, their goals might be nearly impossible to achieve. All we can do, public health officials and travelers alike, is watch and wait.
Carrie Arnold has been following the chikungunya outbreak since 2008, when she wrote about it as her thesis topic at Johns Hopkins University. She lives outside Norfolk, Virginia, and is working on her third book.