News of the WeekEpidemiology

Tropical Disease Follows Mosquitoes to Europe

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Science  14 Sep 2007:
Vol. 317, Issue 5844, pp. 1485
DOI: 10.1126/science.317.5844.1485a

For years, medical entomologists have worried that the astonishing ascent of the Asian tiger mosquito (Aedes albopictus) might bring not only nasty bites but also new public health surprises. After all, the mosquito is a known vector for more than 20 viral diseases.

They were right. This summer, the mosquito, which has become firmly established in southern Europe, has infected almost 200 people in Italy with chikungunya, a painful viral disease. It's the first known example of chikungunya transmission outside the tropics—and it's making scientists wonder whether A. albopictus has the potential to touch off much larger outbreaks in Europe and the United States.

Chikungunya is rarely fatal but can cause severe fevers, headaches, fatigue, nausea, and muscle and joint pains. People started falling ill in Castiglione di Cervia and Castiglione di Ravenna—two villages separated by a river in the province of Ravenna—in early July, says Antonio Cassone of the Istituto Superiore di Sanità (ISS), a national government lab in Rome. But most patients' symptoms were mild and resembled those of other diseases, such as the Toscana virus, so health officials didn't notice for a while. Samples reached ISS on 27 August, and the virus was identified the next day.

Epidemiological detective work suggests that the index patient was a man who traveled to one of the villages and became sick there, after having been infected in India. Isolation and sequencing of the virus are under way to confirm that theory, Cassone says. One patient, an 83-year-old man with severe preexisting medical problems, has died.

Chikungunya sickened more than one-third of the almost 800,000 inhabitants of La Réunion, a French island in the Indian Ocean, in 2005 and 2006 (Science, 24 February 2006, p. 1085). India suffered an explosive outbreak in 2006 with more than 1.25 million cases, although some believe the real toll is much higher. Several European countries had seen “imported” cases of chikungunya lately, but local transmission in Europe has never been observed before. “It's fascinating,” says entomologist Paul Reiter of the Pasteur Institute in Paris.

A daytime biter, A. albopictus originated in Southeast Asia and has made impressive strides across the globe in the past 2 decades. It was first found in the United States in secondhand tires imported from Asia in Houston, Texas, in 1985; today, it has spread to more than 20 southern and eastern states. In Europe, the mosquito has appeared in Mediterranean countries from Greece to Spain and as far north as the Netherlands. Its eggs often hitch a ride with plants shipped in water containers, such as the popular Lucky bamboo.

Six-legged stowaway.

Aedes albopictus frequently travels in shipments of plants such as Lucky bamboo.

CREDITS (TOP TO BOTTOM): TIM ROBERTS/GETTY IMAGES; GETTY IMAGES

It's too early to tell whether chikungunya now has a permanent foothold in Europe. New cases have slowed to a trickle, says Cassone, in part because the mosquito population is dwindling as temperatures drop. A critical question is whether infected mosquitoes can survive the winter or pass on the virus to their offspring via their eggs, says Reiter. “If they can, we might see a rip-roaring epidemic next year,” he says. Even if they can't, any newly imported case could kick off an outbreak in the future.

There are no drugs or vaccines against chikungunya, but the outbreak at La Réunion triggered renewed interest in an old vaccine candidate developed in the 1980s by a U.S. Army lab in Fort Detrick, Maryland. Scientists at three French government institutions are now working on that vaccine, and new clinical trials might begin before the end of 2008, says epidemiologist Antoine Flahault, who chaired a French task force on chikungunya last year.

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