A few days ago, health authorities in southern France announced that they’d found two cases of the mosquito-borne disease chikungunya in the Var, in Provence. Chikungunya is a nasty disease; it causes high fevers and severe joint pain, and its name comes from words in a Mozambiquan dialect that describe victims being “bent over” in spasms. Chikungunya perks along steadily in the tropics — Africa, Asia, India — and every year, a few travelers arrive back in the temperate zones suffering from infections they picked up on holiday (e.g., into Europe from the Maldives and from Thailand, both in 2009).

There was something unusual about the cases in the Var, though. Neither of the victims, 12-year-old girls who are friends and live in the town of Frejus, had been outside France. They were the first locally acquired cases of chikungunya that France has ever recorded. And they happened to follow, by two weeks, the discovery of France’s first locally acquired cases of dengue, which is, after malaria, the most serious mosquito-borne disease in the world.

So, on the one hand: Few cases, everyone treated, nobody died.

On the other hand: An early warning signal worth listening to.

Unlike some other vector-borne diseases, chikungunya and dengue are carried only by humans; there are no intermediate hosts, no birds or deer or horses to keep the organism circulating in the environment. A human gets bitten by a mosquito; the mosquito bites someone else; the infection passes on. That tight cycle of transmission requires only a few things: that the human be recently infected, so that the viral load in the blood is relatively high; that the mosquito be one of the few species that transmit the disease; and that the mosquito population be robust — and, ideally, not killed off by winter temperatures.

Put them all together, and you get the Democratic Republic of the Congo in 2004, Reunion Island in 2006 and Singapore in 2008 — all significant epidemics, all tropical locations. But you also get Ravenna, in northeast Italy, where there was an outbreak of more than 200 cases of chikungunya in summer 2007. Ravenna is hardly tropical. Neither is Frejus. But both of their outbreaks started with a recently arrived traveler: a visitor from India in Ravenna, and in Frejus apparently a 7-year-old girl who was in Asia with her family. And both areas share something that gravely worries global-health planners: an apparently increasing population of the key mosquito species, which have ranged further north as average temperatures rise.

In Europe, chikungunya (and dengue) are being spread by Aedes albopictus, the Asian tiger mosquito. It used to be only a tropical species, but it was first spotted in Europe in 1979, in Albania, and it arrived in Italy in 1990. It’s not a long-distance flyer, just a smart adapter: It’s moved around the world, including to the United States (more on that in a minute), via shipments of used tires and sales of those “lucky bamboo” stalks you can find in any gift shop. It has landed in places as far north as Germany and the Netherlands, and has established itself in Albania, Croatia, France, Greece, Monaco, Montenegro, Slovenia and Spain, with the densest populations in Italy.

Where will it go, facilitating disease spread as it moves? The European CDC took an in-depth look at the possibility a year ago. In a report plugging in a variety of climate change models, they concluded there isn’t really, any good news. On the left, in red, is their most conservative short-term estimate of the mosquito’s likely range (for 2010; the maps were generated in 2008 and the report published in 2009); on the right, the most pessimistic estimate, for 2030. Either way, that’s a lot of potential disease transmission.

So what about the United States? By 2006, 37 travelers had arrived in 17 states and Washington DC while infected with or recovering from chikungunya; five of them were viremic enough to be potentially infectious. And in 2008, 14 church workers came back to the US with active dengue— but by luck, returned to the cold-climate states of Minnesota and Iowa, where mosquitoes don’t live very long. They persist for much more of the year in Key West: Between September 2009 and April this year, 28 people, some of them snowbirds from northern states, were infected with dengue in the first local, sustained US outbreak since 1945.

When the European CDC was calculating the potential current range of the mosquitoes that carry dengue and chikungunya, one of the measures they used was mean annual temperature: At a mean of 11 C (52 F), Ae. albopictus are more likely to survive the winter and so hang on from year to year. When you map the United States by mean annual temperature, this is what it looks like; anywhere in yellow is warm enough, long enough, to keep the mosquitoes going.

So, to recap: First Italy, and then France, experienced infected travelers returning to areas where newly established mosquito populations were large and long-lived enough to sustain the importation of disease. In the US, we know we have the travelers; we can see we have the mosquitoes; and given the Key West dengue outbreak, we know transmission of disease by the same mosquito vector has already happened.

In an early sign of what might come next, the Iowa Electronic Health Markets— a prediction engine for disease outbreaks — has just opened a market on the likelihood of dengue occurring in the US.

It might be time to buy a little more DEET.

Maps from Technical report: Development of Aedes albopictus risk maps, ECDC, 2009. Image courtesy of Flickr user Matteo Dudek. Inspiration from ProMED Mail.