Health officials in France were investigating two suspected cases of deadly mouse-borne hantavirus in people who may have been exposed at Yosemite National Park this summer.
Some 1,923 Europeans in 18 countries may be at risk of contracting hantavirus pulmonary syndrome from visits to the U.S. national park in California between June and August, according to an assessment by the European Centre for Disease Prevention and Control on Wednesday.
"In France, the French National Institute for Public Health Surveillance has reported an ongoing investigation into two potentially exposed cases," the European CDC said.
The disease has so far been confirmed to have killed two men and sickened four other people, all U.S. citizens, prompting the U.S. authorities to issue a health alert.
But officials are concerned that more Yosemite visitors could develop the lung disease over the next month. Most of the victims identified so far were believed to have been infected while staying in one of 91 "Signature" tent-style cabins in the park's popular Curry Village camping area.
Of the 10,000 people thought to be at risk, as many as 2,500 live outside the United States, park service epidemiologist David Wong said. U.S. health officials sent warnings to 39 other countries earlier this week that citizens who stayed in Yosemite should be on the lookout for symptoms of the lung disease.
Of the European visitors to Yosemite this summer, the greatest number came from France, at 502, the European assessment said. It said 342 British people also could have been exposed, along with 250 Germans and large groups from the Netherlands, Italy and Belgium.
There is no cure for hantavirus, which kills 36 percent of those it infects, but early detection through blood tests greatly increases survival rates. The disease has never been known to be transmitted between humans.
Last week, park officials shut down the "Signature" tent cabins after finding deer mice, which carry the disease, infesting the double walls.
Early symptoms of the disease include headache, fever, muscle aches, shortness of breath and coughing. The virus may incubate for up to six weeks after exposure and can lead to severe breathing difficulties and death.