(NEXSTAR) — Federal health officials warned doctors around the U.S. this week to look out for signs patients may have dengue fever, a virus spread by mosquitoes that’s surging worldwide. Cases in the U.S. are three times higher than this time last year.

Most of the cases in the U.S. are in people who recently traveled abroad to regions where dengue is spreading quickly. Only one state so far, Florida, has evidence of local transmission, the Centers for Disease Control and Prevention says. U.S. territories Puerto Rico and the Virgin Islands are also seeing local spread of the disease.

Cases associated with travel are more common. Thirty-eight states plus D.C. have reported at least one case of dengue fever this year.

The map below shows which states have and have not reported confirmed cases of dengue to the CDC: gray indicates zero known cases, pale green indicates just one to four confirmed cases, teal represents five to 49, and blue is for states with 49 to 250 cases of dengue fever.

Florida has 191 confirmed cases, New York has 134 and Massachusetts has 50, according to CDC tracking. At least 2,241 cases have been confirmed nationwide.

For an interactive version of the map above, check the CDC’s website here.

In a health alert Tuesday, the Centers for Disease Control and Prevention advised doctors to know the symptoms, ask questions about where patients recently traveled and consider ordering dengue tests when appropriate.

The virus is spreading fastest in tropical, warm countries, where warm weather mosquitoes that carry the disease thrive. These mosquitoes are expanding their geographic range because of climate change, experts say.

Across the world, more than 6.6 million infections were reported by about 80 countries last year. In the first four months of this year, 7.9 million cases and 4,000 deaths have been reported, according to the World Health Organization. It’s been particularly intense in the Americas, including in Brazil and Peru.

About 1 in 4 dengue fevers are symptomatic, the CDC says. Symptoms, if they do occur, show up typically five to seven days after infection. People often feel fever and other flu-like symptoms such as nausea, vomiting, aches, rashes, joint and bone pain, and headaches.

Severe cases can cause serious bleeding, shock and death. There is no widely available medicine for treating dengue infections.

The Associated Press contributed to this report.