The first cases in the recent outbreak occurred last summer and fall. In August, a New York woman recently back from a Key West vacation came down with the characteristic dengue symptoms — fever, wicked headache, chills, muscle and joint pain, and bloody urine. An alert doctor in Rochester, N.Y., diagnosed dengue fever.
Around the same time, the virus showed up in a woman and a married couple in Key West, none of whom had traveled to areas where dengue is common.
These cases triggered an investigation by the Centers for Disease Control and Prevention and Florida health officials. That uncovered 23 more cases in Key West last summer and fall. Everyone recovered.
Taking the probe another step, the CDC did blood tests on randomly selected Key West households for antibodies to dengue virus, a marker of past infections. Out of 240 people tested, five percent had evidence of recent infection.
The outbreak has subsided, but the virus is still around. On April 9, a 41-year-old Key West man was hospitalized with bloody urine and abnormal blood counts. He also had the dengue virus, and hadn't traveled outside the area. That brings the Key West outbreak to 28 cases.
Dengue fever is rarely fatal, though it can be. But it's often very unpleasant, and dangerous in people with impaired immune systems and other disorders. It's the most common mosquito-borne virus in the world, causing up to 100 million infections and 25,000 deaths each year.
Until this outbreak, Florida hadn't seen dengue fever since 1934. The U.S. as a whole hasn't seen many infections since 1945, except for occasional outbreaks along the Texas-Mexico border and a 2001 outbreak in Hawaii, imported from Tahiti.
By some estimates, several million Americans, mostly immigrants and the poor, have illnesses more commonly seen in the developing world, such as dengue fever and Chagas disease. The conditions often go unrecognized by American doctors.
Infectious disease specialists have been watching for more dengue in the southern U.S. Two species of mosquitos that carry the dengue virus are widespread in this country. Dengue is the most common cause of fevers among Americans returning from the Caribbean, South America and Asia. An infected traveler can touch off a local outbreak if bitten by a stateside mosquito when there's a lot of dengue virus in his or her blood.
Perhaps most ominous, cases of dengue fever elsewhere in this hemisphere – in the Caribbean, Central and South America – have jumped from around 1 million to 4.8 million since 2000. That's why the U.S. made dengue a reportable disease last year.
Officials don't know what caused dengue to pop up in Key West. But all the ingredients are there –- abundant mosquitos of the right kind, lots of tourists exposing lots of skin, and what CDC calls "a proliferation of man-made containers able to serve as mosquito-breeding sites." Lax mosquito spraying programs has also played a role, officials say.
The Florida outbreak might have gone unnoticed if it hadn't been for that astute physician back in upstate New York. So the CDC is urging doctors all over the country to think dengue if they have another patient with the right combination of symptoms and travel history.