The first two locally acquired cases of a painful mosquito-borne viral illness, chikungunya, have been reported in Florida, health officials confirmed on Thursday.
One case was reported in Miami Dade County and the other in Palm Beach County.
The U.S. Centers for Disease Control and Prevention is working closely with the Florida Department of Health to determine how the patients contracted the virus, officials announced.
Chikungunya has surfaced widely across the continental United States but until now the cases have not been transmitted by local mosquitoes, which raises the threat. All prior reported cases involved recent travellers to the Caribbean, where the virus is raging.
"The arrival of chikungunya virus, first in the tropical Americas and now in the United States, underscores the risks posed by this and other exotic pathogens," said Dr. Roger Nasci, chief of CDC's Arboviral Diseases Branch.
"We encourage everyone to take precautions against mosquitoes to prevent chikungunya and other mosquito-borne diseases by draining standing water, covering your skin with clothing and repellent and covering doors and windows with screens," said Dr. Anna Likos, Florida's epidemiologist and disease-control and health-protection director.
Since 2006, the United States has averaged 28 imported cases of chikungunya per year in travellers returning from countries where the virus is common, the CDC said.
"To date this year, 243 travel-associated cases have been reported in 31 states and two territories," it said, adding that Puerto Rico has reported 121 cases of locally acquired chikungunya.
Chikungunya has rapidly spread in the Caribbean in recent months, sending thousands of patients to hospitals with painful joints, pounding headaches and spiking fevers.
Symptoms surface within three to seven days after a bite from an infected mosquito and typically dissipate within a week. There is no vaccine and the virus is not deadly.
"It is not known what course chikungunya will take now in the United States," the CDC said.
The two species of mosquito known to transmit the disease are commonly found across the U.S. Southeast. Local transmission occurs when a mosquito bites someone who is infected with the virus and then bites another person.
Officials say they expect the virus will behave like dengue virus in the U.S., where imported cases have not caused widespread outbreaks.
"None of the more than 200 imported chikungunya cases between 2006 and 2013 have triggered a local outbreak. However, more chikungunya-infected travellers coming into the United States increases the likelihood that local chikungunya transmission will occur," the CDC said.