Two of the four cases are in Miami-Dade County while the other two are in Broward County. None of the four individuals has traveled to Zika-affected areas. Sexual transmission has not been ruled out.
"We are looking into other modes of transmission. We're conducting this investigation as we would other mosquito-borne viruses, such as dengue (fever)
," Mara Gambineri, communications director for the Florida Department of Health, wrote in an email to CNN.
Two of the four cases had been announced last week (one each in Miami-Dade and Broward counties).
Officials are going door-to-door asking residents to provide urine samples and other information in an effort to determine how many people may be infected with the virus. It is possible that someone could unknowingly be infected since 80% of those infected have no symptoms.
If any of these four cases is determined to be an infection transmitted by local mosquitoes, it would signify the first local mosquito transmission of Zika in the continental United States. Federal health officials have said local transmission should be expected, although they don't anticipate that it will be widespread.
"Evidence is mounting to suggest local transmission via mosquitos is going on in South Florida," Tom Skinner, senior press officer at the Centers for Disease Control and Prevention, wrote in an email. "These cases fit similar transmission patterns (of) mosquito borne diseases like chikungunya that we've seen in South Florida in years past."
The CDC is assisting state and local health officials with the investigation in Florida.
As a precaution, the U.S. Food and Drug Administration, which oversees blood safety, has asked blood donation centers in the two affected counties to stop collection immediately until officials can either screen the blood for the virus or implement a process to deactivate it. The same precautions are also being recommended for South Florida counties surrounding Miami-Dade and Broward.
In addition, individuals who have traveled to these two counties are asked to delay blood donation for four weeks after their return. The FDA said in a statement that it is "a prudent measure to help assure the safety of blood and blood products."
, there are
383 cases of the virus in Florida; 55 of them are pregnant women.
On Thursday, the CDC reported 1,658 cases of the virus in the continental United States and Hawaii. None of those cases is a result of local mosquito transmission. Fifteen of those individuals were infected by sexual transmission, and there is one case of a laboratory-acquired infection.