In warning the public last week about active transmission of Zika virus by mosquitoes in Miami, the Florida Department of Health drew a one-square-mile perimeter around a neighborhood north of downtown and advised pregnant women to avoid travel to the area.
State health officials have identified the area around Wynwood as the only one in the continental United States where Zika is spreading by mosquitoes, despite evidence of locally transmitted cases emerging elsewhere in South Florida.
And even though the mosquito most capable of transmitting Zika virus, the Aedes aegypti, typically doesn't travel more than 150 meters — or about 500 feet in its lifetime — one-square-mile is as precise as the health department would get in identifying Florida’s only area with ongoing transmission, said Sarah Revell, a spokeswoman.
“That is a pretty specific area,” she said. “We've identified the street boundaries, and we've provided a map. So we have tried to be as specific and accurate as possible so the public is aware and informed.”
Given the devastating birth defects that congenital Zika virus can cause, however, Aileen Marty, a physician and professor of infectious diseases at Florida International University, said it would be wiser for the health department to share more information.
“Just think about the tragedy of every single child born with microcephaly,” the condition that causes babies to be born with smaller heads and undeveloped brains, Marty said in July before the state confirmed the locally transmitted Zika cases. “I don’t like secrecy. I think overall it hurts.”
In discussing the 10 additional local Zika cases in Miami this week, the Centers for Disease Control and Prevention’s Director Tom Frieden told reporters that all of the infections had been transmitted by mosquitoes within a 150-meter area surrounding an unidentified workplace.
Zika virus is transmitted to people primarily through the bite of an infected Aedes aegypti and Aedes albopictus mosquito. These are the same mosquitoes that spread dengue and chikungunya viruses.
Florida health officials on Tuesday clarified that at least 12 of Miami-Dade’s local cases acquired the disease inside of the one-square-mile area — but not at a single site.
“It's probably best to communicate to the public the one-square-mile radius because it’s not just that one work site,” Revell said.
Florida law authorizes the health department to keep confidential the information gathered as part of an epidemiological investigation, but Revell said that’s not the only reason why the agency has declined to identify more specific locations of local transmission.
“Sometimes these cases do lead us to believe it was an isolated incident,” she said..
The health department has identified 15 locally transmitted Zika cases to date in South Florida — including 13 in Miami-Dade and two in Broward counties. Of those, one case in Broward and 12 in Miami-Dade likely occurred inside the one-square-mile zone identified by the health department.
The other two cases appear to have been isolated incidents that occurred outside the designated area, though the newest one in Miami-Dade is still under investigation.
“If we do identify another area of local transmission, we will put out an advisory,” Revell said.
13 Number of locally-transmitted Zika cases associated with Wynwood area
The health department confirmed Florida’s first locally transmitted Zika cases on July 29, involving four people — two in Miami-Dade and two in Broward. Based on where those four people spent most of their time, the health department investigated three locations, but found ongoing transmission at only one site, the identified area around Wynwood.
Revell said the health department has tested more than 340 people in Miami-Dade and Broward since launching its investigation on July 7, including family members and neighbors of the first individuals infected by local mosquitoes. Those tests turned up at least six more people who tested positive for Zika but were asymptomatic.
But two of the original four local cases — one in Miami-Dade, and one in Broward — have led to “deadends,” Revell said, meaning that there were no other close contacts or neighbors who tested positive for the virus.
For Debbie Wehking of West Kendall, knowing that Zika is actively spreading in a one-square-mile area helps her decide whether or not to take out-of-town guests to Wynwood for an art tour or lunch.
“It’s helpful to have specific areas,” she said, “and I think one-square-mile is specific enough. ... Then you’re aware that you need to be particularly careful, or you can choose not to go there.
Wehking, who is principal of the Brucie Ball Educational Center, said she prefers not to have specific locations identified as areas of ongoing Zika transmission because it could create a false sense of confidence.
“That might actually be a disservice,” she said, “because if I think that I should stay away from restaurant A because that’s where someone was bitten, and I go to restaurant B instead, well that mosquito might be in restaurant B, too.”