Zika case in Florida could have come from local mosquito, a first in continental US

Health officials in Florida are investigating what could be the first case of mosquito-borne Zika transmission in the continental United States.

The state health department put out a brief statement Tuesday evening saying they were “conducting an investigation into a possible non-travel related case of Zika virus in Miami-Dade County.” The county already had 88 travel-related cases of Zika — the most in the state.

The virus can be transmitted through sex, as well as by mosquitoes, but the release did not mention that as a possible mode of transmission. The health department did not immediately respond to questions.

Zika, which is primarily spread by Aedes mosquitoes, has swept through much of the Americas since cases first emerged in Brazil last year. The virus typically causes no symptoms or only a mild illness. But it can cause serious birth defects in fetuses when it infects pregnant women, including a condition called microcephaly in which the brain is underdeveloped and head is abnormally small.

There have been more than 1,300 Zika cases in the United States. Almost all of the patients were infected while traveling in a Zika-infected area. A handful of cases were the result of sexual transmission.

So far, there is no confirmed case of local mosquito transmission of the virus; Florida’s would be the first.

Federal authorities have a 58-page plan for responding to even a single local transmission.

The most likely route for local transmission is that someone who is infected while traveling returns home and is bitten by a local mosquito. If that mosquito becomes infectious itself, it can then infect other people it bites.

Still, most experts believe that any local spread of the virus will be contained, nothing like the wide spread that has been seen in Latin America and the Caribbean. Related viruses also spread by Aedes mosquitoes, including dengue and chikungunya, have had limited impact in the US, with just a few reported cases of local transmission of those viruses in Florida and South Texas.

People in the continental United States are generally less exposed to mosquitoes than in other regions thanks to screened windows, the use of air conditioning, and better home construction. Densely packed urban areas in the rest of the Americas also make it easier for a mosquito to infect several people.

In the continental United States, Aedes aegypti, the primary driver of the virus, are found in the highest numbers in southern states and along the Gulf Coast.

Florida’s health department said it is working with the Centers for Disease Control and Prevention on its investigation and that crews have already launched mosquito control efforts in the area where the possible transmission occurred.

“Florida is the lead on an investigation involving a case of Zika in someone with no travel history to a country with active Zika transmission,” a CDC spokesman said. “CDC is consulting with Florida on the case.”

Helen Branswell contributed reporting.