Mosquitoes in the continental United States may now be spreading the Zika virus.

Health officials in Florida said Wednesday they were investigating two Zika cases that could have been spread by local mosquitoes, in addition to the two similar cases they announced last week. None of the infected individuals has been confirmed to have acquired the virus from mosquitoes, but it seems increasingly likely that a local outbreak is occurring.

There have been more than 1,400 Zika cases so far in US states, but the vast majority of them have been related to travel outside the continental United States. Fifteen of the cases were sexually transmitted, and one was acquired in a laboratory.

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The primary drivers of the virus worldwide are Aedes aegypti mosquitoes, and Florida has been considered among the places most at risk of Zika outbreaks in the United States. It’s one of the few places in the United States that has seen local spread of related mosquito-borne viruses in the past, such as dengue and chikungunya.

Florida has seen more than 380 cumulative travel-related cases of Zika, including 53 in pregnant women.

Officials have not released much information about the new cases, only describing them as “possible non-travel related.” They have also not ruled out sexual transmission.

In a statement Wednesday, the officials said the pattern of the new cases “is consistent with other mosquito-borne virus investigations,” including a dengue outbreak in 2013.

All four cases are in Miami-Dade and Broward counties, which are adjacent. Those two counties have seen the most travel-related Zika cases of any Florida counties.

The state’s investigation, with which federal health officials are assisting, includes testing other residents near where the patients live and trapping and testing mosquitoes in that area. Officials have not found any Zika-positive mosquitoes since they started their investigation, but experts say that depending on the number of infected mosquitoes, that could be like finding a needle in a haystack.

“Residents and visitors are urged to participate in requests for urine samples by the department in the areas of investigation,” Florida officials said in the statement Wednesday. “These results will help the department determine the number of people infected.”

Zika generally only causes a mild illness, but it can cause serious defects in fetuses when it infects pregnant women.

It’s not clear if the Florida cases — if they do in fact stem from local mosquitoes — are connected. Aedes aegypti mosquitoes only fly a few hundred yards in their lifespans.

Local transmission could occur when 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.

Experts do not expect a wide outbreak of Zika in the continental United States. Residents are not as exposed to mosquitoes thanks to the use of air-conditioning and screens on windows, and cities are not as densely packed as they are in places in Latin America and the Caribbean where Zika has spread widely.

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