For the first time, the Zika virus might have been transmitted from mosquitoes to humans inside the United States, according to Florida Governor Rick Scott. Scott confirmed that four cases under investigation in his state were not brought in from outside the country.

The four cases, which occurred in Miami-Dade and Broward Counties, were "the result of local transmission," and are "likely mosquito-borne," according to the Florida Department of Health. However, health officials have yet to locate mosquitoes infected with the virus, and have yet to rule out sexual transmission of the virus.

If the virus was transmitted by mosquito, it would mark the first time Zika has been transmitted in such a fashion in the United States. If a local reservoir of Zika has taken hold in southern Florida, that would complicate containment of the virus.

If transmitted by mosquito, this would be the first instance in the United States

On Wednesday, the US Food and Drug Administration requested that blood donations be halted in both counties until proper donor screening protocols can be established, and has asked other blood collection locations to screen donors who have visited the two counties in order to prevent the virus from entering blood banks.

Since the Zika outbreak began in Brazil in April 2015, the virus has spread throughout Latin and Central America. Zika can cause mild symptoms such as fever, joint pain, and rashes. But it's also been linked to Guillain–Barré syndrome, a severe neurological disorder that can lead to paralysis and death.

Zika can also cause microcephaly in infants when the mother is infected with the virus. The health effects of the virus have prompted the CDC to issue warnings to pregnant women traveling in regions where the outbreak is currently ongoing. In the US, babies were born with birth defects due to infected mothers who had traveled to Zika-infected countries.

Correction: this article originally referred to Miami-Cade County. The correct name is Miami-Dade. We regret the error.