TAMPA (FOX 13) - The Florida Department of Health says they now believe four cases of Zika being investigated in South Florida were contracted in the U.S.
The Department of Health said it also believes there is an active transmission of the Zika virus in one small area in Miami-Dade County, north of downtown.
No mosquitoes trapped in the Miami area have tested positive yet for the Zika virus, but the state health department believes there are infected mosquitoes in the area after one woman and three men in Miami-Dade and Broward Counties contracted the virus.
More than 1,650 Zika infections have been reported in the U.S., but the four patients in Florida would be the first not linked to travel outside the U.S. mainland.
Zika primarily spreads through bites from tropical mosquitoes. In most people, the virus causes only mild illness, but infection during pregnancy can lead to severe brain-related birth defects for the fetus.
"If you live in this area and want to be tested, I urge you to contact the county health department which stands ready to assist you," Florida Governor Rick Scott said Friday.
There is also concern among folks in Tampa Bay that mosqiutoes there will eventually transmit the virus. Mosquito experts said they have been preparing for that possibility.
"We have already increased out mosquito applications this summer probably to one of the highest levels ever," said Ron Montgomery, with Hillsborough County's Mosquito Control.
So far there have been about 40 cases in Tampa Bay, all are travel-related.
Montgomery said the county will ramp up efforts to trap, kill and test the type of mosquitoes that are carriers.
"We've been planning, preparing for Zika for quite some time and we maintain a robust posture of enhanced surveillance and I think we're not unique in that respect throughout the state of Florida," he said.
Gov. Scott urged pregnant women or women thinking of becoming pregnant who live in the impacted area near Miami to contact their OB/GYN for guidance and to receive a Zika Prevention Kit.
Scott has directed the Department of Health to immediately work with commercial pest companies to increase spraying and mosquito control.
"Floridians can do their part by draining standing water surrounding their homes, as it can serve as breeding grounds for the mosquitoes that are capable of transmitting the virus," Florida Commissioner of Agriculture Adam H. Putnam said Friday.
OneBlood said Friday that effective immediately, it will test blood donated through OneBlood's services in Florida, Georgia, Alabama, and South Carolina for Zika virus.
CDC director Dr. Tom Frieden said, although the four individuals who likely acquired the Zika virus infection locally lived in different locations, they were employed at different work sites that were described as "close by" one another. The infections likely occurred in early July, but did not manifest in symptoms until a week or so later. By mid July, the Florida department of health responded with "aggressive" mosquito control.
Currently, the CDC is not issuing any travel advisories for pregnant women to avoid travel to the area where the local transmission occured. Dr. Frieden did recommend that pregnant women who live in, or who are visiting areas where the aedes aegypti mosquito is present, to take precautions in avoiding mosquito bites.
Because 4 out of 5 cases of Zika may be asymptomatic, there is no way to know if it is present in any community. Just because a person has no symptoms, it doesn't mean the virus will not be harmful.
Dr. Frieden said cases of microcephaly have occurred in babies of mothers who reported no symptoms. Sexual transmission and spread of the disease through mosquito bites can also occur in people who never develop the rash, fever, joint or muscle aches, or red eyes associated with about 20% of Zika infections.
THE ASSOCIATED PRESS CONTRIBUTED TO THIS REPORT.