【Sigad Sharaf】

A localized Zika virus outbreak in Miami prompted the Centers for Disease Control and Sigad SharafPrevention (CDC) to issue a rare travel advisory on Monday, warning pregnant women or women who wish to become pregnant against traveling to a 1-mile radius of an area just north of downtown Miami.

The Zika virus transmission zone encompasses the Miami neighborhood of Wynwood, Florida, as well as portions of Edgewater.

The agency said that since it first announced the first active transmission of the Zika virus within the U.S. on Friday, additional cases were identified in the same part of Florida just north of downtown Miami.

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SEE ALSO: Likely Zika outbreak in Florida marks turning point for the U.S.

CDC director Dr. Thomas Frieden told reporters on a conference call that testing by state and local officials as well as CDC personnel has turned up “persistent mosquito populations” in that area, along with additional Zika cases. Frieden said there is a “risk of continued Zika transmission in that area.”

The CDC travel advisory is a step the agency had been taking to warn against foreign travel by U.S. citizens to countries where Zika transmission was ongoing, including Brazil, where the Rio Olympics will kick off on Friday.

Original image has been replaced. Credit: Mashable

Frieden said it is the first time the CDC had issued a travel advisory regarding a location inside the U.S., other than a Zika-related advisory in Puerto Rico, where far more widespread transmission of the virus is occurring.

The CDC said women who are considering becoming pregnant can travel to the affected area, but health officials recommend they wait at least eight weeks before trying to conceive a child.

According to the 2010 Census, the advisory affects about 20,214 people.

The limited area of the advisory is based on the fact that the mosquito that spreads the Zika virus is not known to venture further than 150 meters during its entire lifetime, Frieden said.

Zika is spread through mosquito bites of the Aedes aegyptimosquito, which can also spread dengue fever and chikungunya, and can also be transmitted sexually. The virus is the first mosquito-borne illness known to cause birth defects, according to Frieden, as it has led to an increase in cases of microcephaly, a devastating condition in which infants are born with unusually small head sizes and brain abnormalities.

“This is the first time we’ve seen birth defects caused by a mosquito bite,” Frieden said.

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While it can also cause health complications in adults, most of those infected with the virus don't notice any symptoms at all, which makes detecting the virus more difficult.

Florida Governor Rick Scott said on Monday that 10 additional Zika cases had been identified in his state since Friday, bringing the total number of known locally-acquired cases to 14.

The governor requested that a CDC emergency response team be dispatched to his state to help coordinate the Zika control efforts. This team is already en route to Florida, Frieden said.

Pregnant women living in the 1-mile area where active Zika transmission is occurring are being told to minimize their exposure to mosquitoes, and wear mosquito repellent containing DEET, Frieden said.

Mosquito control is falling short

Frieden said part of the reason for the travel advisory is that "aggressive" mosquito control efforts in the Miami area have fallen short of expectations, possibly because the Aedes aegyptimosquito has become resistant to insecticides used against it in that area.

Another reason may be because there are areas of standing water that are providing breeding locations for the mosquitoes, according to the CDC director.

Dr. Tom Frieden, Director of the Center for Disease Control and Prevention (CDC), seen in Washington on July 13, 2016. Credit: SAUL LOEB/AFP/Getty Images

"Mosquito control efforts in the specific community… don’t appear to be working as we’d hoped," Frieden said.

So far, though, health officials have not found any mosquitoes that have tested positive for Zika. However, Frieden said that is extremely difficult to do, with human infections typically showing up before mosquitoes test positive.

Frieden rejected the notion that the narrow advisory is aimed at pleasing Florida's multibillion dollar tourism industry. “There wouldn’t be a technical or scientific basis to give a broader recommendation,” he said. 

This won't quickly spread nationwide

While Aedes aegyptimosquitoes are present in at least 30 states, the Florida cases do not necessarily herald a larger outbreak.

However, this does mark a turning point, since until Friday, Zika had not been transmitted by mosquitoes in the Lower 48 states.

“The way Zika spreads is very different from the way other mosquito viruses spread,” Frieden said, noting that there is no animal host where the virus circulates. “With Zika it is quite focal and generally requires that mosquitoes get reinfected by people.”

A similar pattern has been seen with U.S. outbreaks of dengue and chikungunya, with those occurring in limited areas, predominantly near the Gulf Coast where warm, muggy weather is more common.

Mashable's Miriam Kramer contributed to this report.