Miami's South Beach has been identified as a second site of Zika transmission by mosquitoes on the U.S. mainland, and containing it there will be difficult because high-rise buildings and strong winds make it impractical to spray the neighbourhood from the air, officials said Friday. 

Five cases of Zika have been connected to mosquitoes in this area of Miami Beach, bringing the state's caseload to 36 infections unrelated to travel outside the U.S., Florida's governor and health department announced Friday. 

The discovery prompted the U.S. Centers for Disease Control and Prevention to announce that it was expanding its travel warning for pregnant women to include the Miami Beach area known for nightclubs, pedestrian thoroughfares and beaches.  

The CDC previously warned pregnant women to avoid the Wynwood arts district in Miami. In its statement Friday, the agency said pregnant women may also want to consider postponing nonessential travel throughout Miami-Dade County if they're concerned about potential exposure to the mosquito-borne virus.

"We're in the midst of mosquito season and expect more Zika infections in the days and months to come," said CDC director Dr. Tom Frieden. "It is difficult to predict how long active transmission will continue."

Aerial spraying and door-to-door operations on the ground have cut mosquito populations in Wynwood by up to 90 per cent, but Zika may be continuing as mosquitoes breed, Frieden told reporters Friday. 

"The mosquitoes are persistent, and we won't know for a couple of weeks whether these aggressive measures have worked," Frieden said.  

Aerial spraying isn't practical over South Beach because of the height of its buildings and strong winds over the narrow strip of land, Frieden said. Officials will be limited to spraying for mosquitoes at ground level in the highly populated area. 

"Miami Beach does have a series of characteristics that make it particularly challenging," Frieden said.

Two of the people infected in Miami Beach are Miami-Dade County residents, and three are tourists, including one man and two women, Scott said. The tourists are residents of New York, Texas and Taiwan.

The new area of infection in South Beach is roughly 1.5 square miles between 8th and 28th streets, according to Florida's Department of Health. 

Three vacuum trucks purchased to help Miami Beach fight rising sea levels have been used since the beginning of the year to drain water in low-lying areas where mosquitoes could breed, said Roy Coley, the city's infrastructure director. 

The city also has been sending workers to fill potholes collecting water in alleys and fix leaky beach showers, in addition to applying pesticides to the area's many construction sites and flood-prone residential streets, Coley said. 

Possible infections outside Wynwood and Miami Beach also are being investigated.

Zika infection can cause severe brain-related birth defects, including a dangerously small head, if women are infected during pregnancy. But the virus only causes mild, flu-like symptoms in most people, making it difficult to confirm local transmissions, the CDC said.

"For this reason, it is possible that other neighbourhoods in Miami-Dade County have active Zika transmission that is not yet apparent," the CDC's statement said. 

Elsewhere on Friday, the CDC updated its guidance for health care providers caring for infants born to mothers with possible Zika virus infection during pregnancy.

Zika linked to birth defect

The current Zika outbreak was detected last year in Brazil, where it has been linked to more than 1,700 cases of microcephaly. The birth defect is marked by small head size that can lead to severe developmental problems. The virus is also associated with other complications in babies.

About 80 per cent of people infected with Zika virus have no symptoms. In others, the infection is mild, with symptoms that include fever, headache, conjunctivitis or pink eye and skin rash, along with joint and muscle pain.

For people in Canada, the risk continues to be low since the mosquito species that normally transmits Zika virus cannot survive the cold weather, the Public Health Agency of Canada says. 

Travellers are advised to protect themselves from mosquito bites at all times. 

As of Thursday,  220 travel-related cases, two sexually transmitted cases and three reports of maternal-to-fetal transmission have been detected in Canada.

Ottawa is recommending that pregnant women and women who plan to get pregnant avoid travelling to South Florida after more cases of Zika virus linked to mosquitoes were reported in the area.

Canada warns some against travel to South Florida

The Public Health Agency of Canada issued a travel health notice Friday after Florida health officials announced five cases of Zika had been connected to mosquitoes in Miami Beach.

The agency recommends that pregnant women and those who plan to get pregnant avoid travelling to South Florida. 

It said it's working closely with the U.S. Centers for Disease Control to provide Canadians with the most up-to-date information possible regarding the virus.

With files from CBC News