The U.S. Food and Drug Administration has approved genetically modified mosquitoes as part of a trial to control the Zika outbreak in the Florida Keys.

The agency announced the approval Friday, saying it would not have a significant impact on the environment. Meanwhile, aerial and ground spraying of pesticides continues.

The British company Oxitec developed the mosquitoes, which are modified so their offspring die before reaching reproductive age.

No mosquitoes will be released immediately. Rather, officials in the Keys will hold a nonbinding public vote on the plan in November.

Trials of the modified mosquitoes in Brazil, Panama and the Cayman Islands suggest they reduced local populations of Aedes aegypti mosquitoes by more than 90 per cent, Oxitec says. The mosquito species also spreads dengue, yellow fever and chikungunya.

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

Earlier this week, health officials in Canada and the U.S. recommended that pregnant women and those planning to conceive avoid travelling to a small area north of downtown Miami where Florida officials report 15 non-travel related infections of Zika.

It's a new approach to limit a travel advisory to such a localized area rather than an entire country, said Dr. Gregory Taylor, Canada's chief public health officer.

"The key is to follow this closely as the Florida officials are doing, testing a lot of people and testing a lot of mosquitoes to look for the virus," Taylor said in an interview with CBC's Power and Politics.

In Miami, the area with demonstrated spread of Zika is contained in a buffer zone of one square mile. That's where workers are spraying insecticides from the air and on the ground to control both the adult and larval forms of mosquito that transmit the virus.

Zika Florida

Pregnant women should protect themselves from mosquito bites, says Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. (Wilfredo Lee/Associated Press)

Zika is unprecedented as a mosquito-borne disease that can cause birth defects. That's why it's key to protect pregnant women, said Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention.

Pregnancy precautions

"We've made recommendations that pregnant women shouldn't travel to that one-mile area," Frieden told reporters on Thursday. "Understand that as long as there is Zika spreading anywhere, pregnant women should take steps to protect themselves from mosquito bites."

About four million Canadians visit Florida every year.

Taylor also advised those who are trying to get pregnant to not go to the affected area in Miami, or to practise contraception while they're there.

"The estimate so far is somewhere between one and 12 or 13 per cent of pregnancies or outcomes are bad," Taylor said.

While the majority of pregnancies are going to be fine, Taylor said, the numbers are still relatively high for having a severely affected child.

Since Zika has been detected in semen up to 90 days post-infection, abstinence for six months or barrier methods of contraception are also recommended. 

Among the 187 Canadians who've contracted Zika, two case were sexually transmitted. One case involves a baby who contracted it from a mother infected during the first trimester of pregnancy. 

At Toronto's Pearson Airport, some people boarding a flight to Miami on Friday said they planned to take precautions.

Zika Florida

A Miami-Dade County mosquito control worker sprays around a home in the Wynwood area of Miami. (Alan Diaz/Associated Press)

"It's been quite a big issue," said Charlotte Toogood. "There are definitely things like covering up, taking bug spray , screens and so on. Just any extra steps that you can."

Andrea Crawford said she's not too concerned.

"We are taking precautions. None of us are in our child-bearing years, or they better not be," she said glancing at her daughters. "I think we're going to be good, we'll be fine."

Frieden said early tests suggest the first spraying of an insecticide from airplanes by crews in Miami-Dade County significantly reduced the number of mosquitoes in the affected area.

The vast majority of people infected with Zika have a mild infection and 80 per cent have no symptoms, Taylor said.

He said the risk for people in Canada continues to be low, since the mosquito species that normally transmit the Zika virus cannot survive cold weather.

What's more, tests so far suggest six domestic mosquito species aren't able to transmit the virus, including the one responsible for 90 per cent of bites, Taylor said.

With files from CBC's Marcy Cuttler and Reuters