The World Health Organization has warned that the Zika virus is “spreading explosively,” and with three cases now confirmed in Canada, travellers are being told to consult their doctors.

Health Minister Jane Philpott told reporters in Ottawa Thursday that all three cases reported in Canada were acquired outside of the country, and that risk of the mosquito-borne virus spreading locally is low.

However, Philpott is encouraging Canadians planning to travel internationally to read the warning on the Public Health Agency of Canada’s website. The warning states, in part, that pregnant women and those considering becoming pregnant should discuss their travel plans with health care providers.

Two of the cases were confirmed in B.C. travellers. One had been to El Salvador and the other to Colombia, according to the BC Centre for Disease Control. The third case was in an Alberta traveller who returned from an unidentified country.

Pregnant Vancouver woman Elana Lanteigne said Wednesday that she had decided not to go on a trip booked to the Dominican Republic after cases were confirmed there and her doctor had warned against going.

However, she had been told she would not be getting a refund. “I’m hoping travel companies become a bit more compassionate,” she said.

On Thursday, Air Canada, Sunwing and Air Transat announced they will allow changes to flights for pregnant passengers who provide doctor’s notes. WestJet, meanwhile, said it will provide refunds in the form of travel credit to anyone booked on flights to affected countries.

Minister Philpott’s warning came just as the World Health Organization announced it will host an emergency committee on Monday to decide if the outbreak should be declared an international health emergency.

In a statement Thursday, WHO Director-General Dr. Margaret Chan said there is an “extremely high” level of concern about the virus, which may be linked to a serious birth defect and neurological problems in some parts of South America.

“Questions abound. We need to get some answers quickly,” she said.

Although there is still no proof that Zika is responsible for a sudden spike in the number of babies being born in Brazil with abnormally small heads – a condition called microcephaly -- a causal relationship between the virus and birth defects “is strongly suspected,” the WHO statement said.

“The possible links, only recently suspected, have rapidly changed the risk profile of Zika, from a mild threat to one of alarming proportions,” Chan said.

The virus is also being linked to an increase in cases of Guillain-Barre syndrome, a condition in which the body’s immune system attacks the nervous system.

The Zika virus has spread to more than 20 countries in the Americas in the last year. It is transmitted by a species of daytime mosquito called Aedes aegypti.

Earlier this week, the WHO said the virus is likely to spread to every country in the Americas, except Canada and continental Chile.

Dr. Chan said Thursday WHO is concerned about the lack of immunity in the newly affected areas in the Americas, as well as the absence of a vaccine, any specific treatments, or any rapid diagnostic tests.

“For all these reasons, I have decided to convene an Emergency Committee under the International Health Regulations,” she said, adding that the committee would meet in Geneva.

“I am asking the committee for advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere. I will also ask the Committee to prioritize areas where research is most urgently needed.”

An international team of researchers including Laval University’s Gary Kobinger, are among those working to develop a vaccine.

Dr. Vanessa Allen, the chief of microbiology with Public Health Ontario, told Canada AM on Thursday that it usually takes a decade for a vaccine to be developed, tested and produced.

Allen said she feels the concern about Zika virus is justified.

“It’s in a population that has never been exposed to this virus,” she said. “A lot of people are getting sick and I think it’s appropriate for us to mobilize in this case. I think there’s a lot still to be learned about the virus.”

Allen said her hope is that the wealth of knowledge around the world can be mobilized to begin working on ways to protect people from the virus.

That process could be expedited the way it was for the Ebola virus, she said. In that case, a vaccine went to the human trials stage much faster than normal. But she said a vaccine is likely a long way off, even in the best-case scenario.

“Certainly, it would not be in the phase of several months; it would be several years,” Allen said.

Millions of asymptomatic cases

In a conference call with journalists Thursday, Sylvain Aldighieri, head of WHO's epidemic response team in the Americas, said there could be as many as 3 to 4 million cases of Zika in the Americas over the next year.

He said that estimate was based on the previous spread of dengue fever, which is also carried by the same mosquito species.

But Aldighieri noted that the vast majority of those cases will be asymptomatic, meaning they won’t show the typical rash and fever that Zika infection can cause in 20 per cent or patients. This "silent circulation" of the disease may make tracking the virus’ spread more difficult, he said.

Dr. Bruce Aylward, who runs WHO's outbreaks and health emergencies department, noted in the conference call that there is a lot of uncertainty on these estimates which are likely to change as more is learned about the virus and its spread.

He added that WHO is working with both the global scientific community and industry leaders to help develop a Zika virus test, treatments and perhaps a vaccine. He added that Monday’s emergency meeting will also look to prevent “inappropriate” travel or trade bans.

With a report from CTV National’s Peter Akman and files from The Associated Press