The mosquito-borne Zika virus is "spreading explosively," and could infect three to four million people this year, the World Health Organization says.

Most people who are infected won't have any symptoms of the mild illness, such as fever, headache, rash and eye irritation.

On Thursday, WHO director general Dr. Margaret Chan said it will hold an emergency meeting on Feb. 1 to decide if it should be declared a public health emergency of international concern.

One reason for alarm is a possible link between Zika infections and a birth defect called microcephaly — abnormally small heads. 

"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions. The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities," Chan said.

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Margaret Chan, the head of the World Health Organization, WHO, says the Zika virus has gone from a mild threat to one of 'alarming proportions.' (Martial Trezzini/Associated Press)

Health authorities in Brazil have reported a spike in babies born with microcephaly. It's not known if the virus is the cause and if so, what proportion of the birth defects can be linked to it. Studies are underway to look for answers.

Health Minister Jane Philpott encouraged pregnant women or those considering travel to affected areas in South America, Central America and the Caribbean to postpone travel.

If travel is necessary, precautions to avoid mosquito bites are advised, Philpott said.

There are three reported cases of Zika virus known in Canada among people who have travelled.

Based on cases of dengue, another virus spread by the mosquitoes, it's estimated there could be three million to four million Zika infections in the Americas between the southern U.S. and Argentina in the next 12 months, Dr. Sylvain Aldghieri, unit chief of International Health Regulations Epidemic Alert and Response for WHO, told reporters.

Zika-map

Most of the countries affected by the virus are in Central and South America, according to the Centers for Disease Control and Prevention. (CBC)

The estimate includes the 75 per cent of those infected who never seek medical attention, Aldghieri said. 

Since the mosquitoes that carry and transmit the virus do not live in Canada due to the climate, health authorities expect only travel-related cases in this country. 

"If Canadians are interested in going to Central America or South America, Mexico, the Caribbean, and they're not pregnant and they're not considering becoming pregnant in the next little bit, they should just be as concerned as they normally are when they travel to those areas, which is not very concerned at all," said Dr. Isaac Bogoch, a tropical infectious disease expert at Toronto's University Health Network.

As a precaution, Canadian Blood Services said Thursday it will temporarily ban blood donations from those who have travelled to countries where the mosquito-borne Zika virus has become widespread.

Chan cited four main reasons why WHO was "deeply concerned" about Zika:

  • The possible link to birth defects and the rare nervous system disorder called Guillain-Barré syndrome. The syndrome can start with tingling and weakness in the body and progress to muscle weakness and paralysis. Most people recover completely.
  • The prospect of further spread.
  • Lack of immunity among people living in the newly affected areas in the Americas.
  • The absence of vaccines, treatments or quick diagnostic tests for the virus.

Declaring a global emergency can bring more money and resources to quell an outbreak.

The last such emergency was announced for the devastating 2014 Ebola outbreak in West Africa, which eventually killed more than 11,000 people.

"With the Ebola virus outbreak in West Africa, the World Health Organization explicitly stated that they were a little bit slow to come up with an international co-ordinated effort to combat that infection. Perhaps they're learning from that past incident that was just over a year ago and coming up with a rapid solution here," Bogoch said. 

In the U.S., there's been 20 lab-confirmed cases of Zika infection among people who travelled to the affected areas, the Centers for Disease Control and Prevention said.  

The Zika virus is named after a forest in Uganda where it was first detected in 1947.

With files from Associated Press