The spread of polio across borders is an international public health emergency that demands emergency measures such as proof of vaccination for people leaving the worst-affected countries, the World Health Organization says.

Outbreaks of the crippling viral illness in Pakistan, Syria and Cameroon are a particular concern because these three countries are considered at greatest risk of allowing the virus to spread beyond their borders, the UN health agency said Monday.

Polio mainly affects young children. The virus is usually spread through contaminated water.

"The international spread of polio to date in 2014 constitutes an extraordinary event and a public health risk to other states for which a co-ordinated international response is essential," WHO assistant director general Bruce Aylward told reporters. 

"Until it is eradicated, polio will continue to spread internationally, find and paralyze susceptible kids."

This is the first time that the WHO has declared a public health emergency since the 2009 H1N1 flu pandemic.

The declaration follows last week's emergency meeting in Geneva on the spread of polio. Members from the 10 affected countries (Afghanistan, Cameroon, Equatorial Guinea, Ethiopia, Israel, Nigeria, Pakistan, Somalia and Syrian Arab Republic) participated in the information exchange.

Polio Map

The following countries have been flagged by the WHO as areas of concern for polio. (CBC)

Experts are concerned about how civil war, strife and unrest threaten the UN's goal of eradicating polio by 2018.

"Maybe we could call a halt to all these conflicts and say, 'Look, here's a week. We're going to immunize everybody during this week. Everybody put down your guns. Except I don't think they'll value children enough to do that," Dr. Noni MacDonald, a professor of pediatrics at Dalhousie University, said from Vancouver.

"Explain to a three-year-old how come they got polio. It's not their fight. It's not their battle. It's not fair."

Public health experts are also keeping a close eye on how cases of polio are occurring in what WHO called "polio-free but conflict-torn and fragile states." MacDonald gave Ukraine and Sudan as examples.

Eradication 'tantalizingly close'

It's estimated that for every clinical case of polio that's seen, there are likely 100 others who are infected and silently spreading it, MacDonald noted. The virus can be excreted in the feces of people who have no symptoms and the infection can be transmitted to others.

"In Canada, we need high immunization rates to be protected from importing polio, just like we imported measles," MacDonald said.

Public health experts say the spread of the virus can only be prevented if more than 90 per cent of people in a community are vaccinated. In Canada, the theoretical concern surrounds the possibility of infections in pockets where vaccination rates don't meet that threshold for protection.

"People could literally get on a plane carrying the polio virus, not even necessarily be sick with it and bring it to Canada where we don't have perfect vaccination rates anymore," said Dr. Michael Gardam, director of infection prevention and control at the University Health Network in Toronto.

"The point of all of this is that Canada is never zero risk."

There are no treatments to reverse the damage from polio.

"If you can get the vaccine to people, you can eradicate this. It doesn't live in any other organisms besides humans, so it's one of these things that are tantalizingly close," Gardam said.

But 30 years after polio eradication efforts began, stamping out the final cases poses the biggest challenge, MacDonald said in commending India's success. 

In 2013, WHO said there were 417 cases of polio worldwide. As of April 30, the Global Polio Eradication Initiative reported 68 cases, compared with 24 by that time last year.

Polio's high transmission season begins in May and June, WHO said. So far this year, the international spread of poliovirus has occurred from Pakistan to Afghanistan, from Syrian Arab Republic to Iraq and from Cameroon to Equatorial Guinea.

Pakistan is the only country where polio infections have a foothold of regular spread (what public health experts call "endemic") and cases rose last year, accounting for more than a fifth of the 417 cases worldwide in 2013.

With files from The Associated Press and Reuters