09/05/2014 12:03 EDT | Updated 11/05/2014 05:59 EST

Ebola cases could grow by thousands per week if current spread continues: WHO

TORONTO - The World Health Organization is warning that the speed of spread in West Africa's raging Ebola outbreak could escalate in the coming weeks, with thousands of new cases a week by early October.

With the current rate of growth in the hundreds of new cases per week and no easing of the rate of transmission, a jump into the quadruple digits seems to be where this unprecedented outbreak is heading, an epidemiologist working in the office of WHO Director General Margaret Chan said Friday.

"The story is looking quite bleak. There are signs that the numbers of cases are not just continuing to arise in most parts that are already infected in the three main countries, but they are continuing to increase week on week," Christopher Dye, director of strategy in Chan's office, said in an interview with The Canadian Press.

"If we make a simple projection on what has happened over the last, say, 10 weeks ... and make a projection forward, then what we're faced with is not hundreds of cases a week, which is what we see at the moment, but thousands of cases a week going into next month.

"And that would really be an awful scenario because ... health services are already very seriously stretched."

As of Aug. 31, the Geneva-based WHO estimated there had been 3,685 cases in this outbreak, with 1,841 deaths. Both the total cases and deaths figures are greater than the combined totals from all previous known Ebola outbreaks.

Guinea, Sierra Leone and Liberia are at the centre of the epidemic. But cases have also spilled into Nigeria and Senegal. Other neighbouring countries have been told to be on high alert for imported cases.

Dye, who has been seconded to the Ebola response team from the WHO's tuberculosis and HIV-AIDS work, said the agency isn't clear that the growth will hit that rate. There has never been an Ebola epidemic remotely this big, so disease experts are uncertain how long the outbreak will continue to expand.

He noted the three most affected countries are home to about 20 million people — many if not all of whom are likely to be susceptible to infection, if they are exposed to the virus.

"We just don't have a sense of what the maximum number of cases could be under these circumstances," Dye admitted.

Infectious diseases expert Michael Osterholm shared his concern.

"We're in uncharted territories," said Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.

In the past, Ebola outbreaks have mainly happened in remote areas of central Africa, where the virus's opportunity to spread was limited. But in this outbreak, cases are occurring in cities, creating an entirely different dynamic.

"Here, there's almost an endless source of new vulnerable contacts," Osterholm said.

Ebola is transmitted through contact with virus-laced bodily fluids. And the disease generates those en masse; victims experience profuse diarrhea and vomiting. In places where people live in densely crowded neighbourhoods, where they have little access to health care and no real means of isolating themselves when they get sick, the virus has ample opportunity to spread.

"Ebola is like a gun that makes its own bullets," Osterholm noted. "Add all those factors together and they begin to redefine the laws of infectious disease transmission."

Dye said the WHO is looking for ways to figure out how and why the outbreak might peak, but the task is proving to be difficult at the moment.

"We just don't know whether that's what's going to happen," he said of the possibility that cases might increase by thousands per week if the outbreak isn't brought under control.

"That's just assuming that what's going to happen next week is the same as happened last week, the week before and the week before that. Because we have no basis for making any other projection at the moment. ... So all we can do is look at the data as they come in."

He noted that as of now, only portions of the three main countries in the outbreak — Guinea, Liberia and Sierra Leone — are seeing cases. Of the roughly 200 districts in the three countries, he said, only about 14 are experiencing ongoing spread of the virus.

An update issued by the WHO on Friday put some numbers on that situation.

It said Liberia continues to be the most affected country, reporting more than 200 cases a week over the past three weeks. Sierra Leone has reported more than 150 per week for the past two weeks and Guinea reported more than 100 last week.

The overall case fatality rate — the percentage of infected people who die — is 53 per cent, though that is only of known cases. It varies among the affected countries, ranging from 39 per cent in Sierra Leone to 64 per cent in Guinea.

The update said there remains a critical shortage of Ebola treatment centre beds in the three worst hit countries. Based on current capacity and needs, an additional 980 beds are required; the Liberian capital of Monrovia alone needs 760 more treatment beds.

Follow @HelenBranswell on Twitter.