Image caption The infection is thought to get into humans through close contact with bodily fluids

Canadian scientists have shown that the deadliest form of the ebola virus could be transmitted by air between species.

In experiments, they demonstrated that the virus was transmitted from pigs to monkeys without any direct contact between them.

The researchers say they believe that limited airborne transmission might be contributing to the spread of the disease in some parts of Africa.

They are concerned that pigs might be a natural host for the lethal infection.

What we suspect is happening is large droplets - they can stay in the air, but not long, they don't go far. But they can be absorbed in the airway Dr Gary Kobinger, Public Health Agency of Canada

Ebola viruses cause fatal haemorrhagic fevers in humans and many other species of non human primates.

Details of the research were published in the journal Scientific Reports.

According to the World Health Organization (WHO), the infection gets into humans through close contact with the blood, secretions, organs and other bodily fluids from a number of species including chimpanzees, gorillas and forest antelope.

The fruit bat has long been considered the natural reservoir of the infection. But a growing body of experimental evidence suggests that pigs, both wild and domestic, could be a hidden source of Ebola Zaire - the most deadly form of the virus.

Now, researchers from the Canadian Food Inspection Agency and the country's Public Health Agency have shown that pigs infected with this form of Ebola can pass the disease on to macaques without any direct contact between the species.

In their experiments, the pigs carrying the virus were housed in pens with the monkeys in close proximity but separated by a wire barrier. After eight days, some of the macaques were showing clinical signs typical of ebola and were euthanised.

One possibility is that the monkeys became infected by inhaling large aerosol droplets produced from the respiratory tracts of the pigs.

Image caption Pigs could act as a host and amplify the Ebola virus

One of the scientists involved is Dr Gary Kobinger from the National Microbiology Laboratory at the Public Health Agency of Canada. He told BBC News this was the most likely route of the infection.

"What we suspect is happening is large droplets - they can stay in the air, but not long, they don't go far," he explained.

"But they can be absorbed in the airway and this is how the infection starts, and this is what we think, because we saw a lot of evidence in the lungs of the non-human primates that the virus got in that way."

The scientists say that their findings could explain why some pig farmers in the Philippines had antibodies in their system for the presence of a different version of the infection called Ebola Reston. The farmers had not been involved in slaughtering the pigs and had no known contact with contaminated tissues.

Dr Kobinger stresses that the transmission in the air is not similar to influenza or other infections. He points to the experience of most human outbreaks in Africa.

"The reality is that they are contained and they remain local, if it was really an airborne virus like influenza is it would spread all over the place, and that's not happening."

Hidden host

The authors believe that more work needs to be done to clarify the role of wild and domestic pigs in spreading the virus. There have been anecdotal accounts of pigs dying at the start of human outbreaks. Dr Kobinger believes that if pigs do play a part, it could help contain the virus.

"If they do play a role in human outbreaks it would be a very easy point to intervene" he said. "It would be easier to vaccinate pigs against Ebola than humans."

Image caption Workers prepare to disinfect during a recent Ebola outbreak in Uganda

Other experts in the field were concerned about the idea that Ebola was susceptible to being transmitted by air even if the distance the virus could travel was limited. Dr Larry Zeitlin is the president of Mapp Biopharmaceuticals.

"It's an impressive study that not only raises questions about the reservoir of Ebola in the wild, but more importantly elevates concerns about ebola as a public health threat," he told BBC News. "The thought of airborne transmission is pretty frightening."

At present, an outbreak of ebola in Uganda has killed at least two people near the capital Kampala. Last month, Uganda declared itself Ebola-free after an earlier outbreak of the disease killed at least sixteen people in the west of the country.