TORONTO - Why does the Ebola virus only sporadically break out of its hiding place in the jungles of central Africa to infect humans or great apes? New research from the National Microbiology Laboratory in Winnipeg may help scientists answer that question.
The work suggests one or more species of animals that serve as reservoirs for the virus may be able to safely harbour it at low levels, like a chronic infection.
But exposing those animals to some external stressor -- a chemical, climatic change, maybe even pregnancy -- can trigger a rise in virus levels. The animal becomes sick and more likely to shed viruses or be preyed upon by great apes or humans, leading to transmission of the deadly virus.
If proven, the idea could help make sense of the fact that outbreaks of Ebola -- and its cousin, the Marburg virus -- are sort of nature's version of disease "hit-and-runs," said the scientists, who are with the Winnipeg lab's special pathogens research group.
"The issue is, certainly in the jungle, the contacts with fruit bats and monkeys and great apes and things should not be that infrequent," said lead author Dr. James Strong.
Scientists believe one or several species of African bats may be the reservoir of these viruses, though proof positive so far has evaded the scientific community.
"You'd think that if it was always spreading the virus or always capable of having the virus there, you would think that the outbreaks would be more often. But they still are very sporadic," said Strong, a medical officer with the laboratory, which is part of the Public Health Agency of Canada.
"So you almost have to think of this as a mechanism of explaining why the things are so infrequent, even though the contact between some of these animals would be relatively common."
The work was published this week in the journal Proceedings of the National Academy of Sciences.
Working in one of the Winnipeg facility's most stringent biosecurity labs, the scientists infected cells from a variety of animals with low levels of Ebola virus, which is normally highly deadly. They managed to achieve the chronic infection stage in cells from fruit bats and two types of mice.
Then they provoked a more acute disease state by exposing the cells to a chemical compound, though Strong suggested that in the wild the trigger might be changes in moisture levels or the availability or lack thereof of certain foods.
The spike in virus levels in the host animal could increase the chances that the virus will spill over into other species such as primates or people, he said.
Some scientists unrelated to the work said the scenario the findings point to could well turn out to be the reason why viral hemorrhagic fevers like Ebola and Marburg jump into humans so infrequently.
"At least that's an explanation that makes sense. But it's going to have to be proved," said Pierre Rollin, an Ebola expert with the U.S. Centers for Disease Control in Atlanta, Ga. He suggested the next step would be to try to trigger this chain reaction in live bats in a lab setting.
Dr. Billy Karesh, director of the field veterinary program for the Bronx, N.Y.-based Wildlife Conservation Society, noted that a model for this type of disease pattern exists in humans.
Herpes virus and the chickenpox virus lie dormant in people who've been previously infected, but both can reactivate if a person's immune system is weakened by illness or other stressors. A cold sore or shingles could be the result.
Karesh said if the scenario is correct it could explain why it has been so hard to identify the natural sources of these viruses.
"Why is it hard to find a reservoir? Well, maybe it's only in pregnant females for two weeks a year and that's the only time they shed the virus and that's why outbreaks are rare," he said.
"This kind of adds another piece to that puzzle that actually makes good sense."
But another scientist sounded a more cautionary note.
Dr. Daniel Bausch, a virologist and tropical medicine specialist at Tulane University in New Orleans, said the fact that the Winnipeg group was able to show this phenomenon in mice as well as bats made him suspicious that what they are seeing isn't really what happens in nature.
Strong said the work suggests mice should be back on the table as a possible reservoir of Ebola, but Bausch said he thinks they've largely been eliminated as a possible source.
While he insisted the work was good, Bausch said he isn't convinced that the scenario the Winnipeg team mapped out is actually what happens in the wild.
"I didn't see it and go: 'Eureka!... This has to be it,"' he admitted, suggesting that maybe Ebola outbreaks aren't as rare as the world believes, but just happen in places where the loss of a few people in a family might not trigger a cascade of human cases and might never come to the attention of medical authorities.
