The Ebola virus can spread through the air from pigs to macaques, a new study suggests.
Transmission of the virus — which causes an often fatal hemorrhagic fever in people and primates — was thought to require direct contact with body fluids from an infected animal or person. But in the new study, published online November 15 in Scientific Reports, piglets infected with Ebola passed the virus to macaques housed in the same room even though the animals never touched.
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“The evidence that the virus got from a pig to a monkey through a respiratory route is good,” says Glenn Marsh, a molecular virologist at the Commonwealth Scientific and Industrial Research Organization’s Animal Health Laboratory in Geelong, Australia. Marsh was not involved in the new study but has investigated Ebola and other viruses in pigs.
Although pigs transmitted Ebola in the laboratory, there is still no evidence that anyone has been sickened from contact with infected pigs in Africa, where the virus occurs naturally, or that the virus passes through the air under normal conditions, says study coauthor Gary Kobinger, an infectious disease researcher at the University of Manitoba in Winnipeg, Canada. “It’s definitely not an efficient route of transmission.”
Only 13 of the more than 2,200 human cases of Ebola documented since the virus was discovered in 1976 cannot be traced to direct contact with an infected person, animal or body fluid, he notes. If Ebola were able to spread easily through the air, many more cases might result.
The new study raises questions about whether humans can also transmit Ebola by respiratory routes, says Pierre Formenty, of the World Health Organization’s Control of Epidemic Diseases Unit. That is something that will have to be investigated in future outbreaks, he says.
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Kobinger became interested in Ebola in pigs after investigating an outbreak in 2007 in the Democratic Republic of the Congo. Villagers mentioned that some pigs had gotten sick and died early in the outbreak. At the time, there was no evidence that Ebola could infect pigs. Kobinger and his colleagues have since demonstrated that the virus causes disease in pigs in the lab, but no cases have been confirmed in livestock.
“This is all story-telling. Nobody has isolated virus or even detected antibodies from pigs in Africa,” Kobinger says.
But other researchers discovered that pigs on farms in the Philippines could contract a form of the virus known as Reston Ebola. The Reston strain causes disease in macaques but has not been shown to make people sick. Some pig farmers in the Philippines have antibodies in their blood against Reston Ebola, indicating that infected pigs may have exposed farmers to the virus.
Kobinger wanted to know whether pigs could also pass along the form of Ebola found in Africa. Working in a lab designed to contain the most dangerous pathogens, Kobinger and his colleagues infected piglets with the strain known as Zaire Ebola. The piglets were housed next to four cynomolgus macaques, primates often used as stand-ins for humans. A barrier prevented the animals from coming into direct contact with each other.
After about a week living next to infected piglets, two of the macaques fell ill with Ebola. Those two animals were in cages in the path of air flowing from the pigs’ enclosure. It took several more days for the other two macaques to develop the disease.
While the finding could indicate that the virus spread through the air, the researchers can’t rule out that virus may have infected the macaques via water droplets scattered while cleaning the pig cage.
No one is blaming pigs for Ebola outbreaks in Africa now, but Kobinger says the growing pig industry on the continent might want to take a few simple steps to protect their animals. Keeping fruit trees, which attract fruit bats that carry Ebola, away from pig farms is one such measure.
Ebola viruses related to the African strains have been found in orangutans in Indonesia, raising the possibility that other unknown Ebola-like viruses could spill over into pigs and then humans, Marsh says. “That’s concerning.”