Scanning electron microscopic image of Ebola virionsPLOS BiologyAlthough Ebola viruses can cause fatal disease in humans and other primates, pigs can carry the infections with few ill effects. Now, Canadian scientists have shown that apparently healthy pigs can pass the deadliest species of Ebola to monkeys, even without ever coming into contact with them.

The study, published today (November 15) in Scientific Reports, marks the first time that the virus has spread between different species in a lab experiment, and suggests that pig farms could be facilitate such species-hopping in more natural conditions.

However, Gary Kobinger from the University of Manitoba, who led the study, cautioned that “we still don’t know if pigs are playing any role in the natural transmission or ecology of Ebola virus in Africa.”

“An epidemiological survey of wild and domestic pigs in sub-Saharan Africa is now necessary,” agreed Shigeru Morikawa from the...

Ebola has been found in gorillas, chimps, duikers (a small antelope), humans, and recently, pigs. The identity of its reservoir species is unclear, although bats are the most likely candidate. Until recently, no one even knew that pigs could carry Ebola. But in 2009, Roger Barrette found the Reston Ebola virus—the only one of five Ebola species of that does not seem to cause disease in humans—among Philippine pigs and antibodies against it among six pig farmers. More worryingly, Kobinger’s team also showed that Zaire-Ebola virus—the deadliest of the five, with a fatality rate of up to 90 percent in humans—can also infect pigs and spread between them through direct contact. 

“Pigs are remarkably versatile animals when it comes to acquiring and transmitting infections,” said Tara Smith from the University of Iowa, who studies emerging infectious diseases and was not involved in this study. “They have been implicated in the spread of a variety of nasty zoonotic viruses: influenzas, Nipah virus, possibly Hendra virus, and now at least two types of Ebola.”

In pigs, Ebola mainly infects the lungs and airways, which makes them well-suited to spreading the virus through the air. To see if this was possible, Kobinger teamed up with Hana Weingartl from the University of Manitoba. They used nose swabs to infect piglets with Zaire Ebola, then placed them in a room with four cynomolgus macaques. The monkeys lived inside a wire cage within the pig pen, so the two species never made direct contact despite sharing living quarters.

The piglets developed heavier breathing and mild fevers, but were otherwise unharmed by the infection. But the monkeys were not as lucky. After 2 weeks, the pigs had passed the virus to all their neighboring macaques, who developed bloody spots on their chest and limbs and signs of damage in their lungs.

The study shows that the virus can spread without direct contact, but “keep in mind that Ebola is not suddenly an airborne virus, like influenza,” said Kobinger. Instead, the virus could have jumped from pigs to monkeys via small droplets in the air, or larger ones that splashed into the monkeys’ cages when the handlers cleaned the floor of the pigs’ area.

Indeed, the local nature of all known outbreaks suggests that it does not disperse effectively like an airborne virus would. Furthermore, it’s still unclear how common indirect transmission between species is in the real world. It could explain why some Philippine pig farmers were infected with Reston Ebola even though they were not involved in slaughtering the swine, and had not come into contact with contaminated tissues, Kobinger noted. But, he added, “this work was done in controlled conditions, and may not be representative of pigs running outside in the field,” said Kobinger. His team is now headed to Africa, to collect samples from pigs in areas that have had Ebola outbreaks in the past. “We just started this and are looking forward to see the results.”

H. M. Weingartl et al., “Transmission of Ebola virus from pigs to non-human primates,” Scientific Reports, doi: 10.1038/srep00811, 2012.

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