Pig-to-monkey Ebola: is there something in the air?

Ebola has long been known to be a zoonotic virus--one which jumps between species. Though it took several decades to find evidence of Ebola virus in bats, these animals had previously been associated with human index cases of Ebola disease have worked in bat-infested warehouses or traveled to caves where bats roost. Non-human primates have also become infected with the virus, sometimes transmitting the virus to humans when killed primates are butchered for food. Ebola has also been suggested to infect dogs and other wild animals. However, livestock are a newer angle to Ebola virus ecology.

Ebola was first found in pigs in 2008 in the Philippines. This was the Reston virus, named after its discovery in imported Filipino monkeys in a facility in Reston, Virginia, in 1989. Though this virus spread among the captive monkeys, no humans came down with symptoms. However, follow-up studies showed that some humans did develop an immune response to the Reston virus--suggesting they had been infected, even if they didn't realize it. At the time, there was suggestion that perhaps Reston might be spread via aerosol, as the virus appeared to spread amongst monkeys in two different rooms who did not come into physical contact with one another. However, this was not proven at the time and alternative explanations were possible.

When Reston resurfaced in swine and swine farmers in 2008, a similar phenomenon was observed. Though it was not known how the pigs initially became infected with the virus, they did appear to be able to spread it to humans working amongst them, even if those farmers didn't have contact with blood or other secretions (the most efficient way to transmit Ebola viruses). Suggestive of possible transmission from pigs to people via air, but far from conclusive. Since then, two experimental studies have examined airborne transmission of Ebola via pigs.

The first study examined transmission of the Zaire strain of Ebola--the nastiest one, which can kill up to 90% of those infected--within laboratory pigs. Pigs were inoculated with the Zaire virus and housed with uninfected pigs, who were later tested and found to have acquired the virus. Interestingly, when the pigs got sick with Ebola Zaire, the symptoms were mainly respiratory and the virus replicated in the lungs. This was quite unlike what Zaire does in humans and our other primate cousins, where it's a systemic disease and we can find virus in the blood. This suggests that pigs could be infected with even nasty types of Ebola, and we wouldn't realize it.

Last week, Ed Yong reported on a new paper examining transmission of Zaire virus from experimentally-infected pigs to co-housed macaques. Like the previous paper, they observed that Ebola in pigs was a respiratory disease, and that it could spread to other animals (in this case, non-human primates). The macaques they tested developed the symptoms of Ebola that were expected--a systemic disease, with virus isolated from the blood. In this study, they also added in an air sampling component, and were able to detect evidence of virus (via PCR) in the air. However, the authors do note that this could have been aerosolized in other manners than directly from the exhaling pigs (such as during the floor-cleaning process). Finally, even if it does become aerosolized and spread in this manner, as noted in Ed's article, Ebola is not "suddenly an airborne virus, like influenza." Certainly more efficient transmission takes place via close contact with infected secretions during hospital procedures and funeral rites.

Interestingly, the authors note that other experimental studies that have looked specifically at airborne, primate-to-primate transmission of Ebola have come up negative, and that swine are known to generate "infectious short range large aerosol droplets more efficiently then other species." Is there something specific about pig physiology that may make them better respiratory virus shedders? We know that pigs can be intermediate hosts for other viral pathogens as well, such as Nipah virus and of course influenza.  Are pigs playing any role in Ebola ecology, either in Asia or Africa? Might Ebola have more airborne potential than we previously thought? According to Ed, the authors of the second study are currently working on field studies in Africa to examine the pig question outside of the laboratory. The timing may be good for them, as Uganda is currently experiencing another Ebola outbreak;--the country's third Filovirus outbreak in five months.

Reference

Weingartl, H., Embury-Hyatt, C., Nfon, C., Leung, A., Smith, G., & Kobinger, G. (2012). Transmission of Ebola virus from pigs to non-human primates Scientific Reports, 2 DOI: 10.1038/srep00811