Pigs with Ebola Zaire: a whole new can o' worms

Ebola has long been associated with wildlife. From the early days, bats were viewed as a potential reservoir (though it wasn't confirmed that they actually harbored the virus until 2005). Contact with wild animals--particularly primates which were butchered for food--was also long thought to be a risk factor, and now we know that primates can become ill with Ebola and pass the virus to humans.

What hadn't been examined until 2008 were pigs. I mean, it's not exactly the animal you associate with central Africa, where many of the Ebola cases have been concentrated. However, pigs are much more plentiful in the Philippines, where another Ebola subtype--Ebola Reston--is thought to lurk. The Reston strain actually was first documented in the United States, where twice it was associated with outbreaks in primates originating from the Philippines. When the facility in the Philippines was closed down in 1997, Reston disappeared for 11 years--until it surfaced in pigs in 2008.

The ecology of Ebola Reston in the Philippines isn't known--unlike African Ebola strains (and their cousin, Marburg), no bats have been caught in that country and tested positive for the virus, though they probably serve as a reservoir of the virus in the Philippines just as they do in Africa. So it was a huge surprise when pigs from that country tested positive for Ebola Reston--and so did 6 of their human caretakers, suggesting cross-species transmission. (I should note here that the Reston strain has yet to be linked to any symptomatic infections in humans--the pig farmers who tested positive probably had no idea they'd been infected and did not show any clinical signs of illness). Pigs hadn't previously been linked to any Ebola infection, so this brought in a whole other wrinkle when it came to Ebola transmission--the possibility of being exposed to Ebola via contaminated food, and the potential for pig populations to harbor the filovirus (and transmit it to their caretakers, as we have seen with outbreaks of Nipah and Hendra viruses).

A new study delves further into Ebola in pigs. Instead of using the Reston strain, they use the much-more-deadly Zaire strain. This is the one that movies are made about; the one which can cause outbreaks so nasty that they kill up to 90% of those who are infected. Why use Zaire instead of the Reston strain--the one which has actually infected pigs in nature? Well, the researchers wanted to find an animal that's easier to work with than primates (there are all kinds of very strict regulations when it comes to working with non-human primates), so if pigs could work as a good model for human Ebola disease, that would make studying the virus just a bit easier. (In any case, for any live Ebola work, it still needs to be done in a biosafety level 4 environment, meaning complete spacesuits and the whole works).

The authors did 2 studies. In the first, they inoculated 6 pigs with Ebola Zaire, via a combination of intranasal, intraocular, and oral routes of infection. (Interestingly, no injection, which can be a key way Ebola is spread). They had an additional 2 pigs that they inoculated the same way with a saline solution, and housed them separately from the Ebola-inoculated animals. The goal of this experiment was to look at the pathogenesis of a virulent Ebola strains in the pig model. The infected animals all developed fevers and respiratory disease, with some internal hemorrhaging and evidence of airway replication by Ebola. Infectious virus was found at low levels in nasal washes and oral and rectal swabs; one animal also had a low level of virus in the blood. Higher levels of virus were found in various organs, including the heart and bladder, while the highest levels were found in lung tissue.

In the second experiment, they inoculated 3 new pigs in the same fashion, but then added in 4 additional (uninoculated) animals to stay with them, and kept 2 additional control animals in a separate area so that they could investigate pig-to-pig transmission of the virus. They did find viral RNA from the mucosa of all contact animals, and infectious virus was detected from 2 of 4, demonstrating that the virus can be passed among pigs. Not stated in the article was if the authors thought this was due to direct contact with respiratory secretions among the pigs, or via airborne transmission (a much more concerning route of transmission, as in humans, Ebola Zaire doesn't seem to transmit well via air--typically it's spread via close direct contact and bodily fluids).

Notably, pigs didn't seem to develop severe systemic disease from Ebola, as primates do--the main symptoms exhibited were respiratory, which the pathology supports (finding little virus in the blood, but a lot in the lungs). This suggests that even for Ebola Zaire, infection in a pig could be mistaken for other respiratory diseases, such as influenza or PRRS virus (porcine respiratory and reproductive syndrome virus, which the initial pigs in the Phillipenes were co-infected with). So, Ebola may be circulating even more than we realize in the pig population, disguised by its commonplace symptoms.

A commentary published in tandem with the research article ponders the issue of foodborne Ebola, suggesting that this is a remote possibility and noting that butchering infected animals in the wild in Africa has certainly spread the virus. However, solely eating meat as a means of infection hasn't been reported, and cooking likely destroys any risk (similar to influenza viruses). Like influenza virus, Ebola doesn't seem to survive long in most environments, but it's also noted that differences in African food storage (with little refrigeration) versus more typical cold storage may affect that as a risk factor, possibly prolonging the life of the virus when held in the cold. I think foodborne transmission is unlikely, but it can't be completely ruled out right now.

Because of the respiratory symptoms, does this mean Ebola could enter the population via meat from animals that farmers don't consider very ill, or put butchers at a heightened risk of infection during slaughter? This to me is more concerning than simple foodborne transmission. With Reston, at least no human symptoms have been observed, but if pigs (and potentially other animals?) can present with Ebola Zaire as a rather generic respiratory infection...well, that could spell trouble in a lot of different ways. It means that telling individuals to simply avoid sick-looking primates (and bats) is going to be even more woefully inadequate than it already is. Plus, it raises the remote-but-not-completely-outside-the-realm-of-possibility of someone intentionally spreading the virus via animals that are infected in this manner.

Science fiction? Maybe. Probably. Hopefully. But this research opens the door on many new lines of investigation and once again, raises even more questions.

Kobinger GP, Leung A, Neufeld J, Richardson JS, Falzarano D, Smith G, Tierney K, Patel A, & Weingartl HM (2011). Replication, Pathogenicity, Shedding, and Transmission of Zaire ebolavirus in Pigs. The Journal of infectious diseases PMID: 21571728