|Feral Swine - Credit USDA|
While scientists monitor commercial swine herds for signs of influenza evolution, relatively little is known about the carriage of influenza A viruses by feral swine in the United States, and around the globe.
With U.S. feral hog populations tripling over the past 25 years (now est. at 6 million ) scattered across 38 states, these invasive and incredibly destructive creatures are expected to expand to all 50 states in the next few decades (see 2017 study Interpreting and predicting the spread of invasive wild pigs).
Swine, as we've discussed often, can be infected by many types of influenza (including swine, human and avian strains), and may be co-infected by two or more viruses at the same time. This susceptibility - along with influenza's ability to reassort into new subtypes - makes pigs excellent `mixing vessels' for influenza.
Despite less-than-robust surveillance of commercial swine around the globe, over the past couple of years we've seen a number of new `reassortant' viruses detected in pigs. A few recent blogs include:
I&ORV: Triple-Reassortant Novel H3 Virus of Human/Swine Origin Established In Danish Pigs
Emerg. Microbes & Inf.: Pathogenicity & Transmission Of A Swine Influenza A(H6N6) Virus - China
Front. Microbiol.: A Novel H1N2 Reassorted Influenza Virus In Chinese Pigs
Compared to commercial swine, our understanding of what is going on with influenza in feral pigs is almost non-existent. We have seen a handful of studies over the years, including a 2008 survey in the Journal of Wildlife Diseases (see Influenza exposure in United States feral swine populations.) which found:
Of the 271 IAV-positive samples, 236 cross-reacted with swine IAVs, 1 with avian IAVs, and 16 with avian and swine IAVs, indicating that feral swine were exposed to both swine and avian IAVs but predominantly to swine IAVs. Our findings suggest that feral swine could potentially be infected with both avian and swine IAVs, generating novel IAVs by hosting and reassorting IAVs from wild birds and domestic swine and facilitating adaptation of avian IAVs to other hosts, including humans, before their spillover.A 2013 EID Journal Dispatch (see Influenza A Subtype H3 Viruses in Feral Swine, United States, 2011–2012), which from 1,989 serum samples collected from 31 states, found that 182 samples were IAV (Influenza A Virus) positive. They concluded:
Our study demonstrated that subtype H3N2 IAVs are periodically infecting feral swine in the United States. Feral swine are a potential source of IAVs with bidirectional transmission to domestic swine or humans. Detection of an H3N2v-like IAV in the feral swine population demonstrates a potential threat to human health. Continued surveillance is recommended to monitor the distribution and the genomic and antigenic diversities of IAVs in feral swine to better assess the risk.All of which serves as prelude to a new study, published last week in the Journal of Applied Environmental Microbiology, that finds U.S. feral hogs have been exposed to a variety of influenza A viruses, including avian subtypes.
US feral swine were exposed to both avian and swine influenza A viruses(Continue . . . )
Brigitte E. Martina, Hailiang Suna, Margaret Carrelb, Fred L. Cunninghamc, John A. Barochd, Katie C. Hanson-Dorrc, Sean G. Younge, Brandon Schmitd, Jacqueline M. Noltingf, Kyoung-Jin Yoong, Mark W Lutmanh, Kerri Pedersenh, Kelly Lageri, Andrew S. Bowmanf, Richard D. Slemonsf, David R. Smithj, Thomas DeLibertod* and Xiu-Feng Wana*
Influenza A viruses (IAVs) in swine can cause sporadic infections and pandemic outbreaks among humans, but how avian IAV emerges in swine is still unclear. Unlike domestic swine, feral swine are free ranging and have many opportunities for IAV exposure through contacts with various habitats and animals, including migratory waterfowl, a natural reservoir for IAVs.
During 2010--2013, 8,239 serum samples were collected from feral swine across 35 US states and tested against 45 contemporary antigenic variants of avian, swine, and human IAVs; of these, 406 (4.9%) samples were IAV-antibody positive. Among 294 serum samples selected for antigenic characterization, 271 cross-reacted with ≥1 testing virus whereas the other 23 did not cross-react with any testing virus. Of the 271 IAV-positive samples, 236 cross-reacted with swine IAVs, 1 with avian IAVs, and 16 with avian and swine IAVs, indicating that feral swine were exposed to both swine and avian IAVs but predominantly to swine IAVs.
Our findings suggest that feral swine could potentially be infected with both avian and swine IAVs, generating novel IAVs by hosting and reassorting IAVs from wild birds and domestic swine and facilitating adaptation of avian IAVs to other hosts, including humans, before their spillover. Continued surveillance to monitor the distribution and antigenic diversities of IAVs in feral swine is necessary to increase our understanding of the natural history of IAVs.
While agricultural exhibitions at state and county fairs make up the vast majority of the swine-human contact in the United States (followed by commercial farms and abattoirs), feral pigs - which are hunted for meat, sport, and animal control - are increasingly in contact with humans.
Being omnivorous, and free ranging, they also have significant opportunities for contact with IAV infected birds.While the odds of that happening may seem long here in the United States, it was only 7 months ago we were following an outbreak of a rarely reported avian H7N2 virus among hundreds of cats (and 1 veterinarian) across several New York City animal shelters (see J. Virology: Virulence Of A Novel H7N2 Virus Isolated From Cats In NYC - Dec 2016).
The H7N2 virus was likely acquired when an unlucky feline dined on the wrong bird in what is arguably the most urban area of the United States.It that can happen in mid-winter in Manhattan, then it isn't that much of a stretch to imagine it happening among feral hogs in Texas, Florida, or Tennessee.
Just one of the many reasons why I never bet against flu.