It’s now fairly certain that humans have returned the favor and given H1N1/2009 to pigs, another example of a cozy flu swapping relationship now almost a century old. At the time of the 1918 pandemic, pigs were also suffering a serious influenza-like illness which was quickly dubbed “hog flu.” The pigs got sick suddenly with fever and respiratory symptoms but recovered quickly and mortality was fairly low. The same thing occurred elsewhere (Europe, China) during and after the pandemic, so it wasn’t hard to make a connection between the “Spanish flu” and “hog flu.” But in 1918 it wasn’t known that influenza was a disease caused by a virus. It wasn’t until the 1930s that swine influenza was linked to human influenza virus, both the ones circulating in the 1930s and the 1918 virus. For a long time it was thought (and I learned in medical school) that the 1918 pandemic was caused when the virus jumped from pigs to humans. It is only recently, with advanced methods for analyzing the family tree of viruses, that there is good evidence we got the direction wrong: the virus came originally from birds to humans and we then gave it to pigs (Taubenberger et al.; Vana and Westover).
And now it looks like we’ve done it again:
A farmhand who travelled to Mexico and fell ill upon his return apparently infected the pigs with the H1N1 influenza virus, said David Butler-Jones, Canada’s chief public health officer.
?So far, basically what we’re seeing in the pig is the same strain as we see in the humans,? Mr. Butler-Jones said.
?The concern is that if it’s circulating in a pig herd, that any other humans that come onto the farm might be exposed and be at risk.?
The farm worker returned to Canada from Mexico on April 12 and had contact with the pigs two days later. About 220 pigs in the herd of 2,200 began showing signs of the flu on April 24, said the country’s top veterinary officer, Brian Evans of the Canadian Food Inspection Agency.
All of the pigs are recovering or have recovered and the farm worker has also recovered.
One other farm worker subsequently fell ill. It’s not yet known if that person caught the swine flu. (Steve Rennie, Helen Branswell, Bob Weber, Canadian Press via The Globe and Mail)
At the outset we didn’t know how easy it would be for this to happen, but it shouldn’t come as a total surprise. The H1N1/2009 (aka swine flu) has internal genetic segments that are all swine origin, although two of them bear marks of human and avian ancestry. Most Americans also have ancestry from elsewhere at some points in their family trees and even Native Americans belonged to different natural groups (“tribes,” now called “nations”). That doesn’t prevent any of us from being “American” in almost all respects. We are are adapted to the US cultural and economic environment, whatever our ancestry. Maybe a better analogy, though, would be the game of basketball. This is a worldwide sport that operates by essentially the same rules everywhere. But the styles can be quite different in US colleges, the professional leagues (men’s and women’s), secondary schools, Western and Eastern Europe, China, etc. Is a US highschool basketball star interchangeable with a Chinese basketball player? Each is adapted to the style, skill level and specific conditions of his team of origin. Each plays well with his or her original team mates but there’s no guarantee they will play with another team from a far away place, even if the rules are the same.
Now a swine team is playing in a human environment. They played well together in pigs and, with some relatively small adjustments, seem to have made the transition to humans, too. Where and when they learned to play in the human environment we don’t know. But they seem to have retained their ability to play in pigs, too, so maybe it was quite recent.
In the meantime the hog industry has a big problem, mainly economic. The presence of the virus in a country’s pigs could affect exports as other countries ban imports from infected areas; productivity, since the pigs get sick; and public relations, since the animal will be stigmatized. It is less of a public health problem because the virus is already circulating among humans. We get it from each other, not pigs. Even if pigs are infected, cooking the pork kills the virus. While contact with raw pork during preparation is a theoretically possible mode of transmission it is unlikely and is a marginal issue in the big picture.
A tangled web and once again, it is humans who are a danger to pigs, not the other way around. We retain our title as one of the most dangerous species in the biosphere.