Pigs at risk?

Pigs at risk?

A new, and potentially important wrinkle in the H1N1 story emerged over the weekend.  It concerns a pig farmer in Vancouver who returned from Mexico in the second week of April, resumed his normal work routine, and may have infected the swine herd he tends with H1N1.  Reports suggest that 200 or so pigs, out of a herd of 2200, exhibited “loss of appetite or fever”, and the source of the pig’s malaise has been traced to H1N1 infection — a strain identical to the one carried by the farm worker.

This story, if correct, has important implications.  First, it suggests that this influenza strain may have an impact on pig farming and pork production, with the attendant risks of economic and trade disruption.  More subtly, this story hints at the possibility that this H1N1 strain is capable of moving back and forth from pigs to humans and back again with relative ease.  As we discussed in a previous post, the transmission of influenza strains from non-human animal reservoirs to the human population is generally a rare, low probability event.  Similarly, the return trip should also be a unlikely occurrence.  Except, it appears, in this case.

The transfer of influenza strains from one species to another is usually difficult because viruses evolve in response (among other things) to the immune system of their host.  A flu strain shaped by the pig’s immune system is generally not ideally suited to evade the human immune system (although it will quickly begin to evolve in that direction once it gains a foothold in humans).  Conversely, a strain shaped by our immune system, as this H1N1 now is, should have a difficult time upon re-encountering the pig’s immune response.  The same applies to the ability of the virus to invade host cells: if you are good at gaining entry into the cells of a pig, you are probably quite poor at entering human cells.

Influenza strains, in short, usually live and die by the old adage “jack of all trades, master of none”.  But if the report about the Vancouver pig farmer is correct, we may be dealing with a rare exception. This H1N1 strain may have figured out how to remain camouflaged from both pig and human immune responses, and may be carrying the tools to gain entry in to both human and pig cells.  Such a strain deserves our attention, and, if it can indeed move back and forth between humans and pigs, will complicate our efforts to contain its spread.