Last Friday, we reported on Egypt's recent attempt to curb transmission of the human H1N1 epidemic by butchering all 300,000 of its pigs. Experts we interviewed said there was no sound rationale for such a move, because pigs had never been infected with the new virus, which has sickened at least 1085 people in 21 countries – until now.

The Canadian Food Inspection Agency (CFIA) recently announced that a herd of pigs in Alberta might have caught the new virus from a Canadian who had recently spent time in Mexico, ground zero for the current epidemic. Fortunately, both man and pigs have recovered or are in the process getting better, but the incident raises a new question: do pigs now pose a threat to humans?

Rodney Baker, a senior clinician at the Iowa State University College of Veterinary Medicine in Ames, is not too worried about this particular herd of sick pigs, because it's small (only about 2,000 animals he estimates, some 200 of which have reportedly fallen ill) and relatively isolated (about six miles, or 10 kilometers from the next farm). He tells that even if the virus were to take root in the broader pig population, odds are it would become more adapted to pigs and less adapted to humans. The likelihood of the virus genetically morphing into a new, more deadly strain capable of jumping from pigs and back into humans is extremely low.

But the virus doesn't need to change to be a threat, says Israel's former chief veterinary officer Arnon Shimshony. Pigs can pass the original virus right back to humans, particularly if they are coughing and sneezing, and thereby shedding virus particles, as media reports suggest, Shimshony tells us. The Canadian government is wise to quarantine these animals, he adds, but notes that he does not have enough information about the case to offer advice on slaughtering them. 

The good news, Baker points out, is that the pigs do not appear to have passed the virus back to humans, as there are no reports of other farm workers getting sick. He suspects the new virus will continue popping up on pig farms, because farm workers in the U.S., many of whom travel back-and-forth from Mexico, will continue to be exposed to the virus. And for that reason, he says, we need a vaccine for the pigs as well as the people.

See our In-Depth Report for more on the swine flu outbreak.