Could the Outbreak Have Started in Canada?

Jon is a staff writer for Science.

In keeping with the Canadian government’s apparently mistaken hypothesis that the origin of the swine flu outbreak likely had nothing to do with Canadian pigs, what if it did?

On 2 May, the Canadian Food Inspection Agency had this to say about the A (H1N1) outbreak in an Alberta pig herd: “It is highly probable that the pigs were exposed to the virus from a Canadian who had recently returned from Mexico and had been exhibiting flu-like symptoms.” Now it turns out that the farm worker tested negative for the virus.

So what if it was the other way around—that the virus originated in Canadian pigs and infected a human who then traveled to Mexico?

Looking at the epidemiological numbers today, it’s possible that the outbreak started in Canadian pigs.

Canada has 165 confirmed human cases out of a population of 33 million people. The United States, which arguably has similar testing capabilities, has 642 confirmed cases and a population of 304 million. So Canada has 2.3 times as many cases per million population as the United States. Mexico, in comparison, has 822 cases out of a population of 109 million, which means it has 3.5 times as many cases as the United States. Canada has reported that the virus infected 220 pigs, and no other country has yet found it in swine.

Case numbers, of course, are affected by the numbers of samples tested and the capability of the country’s labs, but epidemiologists listen to the best data they have at the moment, and that’s what the numbers are saying right now.

Back to my Canada-human-Mexico speculation. What if that hypothetical carrier infected Mexicans in a rural area—maybe he was a pig trader—and the virus then spread to Mexico City, which has a population of approximately 20 million, much larger than that of any Canadian city? An infected Mexican or Canadian might then have traveled to the United States and started the outbreak here.

A fine line, of course, separates speculation—a dirty word in science—from hypothesis.

And although this what-if may not be, as they say, “highly probable,” it’s as possible as many of the more-accepted theses making the rounds in scientific and public health circles alike.

Posted in Health