There is an old vaudeville joke where a man goes to the doctor complaining about pain in his arm:
Doctor: Have you ever had it before?
Man: Yes, once before.
Doctor: Well, you have it again.
CDC reported on their weekly FluView website on Friday that the Iowa Department of Public Health (IDPH) reported to CDC that in September a boy (age not stated) had a flu like illness from which he fully recovered and for which he hadn’t required hospitalization. In November IDPH determined it was swine flu, but not the pandemic H1N1 but a swine-origin H3N2. According to CDC there was “no clear exposure” to swine, nor was there any evidence of sustained transmission. I checked the IDPH website and couldn’t find any mention of this case so this is all the information we have. It sounds like CDC doesn’t have much information, either. Ace flu reporter Helen Branswell was only able to get this out of CDC spokesperson, Tom Skinner:
He said the fact that the investigation turned up no other cases and that some time has since elapsed suggests there isn’t any ongoing spread. “I think if there was other transmission going on associated with this case we would have picked it up and we haven’t.” (Helen Branswell, Canadian Press; hat tip Mike Coston at Avian Flu Diary and Crof at H5N1))
Well, maybe. This wasn’t caught by the CDC NREVSS virologic surveillance system and was only noticed 2 months after the fact by public health officials in Iowa. We don’t know exactly what IDPH and/or CDC did as a follow-up investigation, but the old saw “absence of evidence is not evidence of absence” springs immediately to mind. If this child really had no exposure to pigs (the phrase “no clear evidence” is ambiguous), then there was an intermediary host or vector between the pigs and the case.
Like the pandemic swine H1N1, which has a seasonal counterpart H1N1, this swine-origin H3N2 has a seasonal H3N2 counterpart. The big question for me is how transmissible is this virus? Lack of further documented cases isn’t very persuasive when it sounds like they haven’t been able to look very easily and the original case was not picked up by the virologic surveillance system. More details on how this was discovered would be useful. Let’s suppose, though, that this virus did dead end in the child, that is, that there was no onward transmission to others. If it didn’t come from a pig, where did it come from and how easily did this boy get infected? September was when pandemic H1N1 was being actively transmitted and circulating in the community, so one wonders what would have happened if this swine H3N2 had appeared 6 months earlier, in virgin soil, without any competition from H1N1. We still don’t understand the mechanism whereby one subtype “crowds out” another, but we know it doesn’t always happen. Is this one just hanging around out there at low levels, waiting for the right conditions to come out of hiding? Unfortunately we can’t tell from the genetic analysis whether this, or any, flu virus is easily transmissible.
More generally, this is just another reason for urgent investment in human and animal surveillance for influenza and other zoonotic diseases (a zoonotic disease is one humans get from animals). Pigs and birds are natural reservoirs for influenza virus, but we haven’t looked very hard in other species, especially livestock.
Maybe this is just another of the sporadic swine to human cases that have been reported in recent years. Of them, H1N1 swine flu is the only one that seems to have gotten a foothold in the population. It’s hard to imagine it’s the only one capable of it, though, and since we understand very little of what allows easy transmissibility, surveillance is our only early warning recourse.