The Recombinomics website reviews the latest info on the Canadian H5 avian influenza detections in wild birds, and speculates that H5N1 may be in the U.S. already. The crippling final line of the status report is:
"Based on the data from Canada, it seems likely that H5 is well represented in the wild bird population in the US. It is unclear why such infections have not been detected and reported."
While it is probably more likely that the H5 influenza found in Canadian birds is H5N2, which is a fairly common LPAI H5 virus in Minnesota.
The answer is probably...um, because we're not really looking that hard for avian influenza viruses. If we were taking thousands of samples from birds all across the country, who knows what we'd find. The question really is, how badly do we want to really know how widespread these viruses are in wild birds? Is it enough to just wait for sick or dead birds to start turning up in parks and golf courses?
While we wait for the U.S. national surveillance plan, we can only hope that we will be offered a long-term, widespread system to detect and track all avian influenza viruses in wild birds--not just a short-term H5N1 detection sytem that will only tell us, too late, that the a dangerous virus has already arrived.