Although HPAI H5 remains primarily a threat to poultry and wild birds, last week the United States recorded its first probable human infection (see Colorado Investigating 1st Human HPAI H5 Infection In the United States) in a work-release inmate at a Colorado prison who was engaged in culling of infected birds.
While this reportedly resulted in only a mild illness - unlike the Asian H5Nx virus which has infected roughly 900 people, killing about half over the past 2 decades - until a little over a year ago, this European lineage of HPAI H5 had never shown any affinity for human hosts.
This doesn't put our HPAI H5 virus in the same league as the Asian H5N1 and H5N6 viruses, but it does command a bit more respect than it used to. Hence the release on Saturday of a CDC HAN Advisory On Human H5 Infection In The United States advising clinicians and public health officials on how to investigate and respond to suspected infections.
Since it first arrived in the United States in mid-January, likely carried in (via Canada) by migratory birds from Europe using the North Atlantic Flyway (see Preprint: Transatlantic Spread of HPAI H5N1 by Wild Birds from Europe to North America in 2021), the virus has spread westward across the continent with remarkable speed.
Compared to the 2015 North American Avian Epizootic (see map below), this outbreak has spread to more than twice as many states, and unlike the last time (see PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl), this virus may establish a foothold on this continent.
Over the weekend the USDA announced that 3 more states (Alaska, Oklahoma & Vermont) have confirmed HPAI H5 in either commercial or backyard flocks.
While we don't know what will happen over the summer, a 2016 study (see Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1), suggests that waterfowl pick up new HPAI viruses in the spring (likely from poultry or terrestrial birds) on their northbound trip to their summer breeding spots - where they spread and potentially evolve - and then redistribute them on their southbound journey the following fall.
We've seen this sort of pattern primarily with birds that overwinter in Siberia, and then migrate to Europe, the Middle East, East Asia, and Africa each fall.
Since 2016, the Eurasian HPAI H5 virus has expanded its avian host range and has become better suited for carriage by migratory birds, which has made for more frequent, and increasingly larger, outbreaks in Europe.
While we've been lucky not to have seen this pattern (yet) in North America, past performance is no guarantee of future results.