Adding to the “that was not on my bingo card” theme of the 2020s, there was a recent fatal H5N2 avian influenza infection in a person in Mexico.

First important point: this was H5N2, not the H5N1 strain we’ve been focused on for the past few years in bird (and spilling over into mammals, including most recently dairy cows). This H5N2 strain is very different, and this is the first reported case of H5N2 infection in a person. H5N2 has caused a few outbreaks in poultry in Mexico so far this year, so there’s likely a wild bird reservoir, and potential for more spillover into domestic poultry, but this strain hasn’t caused human disease before. When we see rare spillovers of avian flu into people, they’re usually in individuals with close contact with infected poultry, but that wasn’t the case here.

The infected person was a 59-year-old who was hospitalized in Mexico City after a short illness with signs including shortness of breath, fever, diarrhea and malaise. Surprisingly, the person has no known exposure to domestic poultry, wild birds or other animals. The person had actually been bedridden for 3 weeks, so their recent exposures could be fairly well characterized. It’s a bit unnerving not to have no idea from where an infection like this came. It could be a one-off that we never figure out and never see again, but we have to try to investigate as much as we can in case it’s an early harbinger of yet another new flu problem.

What could have been the source of the H5N2 virus in this patient?

1) Another person: This patient could have been an infected by another person (healthy or sick) who got the virus from a bird. That would indicate bird-to-human spillover (which we know happens occasionally) followed by human-to-human transmission (a bit of a concerning scenario, especially if it wasn’t a very close household contact). Seventeen (17) human contacts at the hospital were identified and tested and were all negative, including one that reported having a runny nose 4-5 days after the patient got sick. Twelve other human contacts near the person’s home were also tested and were all negative. Even so, it’s possible one of those people was the original human source but had eliminated the infection by the time they were tested. It’s good to have the testing information (and good that they looked), but it does not rule out a human source.

2) Birds: Despite the patient history, I’m not sure we can completely rule out bird contact. I’d want to know more about where the person lived and how people and animals moved around on the property. For example, was there any chance a bird got into the home at some point? If there were open windows, there’s still a chance of direct or indirect bird exposure.

3) Other animals: As for birds, I’d want to know how solid the “no animal contact” determination was, and if it covered more than just birds. Cats are great potential bridging hosts, since they can be infected when hunting wild birds and then have close contact with people. I’d want to make sure there was clearly no dog/cat contact, and if there had been any sick or dead cats seen in the area.

4) Food: This is lower on the list, but contamination of food would have to be considered. I don’t mean properly cooked meat (even if it came from an infected bird), but more likely contamination of prepared food with bird poop (possibly from someone’s hands) or improperly cook meat from an infected bird. I’d investigate where the patient’s food was prepared; if birds could have gotten into the area, contamination of food is possible.

That’s all speculation, and I hate to do too much of that, but it’s important to brainstorm possible sources to come up with surveillance plans.

Hopefully this was just a one-off infection. However, it shows us the ongoing risks from endemic avian flu viruses (not just H5N1) and the need to support good surveillance systems in people, domestic animals and wild animals. Despite continued emergence of infectious diseases, we’re seeing underfunding and active dismantling of our public and animal health surveillance and response systems, and inadequate willingness to address these big issues.