“No clear human-to-human transmission of H5N1 has been documented yet, but this feels the closest to an H5 pandemic that I’ve seen.”
Louise Moncia, avian influenza specialist, University of Pennsylvania
“Predicting pandemics is a little like predicting tornadoes.”
Dr. Richard Webby, director, World Health Organization Collaborating Center for the Ecology of Influenza in Animals
Over 140 different infections emerged during my almost 50 years of practice as an infectious diseases specialist. Thankfully, the emergence of only two pathogens—HIV and SARS-CoV-2—precipitated “black swan events,” that is, exceedingly rare, unpredictable events with enormous impacts. (The HIV/AID pandemic has killed over 42 million people since 1981 and the COVID-19 pandemic has killed more than 7 million people since December 2019.) No one saw these pandemics coming.
The zoonotic infection influenza A virus H5N1 (bird flu) isn’t a “black swan event”; it has long been on the radar screens of infectious diseases specialists and other scientists. Fortunately, to date, it has caused only a single human death—a 65-year-old man in Louisiana who died on January 6, 2025. H5N1’s devastation to animals other than humans, however, boggles the mind. Bird flu therefore is again the topic of a Germ Gems post. (This is my fourth Germ Gems post on bird flu since May 4, 2022, when I posted “Bird Flu: Why We Should Care?”)
What is avian (bird) flu: a recap? Avian influenza, or bird flu, is a broad term that refers to several types of influenza that normally infect birds. Wild waterfowl, including ducks, geese, and swans (yes, even black ones) are highly susceptible to infection. Although bird flu viruses primarily prey on birds, they can also spread to other animals, including humans. Human infections with bird flu viruses are rare, and they’re usually what scientists call dead-end infections because they don’t typically transmit from person-to-person.
Some flu viruses carried by birds cause only mild infections and are classified as low-pathogenic viruses. In contrast, H5N1 is classified as a highly pathogenic avian influenza as it often kills the birds that are infected by it.
Since 2022, H5N1 has killed more than 130 million wild and farmed birds in America across all 50 states. As of January 6, 2025, there have been 66 confirmed human cases of H5N1 bird flu in the U.S., and, as already was mentioned, the only death so far was a 65 year-old Louisiana man with underlying health conditions who had exposure to backyard poultry.
When H5N1 entered North America in late 2021, it quickly caught scientists’ attention because it had broadened its repertoire, spreading beyond birds and infecting a growing variety of mammals. In the current wave of infections, H5N1 has spread to at least 26 countries and to more than 48 species, including foxes, bears, and seals, as well as domestic animals like cats and dogs, and an array of zoo animals. Its spread to 919 dairy herds in the U.S. caused substantial turmoil in the agricultural industry. In February 2024, Dr. Jeremy Farrar, chief scientist at the World Health Organization, called the ongoing spread of H5N1 “a pandemic of animals.”
Dr. Robert Webster, a New Zealand virologist, is credited with being the first to understand that avian flu viruses come from the guts of birds. (See, “Bird poop may be the key to stopping the next flu pandemic. Here’s why.”, CNN, December 27, 2024.) Whether bird poop is the key or not, it is clear that containing if not defeating H5N1 bird flu is a “all hands on deck” emergency calling for multiprofessional research and interventions. Here are several questions the experts are trying to answer.
Why is H5N1 bird flu so mild in humans? Historically, bird flu viruses have caused up to 30% mortality in humans but human infections caused by H5N1 have been surprisingly mild. Characterizing the virulence factors of flu viruses, in general, may provide an answer to this scientific mystery.
How can you prevent getting infected? For the moment, avoiding handling or contact with birds and bird feathers plus vigorous handwashing are essential. Drinking raw cow’s milk should already be on your “never do this” list. A vaccine for bird flu exists, and experts have urged the government to vaccinate farm workers. The decision whether to authorize further development of a human vaccine for H5N1, however, is likely to fall under the purview of soon-to-be president Donald Trump.
How do you know if you’re infected with H5N1? Bird flu in humans causes the same symptoms and signs as other types of seasonal influenza with the exception that conjunctivitis (“pink eye”) seems more prevalent in H5N1 flu. H5N1 is a member of the influenza A virus family, and test kits aimed at detecting influenza A virus should work. But improved testing for H5N1 is being urged. (See “We’re Flying Blind’ on H5N1: Experts See Repeat of CDC’s Missteps on COVID Testing” KFF Health News, June 20, 2024.)
Will H5N1 cause the next human pandemic? This, of course, is the proverbial “$64,000 question” that no one can answer. We do know, however, that the virus is only a single mutation away from its ability to be readily transmitted from person-to-person. Also worrisome is that samples of the virus that infected the man in Louisiana who was killed by H5N1 showed signs of adaptation to human cells.
We can also be fairly certain that if such a viral mutant becomes established, we will indeed see the emergence of the next human pandemic. And if this happens, we can only hope that public health experts and scientists can get on top of the H5N1 pandemic strain well before it kills millions of people as did the influenza A virus that caused the 1918-1919 pandemic. Thus the answer to the most crucial question isn’t black or white, it's grey—rather like cygnets (baby swans) that are grey when they are born.
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