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After an early fall and winter surge, flu season in the U.S. is finally winding down. But virus experts are not ready to turn their attention away from influenza just yet.
A global outbreak of avian influenza, also known as bird flu, has sickened millions of farmed poultry and wild birds, as well as other animals, including seals and foxes. A few cases have even popped up in humans.
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Here’s what you need to know about the evolving situation, and why it’s grabbing the attention of scientists and public health experts.
What is bird flu?
Just like humans can get infected with influenza, different versions of the virus can infect animals, including birds.
Bird flu is caused by influenza A viruses that spread among wild aquatic birds (ducks, geese, gulls, etc.) and can infect poultry (chickens and turkeys). Some of these strains, like the one behind the current outbreak, cause more severe disease than others, especially in poultry. Since 2022, more than 58 million birds in commercial and backyard flocks in the U.S. have been affected by the currently circulating H5N1 bird flu strain.
Sometimes bird flu can infect other animals, too, as is the case with H5N1, which has been detected in about 144 mammals in 23 states since May 2022, according to the Centers for Disease Control and Prevention (CDC). Other countries have also reported infections in mammals.
Matthew Binnicker, director of clinical virology at the Mayo Clinic, says the infections likely happened when these mammals — seals, foxes, minks, etc. — either ate infected birds or encountered a carcass. Mammals exposed to environments with a high concentration of the virus are also susceptible to infection, Tim Uyeki, M.D., the chief medical officer of the CDC’s Influenza Division, said in a recent “Ask the Expert” post.
This spillover has caused some concern among health experts, since mammals “are closer in terms of their cellular properties to humans,” Binnicker says. “And what that tells us is that the virus may be adapting or changing enough where it can now cause infection and disease in these mammalian populations, so it’s just a step closer to adapting to the point where it could readily infect human cells.”