The third and most worrisome reason that this virus is getting so much press is that H5N1 now seems to be transmitting well between individuals of at least one mammalian species. In late 2022, mammal-to-mammal spread occurred in Spain in farmed minks. H5N1 spread very efficiently between the minks and caused clinical signs of illness and death in the mink populations where it was detected.

Sea lions in Peru are also succumbing to H5N1 virus in massive numbers. It hasn’t been confirmed definitively whether the sea lions are spreading the virus to each other or are contracting it from birds or H5N1-infected water.

Here’s the key question: If H5N1 can achieve spread in minks and possibly sea lions, why not humans? We are also mammals. It is true that the farmed minks were confined in close quarters, like chickens on a poultry farm, so that may have contributed. But humans also live in high densities in many cities around the world, providing the virus similar tinder should a human-compatible variant arise.

Birds experience influenza as a gastrointestinal infection and spread flu predominantly through defecating in water. By contrast, humans experience influenza as a respiratory infection and spread it by breathing and coughing.

Over the centuries, some of these avian influenza viruses have been passed from birds to humans and other mammalian species, although this is a relatively rare event.

This is because bird influenza viruses must mutate in several ways to infect mammals efficiently. The most important mutational changes affect the tissue tropism of the virus – its ability to infect a specific part of the body.

Avian flu viruses have evolved to infect cells of the intestine, while human flu viruses have evolved to infect cells of the respiratory tract. However, sometimes a flu virus can acquire mutations that allow it to infect cells in a different part of the body.

Which cells influenza infects is partially dictated by the specific receptor that it binds. Receptors are the molecules on the surface of host cells that a virus exploits to enter those cells. Once viruses are in cells, they may be able to produce copies of themselves, at which point an infection has been achieved.

Both human and bird influenza viruses use receptors called sialic acids that are common on the surfaces of cells. Bird influenza viruses, such as H5N1, use a version called α2,3-linked sialic acid, while human flu viruses use α2,6-linked sialic acid – the predominant variant in the human upper respiratory tract. Thus, to become efficient at infecting humans, H5N1 would likely need to mutate to use α2,6-linked sialic acid as its receptor.

This is a concern because studies have shown that only one or two mutations in the viral genome are enough to switch receptor binding from α2,3-linked sialic acid to the human α2,6-linked sialic acid. That doesn’t seem like much of a genetic obstacle.

With avian influenza viruses, it is not possible to make effective human vaccines in advance, because we don’t know exactly what the genetics of the virus will be if it starts to spread well in humans. Remember that the seasonal flu vaccine must be remade every year, even though the general types of flu viruses that it protects against are the same, because the specific genetic variants that affect humans change from year to year.

Right now, the best way people can protect themselves from H5N1 is to avoid contact with infected birds. For more information about prevention, especially for people who keep domesticated birds or are bird-watching hobbyists, the Centers for Disease Control has a list of guidelines for avoiding H5N1 and other bird flu viruses.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. Like this article? Subscribe to our weekly newsletter.

Read more:
What is spillover? Bird flu outbreak underscores need for early detection to prevent the next big pandemic

Deer, mink and hyenas have caught COVID-19 – animal virologists explain how to find the coronavirus in animals and why humans need to worry

Sara Sawyer is a co-founder of Darwin Biosciences. She receives funding from the National Institutes of Health.

Emma Worden-Sapper and Sharon Wu do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.