20% of US Milk Samples Test Positive for Bird Flu—Should We Be Worried?

It began with a mysterious illness among dairy cows in Northern Texas. Some animals lost their appetite while others began to produce less milk. What little milk that was produced came out thick and discolored.

To the surprise of scientists worldwide, the cows tested positive for a flu virus called HPAI (Highly Pathogenic Avian Influenza) H5N1. Prior to the Texas outbreak, cows had only been infected with the virus as part of lab experiments. Now, it was confirmed in 34 dairy herds in 9 states.

The current spread of H5N1 in US dairy cattle. Source: USDA data from 4/27/24

Worse than that, the reported numbers may be a massive undercount. Already, the FDA has found fragments of H5N1 in 1 in 5 US retail milk samples tested for the virus. A different survey of store-bought milk in the Midwest by Ohio State University yielded an even higher positivity rate of almost 40%.

So what is H5N1?

First isolated from geese in Guangdong in 1996, H5N1 is a distant cousin of the seasonal flu virus we catch every year. Unlike our flu, H5N1 is extremely lethal, and has wiped out millions of birds from China to Egypt. Although its host range is mostly avian, the virus is capable of jumping from birds to humans. Of the more than 800 human infections confirmed since 1997, more than half were fatal.

Fortunately, H5N1 is adapted to spread in birds, limiting its ability to spread from person to person. On the other hand, H5N1 is an RNA virus, making its genetic code highly unstable and capable of changing quickly to infect new hosts. If H5N1 does adapt to humans (something other bird flu viruses did several times in the last century), it could trigger a global pandemic.

The good news is that H5N1 faces a few barriers to becoming a full-blown pandemic. Most importantly, H5N1 poorly binds α2,6 sialic acid, a type of receptor in our airways that differs from those in birds. Without this binding, H5N1 cannot effectively enter our airway cells and become highly contagious between people.

How concerned should we be?

It depends on what you’re most concerned about.

On the one hand, while a high proportion of US milk does contain this dangerous virus, almost all milk in the US is pasteurized before hitting the shelves, which is shown by current tests to safely inactivate H5N1. Therefore, while the average American is likely to ingest dead H5N1 fragments amidst the ongoing outbreak, actually getting it from the milk is not a major concern.

However, the fact that enough cows are infected for 20% of US milk and 40% of milk in the Midwest to contain H5N1 is deeply chilling. Cows are mammals, which means that H5N1 will now be under pressure to obtain mammalian adaptations, including the ability to bind to a2,6 receptors. With this many animals infected, we are effectively playing Russian roulette on a nationwide scale. Additionally, there are millions of cows in the US, meaning countless dairy workers could be exposed. So far, one human who worked with cows has already tested positive, and many more cases may be going undetected.

The fact is that we don’t know if H5N1 will ever get all the mutations it needs to cause a human pandemic. However, since a new strain emerged in 2020, it has been infecting more and more species, including both birds and mammals. The current outbreak, while concerning, is but part of a larger trend—in Argentina, H5N1 just killed 96% of southern elephant seal pups, and an earlier outbreak killed thousands of mink on a Spanish farm. Even if humans aren’t being infected in large numbers right now, H5N1 may soon become permanently entrenched in cows, increasing the number of humans exposures and forcing it to adapt to mammals.

In another scenario, H5N1 could spread to pigs, a species scientists frequently call “mixing vessels” because they can carry different flu strains from birds and humans. If H5N1 becomes as widespread in pigs as it did without warning in cows, it may acquire genes from a human-adapted strain and combine the best of both, producing a mutant that spreads like seasonal flu but kills like H5N1.

Whatever happens next, the current outbreak is a reminder that you can never be complacent when it comes to flu viruses.