Imagine hearing about a bird flu virus that’s causing big problems and has even spread to some farm animals. This virus, called H5N1, has led to millions of birds being killed to stop it from spreading. But it hasn’t stopped there—in 2024, over a dozen farmworkers got infected, with the latest cases happening in Colorado. To understand what’s going on with H5N1, scientists are looking into its past, how it spreads, and ways to keep it under control. Jenna Guthmiller, an immunologist and microbiologist at the University of Colorado, shares what we know about this virus.
H5N1 is a type of influenza A virus, which is the same kind of virus that causes common flu outbreaks. But unlike typical flu viruses, H5N1 mainly targets wild birds, especially waterbirds like ducks and geese. Scientists first found H5N1 in 1959 when it caused a huge outbreak in chickens in Scotland. Since then, the virus has continued to affect birds and occasionally infects people too.
In 1996, researchers realized that waterbirds were the main carriers of H5N1, and since then, there have been four big outbreaks. The current outbreak, which began in 2021, is spreading across the world through migratory birds, reaching six continents and causing millions of domestic birds to be killed to prevent it from spreading.
This bird virus hasn’t only affected birds—it’s now been found in other animals like seals, minks, and even cows. For example, in 2024, the U.S. Department of Agriculture (USDA) found H5N1 in dairy cows. By August, there were 192 infected herds across 13 states. In cows, H5N1 mainly infects the mammary glands (where milk is made) rather than the lungs, which is unusual for flu viruses. Scientists think it spreads through contaminated milking equipment.
From May to July 2024, 13 people got sick from working with infected animals, including dairy cows. It’s still a mystery why H5N1 is spreading so widely, but we know it’s mutating quickly to infect new hosts. Scientists believe a protein on the virus, called hemagglutinin, is changing in a way that helps it infect more animals, including mammals like cows. They even found a specific mutation in this protein that makes it easier for H5N1 to attach to cells in cows.
To slow the spread of this virus, the USDA has set up new testing for cows being transported between states. Colorado, where there are the most infected herds, now requires weekly tests on farms. Even though H5N1 doesn’t spread from person to person, people who work around infected animals are at risk. H5N1 symptoms are similar to a cold, like sore throat, tiredness, and red eyes, and the CDC has resources on bird flu safety.
Animals living near infected farms, like barn cats that drink raw milk, are also at risk. Sadly, some of these cats have died from H5N1. The virus can also spread to other farm animals close to infected cows. To protect themselves, workers on these farms should wear safety gear like gloves and goggles, and they should avoid using the same clothes and boots on different farms.
But there’s good news for consumers — drinking pasteurized milk is completely safe. The pasteurization process kills any potential H5N1 virus in milk, so milk products remain safe for everyone to enjoy.