Bird flu in cows — and now in milk. How worried should we be?

Bird flu is, famously, a disease that infects birds. Yet in recent months, it’s been spreading to all kinds of other animals — including dairy cows. In the last month, government officials detected avian influenza in more than 30 herds across eight US states, raising concerns about the safety of cow’s milk and the potential for the virus spreading in humans. Federal regulations say that any milk from sick cows must be tossed before it reaches the grocery store. But on Tuesday, the US Food and Drug Administration (FDA) said it detected evidence of avian influenza in commercial, pasteurized milk, suggesting bird flu has infiltrated the dairy supply. This doesn’t mean that you’ll get sick from your morning cereal. Here’s what to know about your risk. --- What you’ll learn * Why it’s spreading from birds to cows * Whether it’s safe to drink milk * If humans can get sick from bird flu * Where bird flu comes from --- Does this mean milk is unsafe? The short answer is that there’s still uncertainty, but it is unlikely that you’ll get sick from drinking store-bought, pasteurized cow milk. The FDA and other government agencies said Wednesday that the pasteurization process should render the virus harmless in commercial milk, though officials still have more testing to do. “Our commercial milk supply is safe,” Donald Prater, acting director of the Center for Food Safety and Applied Nutrition at the FDA, said on a press call Wednesday. Independent experts tend to agree. Still, the news raises concerns about what risk bird flu will ultimately pose to humans, especially given the recent detection of influenza in a Texas farmworker. The patient — who was recovering swiftly after infection, sustaining only eye inflammation — was the second person in the US to contract avian influenza. If these people can catch avian flu, what risk does everyone else face? Since the initial outbreaks on poultry farms in 2023, which sent the price of eggs soaring, bird flu has flared up only sporadically among farmed birds. Yet the virus has continued to spread — to different places and to different kinds of animals, including both wild and domestic mammals. Early in 2023, the virus killed thousands of sea lions. This spring, meanwhile, officials found bird flu in Minnesota goats and among 33 dairy cattle herds in eight states (bird flu has likely been spreading among cattle for weeks or months before it was detected). “Although we don’t yet completely understand the factors that led to infection of the dairy cows with HPAI, it is possible that unique viral properties combined with epidemiological and environmental conditions all played a role for the spillover of HPAI into cattle,” said Diego Diel, an associate professor at Cornell University’s college of veterinary medicine. What makes cow-borne bird flu troubling is the obvious fact that humans, too, are mammals. As the virus makes the jump from birds to cows and other mammals, what’s stopping it from jumping to all of us? In the last few years, a couple dozen people worldwide — mostly in parts of Asia — have tested positive for this kind of bird flu, all of which likely came into close contact with infected animals. Symptoms may be nonexistent or subtle, such as eye redness or a mild flu, or they may show up as a fever (above 100 degrees Fahrenheit) and pneumonia and can prove fatal. In the last two decades, more than 800 people have been infected with bird flu globally, and more than half of them died. In the US, however, human infections — let alone deaths — are rare. The first human case of avian influenza from the current outbreak was in 2022, when a Colorado farmer who was involved in killing chickens tested positive. The man said he was tired for a few days but quickly recovered. The person in Texas, meanwhile, tested positive earlier this year, after he was exposed to dairy cattle that were “presumed to be infected” with highly pathogenic bird flu, per the CDC. The patient is taking an antiviral medication and recovering, the agency said. As for the rest of us? First, the good news: The bird flu that infected the farmworker earlier this month in Texas is almost certainly incapable of spreading dangerous infections to other people around the planet, experts say. Simply put, the virus currently does not have the proper biological machinery to easily invade — let alone circulate among — humans. We have very different physiologies than birds and other animals that HPAI readily infects. These workers got sick because they were likely exposed to very high levels of virus. (If you want to go deeper, check out our earlier explainer on this.) The bottom line: The chances of you getting bird flu anytime soon are extremely slim. Bird flu will continue to evolve That said, influenza viruses evolve quickly and unpredictably. Not only do they mutate, but they can also swap entire portions of their genomes with other viruses if two or more of them infect the same host at one time. Under the right circumstances, these changes could give the virus the tools to replicate more easily in mammals, which could make future strains of bird flu more threatening to humans. Scientists are now racing to figure out what, exactly, those circumstances might be. More specifically, they’re looking for signs that the virus is picking up adaptations that make it more likely to replicate within mammalian cells and — critically — to spread from mammal to mammal, such as cow to cow or seal to seal. “There are many unanswered questions about the current HPAI outbreak in cattle, but one of the key questions is whether the virus is being transmitted from cow to cow,” Diel said. A recent study suggests that the virus has adapted to marine mammals and may be able to spread among them. The virus also appears capable of spreading among cows within the same herd, Mike Watson, who leads the US Department of Agriculture’s Animal and Plant Health Inspection Service, said on a call with reporters this week. Still, Watson said, “we have not found changes in the virus that will make it more transmissible to humans.” Obviously it’s not great that another human tested positive for bird flu, or that the virus is showing up in commercial milk. But we’re not on the cusp of (or even close to) another pandemic. This story, however, is not yet over. Scientists are still trying to figure out how the virus is changing and what it means that cows are getting sick. The virus could evolve to become a risk — especially to farm workers and people who work with wild animals — underscoring the importance of monitoring the outbreak closely. How did this all start? Avian influenza, or bird flu, is a general term for the disease caused by influenza viruses that primarily infect and spread among poultry and some wild birds. They’re distinct from viruses that cause the flu in humans, though they’re related. These avian viruses are quite common in the wild. They’ve been circulating for eons among waterfowl, such as ducks and geese, without causing them much harm. Mild forms of infection are called “low-pathogenic avian influenza,” or LPAI, which means they’re typically not deadly to wildlife. The type of avian influenza spreading today is different. Occasionally, a low-pathogenic virus can jump from wild birds to birds on poultry farms. As the virus replicates in densely packed warehouses of farmed birds, it can quickly evolve and adapt in ways that make it highly deadly to poultry. At that point, it gets dubbed a “highly pathogenic avian influenza virus,” or HPAI virus. That’s what’s circulating today — an HPAI virus. The specific version of the virus is called H5N1. Highly pathogenic bird flu has caused outbreaks on poultry farms many times in the past that have killed thousands to millions of farm birds. But what makes this particular form of avian influenza virus so unusual is that it easily spreads and causes severe disease among wild birds and an increasing number of mammals. That’s rare, leading some experts to say that we’re in a new era of bird flu. --- Our world, explained What questions do you want us to answer? Tell us by filling out this form. --- The strain of avian influenza currently spreading descended from a virus that caused an outbreak on a goose farm in Guangdong, China, in 1996. The US first experienced one of these goose farm virus outbreaks in 2014 and 2015, which caused the death and culling of tens of millions of poultry birds and an unknown number of wild birds. Then in 2021, a related version of the virus arrived in the US — the cause of the current outbreak — that appears much better equipped at infecting wild animals. Bird flu became big news in the months that followed, in 2022, when it started spreading through poultry farms. Farmers were forced to cull tens of millions of turkeys and egg-laying hens, often using a gruesome approach. It cost the federal government and the poultry industry hundreds of millions of dollars to manage and sent egg prices soaring in late 2022 and early this year. As the virus continues to spread to poultry flocks and dairy cow herds, it raises complex questions about our food system, and how we can make it healthier for humans — not to mention farmed animals and wildlife. Industrial animal agriculture, with thousands of animals packed together in warehouses, is a breeding ground for bird flu. So, even if this current outbreak among birds wanes, we’ll likely continue to see new outbreaks in the decades to come, across different species. Eventually, that could pose a more serious risk to humans; the more chances bird flu gets to evolve, the more opportunities it has to turn into something dangerous. To prevent or slow the spread of zoonotic diseases, like bird flu and Covid-19, governments, food companies, and farmers will need to invest heavily in surveillance (including of wild birds) and even vaccines, and they may ultimately need to make much more serious changes to animal agriculture. Update, April 24, 5:15 pm ET: This story was originally published on April 2 and has been updated with new detections of bird flu in dairy cows and milk.