In March 2024, a weird disease started appearing in large dairy herds in Texas. And for many weeks it was undiagnosed, until finally a veterinary practitioner noticed on one of her farms that cats were dying and sent samples to a friend who worked in a diagnostic lab. That was the beginning—the first time in the history of the world that avian flu had been found in dairy cattle. – Stephen Wadsworth
Four University of Vermont experts—veterinarians Stephen Wadsworth and Julie Smith, influenza virologist Emily Bruce, and infectious disease specialist Peter Hyson—break down how avian influenza infiltrated dairy cattle herds, what the current risk is, and how that might change.
What symptoms are people experiencing when they contract avian flu?
Emily Bruce: One of the most common symptoms right now is conjunctivitis, or pinkeye. There are similarities between the cells of human eyes and the cells of birds, so the virus can infect human eye tissue easily. When people only get conjunctivitis from an avian flu infection, it can mean the virus hasn’t adapted to replicating in respiratory tract cells.
During the process of milking, there are lots of opportunities for milk from infected cows to get into people’s eyes or faces. In some milking parlors, the aisles for workers are below ground level, so the cows and milking machines are at eye level for workers. When they connect and disconnect the milking equipment, it’s very easy for milk to splash or spray into their eyes.
Are humans getting avian flu in other ways?
Stephen Wadsworth: Tens of millions of infected poultry have been euthanized in the United States in the last year, and when workers cleaned out these massive poultry facilities, some of them contracted avian flu.
Peter Hyson: The people who’ve gotten seriously ill so far seem to have been exposed to birds rather than to infected cows and probably got viral particles in their throats, eyes, or lungs when exposed to the blood, saliva, or feces of sick animals. The trend seems to be that these patients have been infected with the D1.1 variant of the avian influenza virus, which is primarily found in wild birds and poultry and is a different variant than the virus that has been infecting dairy cows over the last year. The person in Louisiana who died of bird flu had been exposed to sick and dead birds in a backyard flock (which means domestic but noncommercial birds). I'm happy to say that to date, we have no evidence of human-to-human spread of avian flu. And infection is still rare. Unfortunately, several instances of spillovers of the D1.1 variant into dairy cows have now been reported. It’s a dynamic situation.
Whether a person can get avian influenza from drinking raw milk is not known, but a recent study found that monkeys who got virus in their airways got sicker than those who just drank raw milk. And if you’re drinking milk, you can certainly choke and get milk in your airway. It's important to add that when you drink raw milk, you are also at risk of infection from bacteria including Salmonella and Listeria, which are normally killed by pasteurization.
Emily Bruce: There are people who’ve been infected with avian flu with no known link to infected animals or other known sources, including a child in California and two people in Saint Louis. This is worrying, because if you can’t trace where people were infected, it suggests avian flu may be spreading in the community, person-to-person. One person, sadly, died in Louisiana, but that person had contact with wild birds, which were the likely source of infection.
Seems like an evolving situation. Why do things keep changing with this virus?
Emily Bruce: The first factor is how viruses reproduce. In humans and other organisms, when a new cell is produced, there’s a proofreading mechanism that ensures the new cell’s DNA matches the old cell’s DNA. But when a new strand of influenza genetic material is produced, there’s no proofreading mechanism. The error rate is much higher, and every time genes are copied inaccurately—in other words, mutation—the resulting virus is different—in other words, evolution. Second, viruses reproduce quickly; each successive generation with new, evolved traits appears within eight hours. Finally, the genetic material of influenza viruses is divided into eight sections, any one of which can mix with a different type of flu to create an entirely new virus.
Now, what if a dairy worker gets avian flu from a cow, and also gets seasonal flu from their kid in kindergarten—and one of their cells gets infected with both viruses? Those two viruses could mix their genetic materials to produce a novel virus that contains the parts of the avian influenza genome that make the host very, very sick, and the parts of the seasonal influenza genome that are really, really good at spreading between people. That’s how new influenza pandemics generally arise in humans—the worst-case scenario.
What animals besides birds and cows (and humans) are at risk?
Stephen Wadsworth: Avian influenza has killed thousands of marine mammals. It was diagnosed in a goat that died and is highly adapted to infecting cats as well.
Julie Smith: This flu virus is nasty. It has affected more bird species as well as more mammalian species than probably any other prior avian influenza—although we can’t know for sure because we haven’t always had the detection technologies we have today.
I think many of us have seen visuals of the coronavirus, with its spikes, and coronavirus can infect multiple species as well. Is the avian flu virus similar?
Emily Bruce: Viruses have these spiky proteins on the surface that let them enter cells to infect them. We can think of these spiky proteins like locks, and they fit into specific keys—proteins called receptors—on a cell’s surface. One of the big things that controls what kinds of animals, tissues or cells a virus can enter is whether the receptor for that virus is available on the surface of a cell. But it’s not all-or-nothing. Sometimes, your key doesn’t quite fit, and you can only open the door one in 100 times. Sometimes, the key fits great, and that virus enters the cell very quickly because it opens the door so easily.
There are several mutations and adaptations that the avian influenza virus needed to function in a mammalian cell. Once some of those mutations happened, probably somewhere in Texas between November 2023 and January 2024, avian flu had the key and made the jump. The virus that had those mutations then began circulating in cows. Fortunately, these viruses do not seem to have gained the mutations that would allow them to spread from human to human via the respiratory route—that would be a much more dangerous scenario. This situation could change, though.
Julie Smith: Recently, three new crossover events have been detected in dairy cows, one in Nevada and two in Arizona. The Nevada event and at least one of the Arizona infections were with the strain that is currently commonplace in wild birds across the country. (Researchers are still working to identify the strain of the most recent spillover event.) This means dairy producers need to remain on the lookout for infection in their herds.
Do we know what the current risk is?
Emily Bruce: I think the important thing to know is that the level of risk can change quickly. An avian flu infection of the airway in humans would be the thing to get really concerned about, because then there’s a greater risk of airborne transmission. There are too many unknowns to predict exactly how likely this is, but we know every infection is a roll of the dice, so do you want to roll those dice once, or 3,000 times? The more opportunities we give the virus to mutate, to join with seasonal flu, the more likely a worst-case scenario is. Right now, we're giving the virus lots of those opportunities. There have been new introductions of avian flu into dairy herds in the last few weeks, in Nevada and Arizona. These are worrying signs for our ability to contain the situation.
Julie Smith: Until the recent detections of the new avian flu variant in cattle in Nevada and Arizona, I had been somewhat confident that these spillover events from wild birds to cattle were so rare that we didn’t have much to worry about when wild birds live near dairy facilities. But this new crossover strain is flying overhead—and landing in new places every day.
Peter Hyson: I think at this point there's a setup for one of these crossover events that could cause the next 1918 flu. I don't think that that virus exists yet, but it's a frightening potential scenario.
What can we do to protect ourselves from avian flu?
Emily Bruce: A few things: don’t permit the movement of cows that haven’t been individually tested for H5N1 across state lines. Mandate testing all milk, to see where the virus is, which is a program the USDA is now running across much of the country. Hand-in-hand with this, we need to financially support dairy farmers, so they don’t take a financial loss for testing and quarantining their animals.
The risks of drinking raw milk are so high I have recommended to my friends and family that they not drink it. To put things in perspective, the amount of virus in raw milk from a cow infected with avian flu is 10,000,000,000 infectious particles per milliliter. I did my master's and my PhD research on flu, it's what my lab works on now, and on my best day in my lab, I can’t grow a concentration of virus that high.
Finally, we could invest in research to understand how likely this virus is to be able to infect the human respiratory tract, and to ensure we have a vaccine that would provide protection against an H5N1 infection.
Stephen Wadsworth: The avian flu virus has an affinity for the mammary system in cows and is shed in the milk of infected cows. Vermont has established a state milk surveillance program. Much of the milk from Vermont is commingled with milk from other states in bottling centers, but Vermont is one of two states in the country that is testing every dairy farm, every month, by taking bulk tank samples rather than processing silo samples. This way, if any bulk milk that Vermont milk is commingled with milk that tests positive for avian flu, we can say with confidence that it wasn't from Vermont herds. The Vermont Agency of Agriculture, Food and Markets has trained 25 technicians for this purpose and used the UVM farm for their training in December. UVM was the first herd they tested, and it was negative, not unexpectedly.
The state, the CDC and industry groups are educating and preparing farmers for enhanced biosecurity programs. Here at the UVM farm, visitors must wear booties and wash their hands. There is no animal contact. We don't let people pet the cows like we allowed in the past. Those are all legitimate biosecurity measures.
Julie Smith: I’d encourage individual dairy operations to be prepared to protect workers. Their health and safety are paramount. Dairy operations should also prepare to support cattle that show clinical signs of avian influenza infection because with proper treatment, most cows will survive.
Peter Hyson: Getting a flu shot is a good idea for everyone and may mitigate the risk of an event in which avian flu combines with a human strain of influenza in an infected human or animal, leading to a new virus that spreads readily among humans (we described this above). A flu shot may not protect you from bird flu, but anything we can do to reduce human influenza might help to lower the chances of this feared outcome. Personal protective equipment is important for farmworkers who are around infected or potentially infected animals—eye protection, gloves, a mask, an apron.
If you find a dead bird, leave it alone and contact your health department as they may test it for avian influenza. The flu is spreading from birds through their feces, saliva, urine. Particles from any of those routes can become aerosolized and windblown, and dairy cows inhaling those particles get sick. The more we can limit flu in any population, be it cows, poultry or human beings, the less likely we are to get a new, more virulent virus.
Final thoughts on this?
Stephen Wadsworth: I think the most important thing to communicate is that we do not have avian flu in Vermont dairy herds—in cows—although H5N1 has been repeatedly detected in the state in wild birds, wild animals and in domesticated birds. We must be very vigilant, but we shouldn’t be full of anxiety. There is a potential risk, but experts are communicating to the public that we aren't on the cusp of another massive pandemic. We shouldn't take it lightly, but we shouldn't be losing sleep over it.
Peter Hyson: There is some good news here: we're a lot better at treating respiratory viral infections than we used to be. COVID resulted in a bit of a space race, if you will, with the rapid creation of mRNA vaccines and antivirals. Have we learned lessons from COVID? Maybe not as many as we should have, but I do think we are well-positioned to respond if such a virus comes along. Likewise, if a new virus does arise, the way we're likely going to deal with it is vaccines. We have decades of evidence that numerous kinds of vaccines, including those for respiratory viruses, are safe, effective and a good means of limiting infections, and severe outcomes of infections. I encourage people to get a flu shot, and if we end up with a new vaccine for a new virus, get that vaccine, too.
Julie Smith: We can’t eliminate avian influenza entirely because wild birds are a reservoir. Instead, I think the important action isn’t to be horrified that it’s happening, but to prepare. We can protect ourselves, and we should at least try.
Emily Bruce: I think this has gone under the radar because no one wants to think about another pandemic, but viruses don't care what we think. It’s better to prepare early.
The researchers also reiterated that this situation is evolving rapidly, and any information may change at any time.
Emily Bruce, PhD, is an assistant professor in the Larner College of Medicine’s Department of Microbiology and Molecular Genetics.
Peter Hyson, MD, is a physician and assistant professor in the Larner College of Medicine’s Department of Infectious Disease
Julie Smith, DVM, PhD, is a research professor in the College of Life and Agricultural Sciences’ Department of Animal and Veterinary Sciences.
Stephen Wadsworth, DVM, is a lecturer and director of the Cooperative for Real Education in Agricultural Management (CREAM) program in the College Agriculture and Life Sciences.