Avian influenza has a grip on dairy herds in California, a few people have fallen seriously ill in North America and researchers have found that a single mutation could give the virus a better foothold in a person's upper airways.
At the same time, those who're tracking avian influenza's footsteps say most of us don't need to be preoccupied with the virus as we go about our day, at least for now.
"We're really on alert," says Benjamin Anderson, an infectious disease epidemiologist at the University of Florida. "It's a tricky thing to communicate."
Fears that H5N1 bird flu could unleash a pandemic go back decades, and previous strains of the virus have proven quite deadly in humans. That has not been the case during this current outbreak. Most infections linked to the strain of bird flu circulating globally in wild birds and U.S. dairy herds — technically known as clade 2.3.4.4b — have been mild.
But two recent cases are a reminder this isn't a foregone conclusion.
First, a teenager in Canada was hospitalized, as was a Louisiana resident who was exposed to backyard flocks. In each, genetic analysis suggests mutations may have emerged as the virus replicated inside of them. However, there's no evidence any of this has led to human-to-human spread.
"The difficult thing is it's hard to know how much of a heads up we're going to get that things are changing," says Richard Webby, a virologist who studies avian influenza at St. Jude Children's Research Hospital.
Here's what some experts are saying now.
Should I be worried about catching bird flu?
Despite its wide reach, the strain of highly pathogenic avian influenza — so named because of its lethality in poultry — driving the current outbreak has not adapted to easily infect humans, meaning it's still "essentially a bird virus," says Webby.
Of the more than 65 human cases in the U.S., almost all of them can be traced back to some kind of close contact with infected poultry, wild birds, or dairy cattle. So, if you're not dealing directly with any of these animals, the chance of catching the virus in its current form remains quite low.
Unlike the seasonal influenza viruses, bird flu doesn't attack the cell receptors that predominate in a person's upper airways. Instead, it prefers "avian-type receptors" deep in the lungs and in the mucus membranes around the eyes, which seems to explain why many recent human cases have presented as conjunctivitis.
"It's not actually very good at infecting people," says Anderson. "What we typically have seen is a lot of viral exposure in order to be infected."
A few cases have popped up with unexplained origins in California, Missouri, and British Columbia. However, there's no indication any of the people contracted the virus from another human, or spread it to anyone else.
For now, the concern is more about what could transpire as more animals and humans get infected, says Kristen Coleman, who researches airborne infectious diseases at the University of Maryland. "It's not an immediate risk, but that's not to say that it won't be a risk in the near future."
That's because every spillover into mammals, including humans, puts added pressure on the virus to adapt to its new host, raising the prospect that certain mutations pop up, which could, under the right circumstance, let the virus more efficiently replicate and infect people.
Scientists already have an idea of what some of the most concerning changes would look like (at least to some extent), and it doesn't appear they've caught on yet, based on testing done in poultry and cattle.
But it's evident this particular strain of H5N1 is sickening a much broader range of mammals than previous ones, says Seema Lakdawala, an influenza researcher at Emory University.
"We don't know why yet, but it could be chance or a precise genetic feature," she says.
What does "close contact" with a bird look like exactly?
During the current outbreak, poultry workers have generally caught the virus while they're culling infected flocks.
This is considered a high risk activity because it can involve handling the animals and spending long stretches of time in facilities where viral particles in their saliva, mucus and poop can hang in the air and contaminate surfaces. For example, workers in Colorado were infected while depopulating birds in hot barns with industrial fans blowing. They struggled to wear personal protective equipment over their mouths, nose and eyes because of the heat.
Historically, human cases of highly pathogenic avian influenza H5N1 have often been traced back to live bird markets. In this context, people tend to be handling, defeathering, butchering and preparing the animals for sale.
High concentrations of viral particles can spew into the air and be inhaled, or coat surfaces and hands, eventually ending up on your eyes and nose, perhaps after you rub your face, says Webby. "If you're just walking past an infected bird, it's extremely unlikely that you're going to get it that way."
Coleman agrees. In a study carried out before this latest outbreak, she sampled the air at a massive live market in Vietnam and discovered it was filled with aerosolized bird flu virus. They could even grow infectious viruses from their samples. Yet, none of the people they tested there had active infections.
Okay, I'm not butchering or touching any birds, but do I need to stop using my bird feeder? What about touching bird poop?
For the average person, it's unlikely you'd get infected from a stray piece of bird poop or from touching your bird feeder, although there isn't necessarily data on these specific scenarios, says Anderson.
How long H5N1 bird flu survives on objects depends on a variety of factors. For example, it can survive for days or even weeks under certain colder conditions and in water. Contracting the virus from a contaminated surface is a legitimate concern for people who are "handling equipment associated with animal care on poultry or dairy farms," says Lakdawala, adding that people should be cautious at petting zoos and agricultural fairs, too.
Bird poop can harbor high loads of the virus, possibly for long stretches of time, but Webby doubts it would be a concern for more than a few days after being exposed to the elements on your porch or deck.
"If you had things that had bird poop on them, yes, you would want to avoid that or disinfect it," says Anderson.
You could do the same for your bird feeder. He says just use common sense hygiene principles like washing your hands and avoid touching your face before hand washing.
People who have backyard flocks should be particularly cognizant of the risk, especially if they live in the path of migratory birds. Do your best to keep your poultry from mixing with wild birds. If you're concerned, you can wear an N95 mask around them.
What's the deal with dairy cattle? Do I need to worry about milk?
Unless you're spending time with dairy cattle or raw milk, there's no reason to be concerned.
The spread of bird flu in dairy cattle took scientists by surprise. The best guess is a spillover from birds into dairy cattle in late 2023 was enough to set in motion the nationwide outbreak in dairy cattle today.
The U.S. Food & Drug Administration has found pasteurizing milk can effectively inactivate the virus. The same is true for pasteurized eggs products, and if eggs are cooked to 165 degrees
There's now "strong" evidence that contact with raw milk, laden with high concentrations of virus, is how roughly 40 dairy workers have become infected, says Anderson. Most likely this is happening in the milking parlor when people are sprayed in the face, or viral particles are aerosolized and inhaled.
Research has shown that raw milk can "stabilize the virus on a variety of surfaces for long periods of time," says Lakdawala. Hence the warnings against drinking raw milk.
Even relatively small amounts can be deadly for mice, according to lab experiments done by Peter Halfmann and his colleagues at the University of Wisconsin–Madison. "But we don't know how this exactly would work in humans," he says. "It's still a big black box."
While the exact risk is unknown, Lakdawala considers consuming raw milk a "high risk activity," especially in states like California where bird flu has been found in some of these products.
Dr. Jürgen Richt, a veterinary microbiologist at Kansas State University, echoes that sentiment.
"Is the mouse a good model for humans? I don't know," says Richt. "But don't become a guinea pig."
What should I be doing to protect my pets?
The basic principles for keeping yourself safe from bird flu also apply to your pets.
Coleman's advice: Give your cats a little bit more supervision when they're outside. Keep them away from wild birds (dead or alive) and rodents, especially if you live on a farm or have backyard poultry.
Coleman recently reviewed the data, and found this current strain of bird flu looks to be more deadly in felines than it is for H5N1 overall, although it's hard to say because of limited data.
She says cats are largely infected with bird flu by consuming raw contaminated meat and by preying on wild birds, including during the current outbreak.
In Oregon, a house cat died after consuming raw pet food, riddled with the virus, which prompted a nationwide recall. It's possible that contaminated meat was behind an outbreak that wiped out 20 cats at a wildlife sanctuary in Washington state. In Los Angeles, raw milk is implicated in the death of several house cats.
Dogs are also susceptible. Last year, a dog in Canada died from the infection after it was found chewing on a dead goose.
While pets can develop respiratory symptoms, the infection can manifest as a neurological illness, seeming to mimic rabies, she says. "If your cat or your dog starts acting funny, maybe circling or acting rabid, make sure to contact a veterinarian right away."
There have only been a few documented cases of feline-to-human transmission of avian influenza, after people tending to the animals had direct contact, but not during this current outbreak.
Anderson says "there's no need to panic" that your pet is going to be getting H5N1 tomorrow, especially if you keep them from these known sources of infection.
"It's just about being aware," he says.
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