GEOFF BENNETT: The continuing outbreak of bird flu in the U.S. has alarmed researchers and prompted new efforts to track the virus that's already caused the deaths of tens of millions of birds from Europe to Antarctica.

But, as William Brangham reports, as H5N1 continues to jump into mammals, most recently dairy cows, many scientists are concerned that we're not watching closely enough as this virus continues to spread.

A warning: This story contains scenes of animals in distress.

WILLIAM BRANGHAM: Evolutionary biologist Michael Worobey at the University of Arizona is one of many scientists around the world trying to untangle bird flu's latest twist, how and when it spread to dairy cows.

MICHAEL WOROBEY, The University of Arizona: The jump into cattle probably took place between mid-November and mid-January, and so we're months into this already.

WILLIAM BRANGHAM: And since then, it's spread like wildfire, infecting dairy cows in at least 46 herds across nine states.

MICHAEL WOROBEY: It seems to be spreading cow to cow in some fashion, but we don't know exactly how that is.

For example, it could just be mechanical transmission as one cow leaves a milking machine, leaves virus on it, and then the next cow comes in.

Or is this going respiratory, like flu does with humans?

WILLIAM BRANGHAM: In late April, the USDA mandated that milking dairy cows being transported across state lines need to be tested for bird flu.

Do you think we have got now enough surveillance out there to know what this virus is doing and where it's moving?

MICHAEL WOROBEY: I think we still have a long way to go, honestly.

We are still sort of dealing with a pretty limited number of samples from a limited number of farms.

And that limits exactly how much we can understand.

For example, you can actually figure out, just like we did with COVID, the number of people infected is doubling every two days.

We still don't know that with cattle.

WILLIAM BRANGHAM: But even the initial discovery that bird flu had jumped species and was now circulating in cows was thanks to a bit of epidemiological work by a handful of veterinarians.

DR. DREW MAGSTADT, Clinical Associate Professor, Iowa State Veterinary Diagnostic Laboratory: The main common denominators with the cattle were a sudden decrease in feed intake, a sudden decrease in milk production, variable fevers, variable manure consistency.

WILLIAM BRANGHAM: Dr.Drew Magstadt is a veterinarian at Iowa State'S Vet Diagnostic Lab.

He was helping colleagues in the Texas Panhandle and Kansas who were dealing with a bunch of sick cows, but then a new clue emerged.

On these same farms where cows were sick, a lot of cats had gone missing, gotten sick, or had died.

So, you thought, we have to check H5N1, the bird flu, because it's been in this area, but you really didn't think that was going to be the case.

DR.DREW MAGSTADT: Well, yes.

And we didn't end up ruling it out.

We ended up finding the virus.

The intriguing part here is that this virus in cattle doesn't seem to be causing any mortality.

After several weeks, the animals recover.

It's very different from the infection in other mammalian species.

WILLIAM BRANGHAM: In fact, unlike cows, this bird flu has been deadly to the nearly two dozen other mammal species that have been infected in this U.S., from a polar bear in Alaska, to a mountain lion in Colorado, to raccoons and foxes.

Many of those animals were likely infected by eating dead animals that were carrying the virus.

But, by far, the biggest impact here in the U.S. has been on birds.

Since this strain of avian influenza first arrived in the U.S. in early 2022, brought here by migratory birds, more than 90 million domestic birds, mostly chickens and turkeys, have died or been intentionally killed across 48 states.

And unlike previous outbreaks, this variant has affected more wild birds and spread across a wider geographic area, crossing down into South America at the end of 2022.

DR. RALPH VANSTREELS, University of California, Davis: I think the alarm really went off when it reached Peru, and that's a massive seabird community, and we saw just unprecedented mortality in the seabirds there.

WILLIAM BRANGHAM: U.C.

Davis' Dr. Ralph Vanstreels is a wildlife veterinarian based in Argentina, and he watched as the virus arrived and decimated bird populations, and then made another jump into mammals.

An estimated 24,000 sea lions died from the outbreak.

It was one of the earliest known mass mortality events from bird flu in mammals.

DR. RALPH VANSTREELS: At first, we weren't sure if each of those sea lions was getting infected by eating a bird.

But it became pretty obvious pretty quick that this was spreading from mammal to mammal.

WILLIAM BRANGHAM: In Argentina, Dr. Vanstreels estimated that the virus killed 17,000 elephant seal pups and an unknown number of adults, a blow to that population that may take decades to recover.

DR. RALPH VANSTREELS: It's definitely very distressing, I can say.

We have worked with these animals for many years.

So we have known these populations, we have seen these colonies.

And on the one hand, we are prepared, because we expected it.

On the other, nothing can prepare you for it.

WILLIAM BRANGHAM: Earlier this year, researchers confirmed that bird flu had spread all the way to Antarctica, primarily affecting birds known as skuas, but, so far, Antarctica's iconic penguins haven't been affected.

DR. RALPH VANSTREELS: But this virus can mutate quite quickly, right as it did when it started infecting mammals.

So it could mutate again and start infecting penguins.

So we're not quite out of the woods yet.

WILLIAM BRANGHAM: And it's that possibility of mutation, where the virus adapts and becomes better suited to spreading from mammal to mammal, that has many on edge, particularly now that H5N1 was discovered spreading in all those dairy cows.

While experts stress it's still very unlikely that this outbreak will lead to the next human pandemic, and government officials say pasteurization kills the virus in milk and dairy products, there are real risks for those who work in close contact with cows.

In March, a farmworker in Texas was infected, but had mild symptoms and recovered.

The CDC says it's monitoring people exposed to infected cattle, but admits that only 33 people have been tested.

MICHAEL WOROBEY: What we have is a situation where the virus, in a sense, has more shots on goal to jump from a related species, a mammal like us.

And now people are no doubt being exposed on a daily basis in pretty large numbers.

WILLIAM BRANGHAM: Worobey says it's a stark reminder that we have not learned as much as we hoped from the COVID pandemic.

MICHAEL WOROBEY: We need to be spending billions more to do things like routinely monitor not the tip of the iceberg of cattle who are visibly ill, but routine monitoring to just find, OK, is there something that shouldn't be spreading in this animal species or in humans?

And we are still not doing that.

WILLIAM BRANGHAM: And with more than nine million dairy cows in the U.S. alone, getting eyes on where this virus may go next remains a monumental challenge for animal and human health.

For the "PBS NewsHour," I'm William Brangham.