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GEOFF BENNETT: The continuing 
outbreak of bird flu 
in the U.S. has alarmed

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researchers and prompted 
new efforts to track the 
virus that's already caused

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the deaths of tens of 
millions of birds from 
Europe to Antarctica.

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But, as William Brangham 
reports, as H5N1 continues 
to jump into mammals,

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most recently dairy cows, many 
scientists are concerned that

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we're not watching closely 
enough as this virus 
continues to spread.

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

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WILLIAM BRANGHAM: Evolutionary 
biologist Michael Worobey at the

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University of Arizona is 
one of many scientists 
around the world trying

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to untangle bird flu's 
latest twist, how and when 
it spread to dairy cows.

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MICHAEL WOROBEY, The University 
of Arizona: The jump into 
cattle probably took place

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between mid-November and 
mid-January, and so we're 
months into this already.

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WILLIAM BRANGHAM: And since 
then, it's spread like wildfire,

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infecting dairy cows in at least 
46 herds across nine states.

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MICHAEL WOROBEY: It seems 
to be spreading cow to 
cow in some fashion,

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but we don't know exactly 
how that is. For example,

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it could just be mechanical 
transmission as one cow 
leaves a milking machine, 
leaves virus on it,

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and then the next cow comes in. 
Or is this going respiratory, 
like flu does with humans?

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WILLIAM BRANGHAM: In late 
April, the USDA mandated 
that milking dairy cows being

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transported across state lines 
need to be tested for bird flu.

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Do you think we have got 
now enough surveillance

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out there to know what 
this virus is doing 
and where it's moving?

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

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

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from a limited number of 
farms. And that limits exactly 
how much we can understand.

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For example, you can 
actually figure out, just 
like we did with COVID,

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the number of people 
infected is doubling every 
two days. We still don't 
know that with cattle.

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WILLIAM BRANGHAM: But even 
the initial discovery that

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bird flu had jumped species and 
was now circulating in cows was

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thanks to a bit of 
epidemiological work by a 
handful of veterinarians.

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DR. DREW MAGSTADT, Clinical 
Associate Professor, 
Iowa State Veterinary 
Diagnostic Laboratory: The

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main common denominators 
with the cattle were a sudden 
decrease in feed intake,

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a sudden decrease in milk 
production, variable fevers, 
variable manure consistency.

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

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Vet Diagnostic Lab. He 
was helping colleagues 
in the Texas Panhandle

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and Kansas who were dealing 
with a bunch of sick cows, 
but then a new clue emerged.

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

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So, you thought, we have to 
check H5N1, the bird flu, 
because it's been in this area,

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but you really didn't think 
that was going to be the case.

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DR.DREW MAGSTADT: Well, 
yes. And we didn't end up 
ruling it out. We ended

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up finding the virus. The 
intriguing part here is 
that this virus in cattle

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doesn't seem to be causing any 
mortality. After several weeks,

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

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WILLIAM BRANGHAM: In fact, 
unlike cows, this bird 
flu has been deadly to the

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nearly two dozen other 
mammal species that have 
been infected in this U.S.,

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from a polar bear in Alaska, 
to a mountain lion in Colorado, 
to raccoons and foxes. Many of

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

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

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this strain of avian 
influenza first arrived 
in the U.S. in early 2022,

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brought here by migratory 
birds, more than 90 
million domestic birds,

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mostly chickens and turkeys, 
have died or been intentionally 
killed across 48 states.

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And unlike previous outbreaks, 
this variant has affected more 
wild birds and spread across

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a wider geographic area, 
crossing down into South 
America at the end of 2022.

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DR. RALPH VANSTREELS, 
University of California, 
Davis: I think the alarm

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really went off when it 
reached Peru, and that's a 
massive seabird community,

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and we saw just unprecedented 
mortality in the seabirds there.

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WILLIAM BRANGHAM: U.C. 
Davis' Dr. Ralph Vanstreels 
is a wildlife veterinarian 
based in Argentina,

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and he watched as the virus 
arrived and decimated bird 
populations, and then made

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another jump into mammals. 
An estimated 24,000 sea 
lions died from the outbreak.

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

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DR. RALPH VANSTREELS: At first, 
we weren't sure if each of those

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sea lions was getting infected 
by eating a bird. But it became

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pretty obvious pretty quick 
that this was spreading 
from mammal to mammal.

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WILLIAM BRANGHAM: In 
Argentina, Dr. Vanstreels 
estimated that the virus

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killed 17,000 elephant seal pups 
and an unknown number of adults,

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a blow to that population that 
may take decades to recover.

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DR. RALPH VANSTREELS: 
It's definitely very 
distressing, I can say. 
We have worked with these

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animals for many years. So we 
have known these populations, we 
have seen these colonies. And on

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the one hand, we are 
prepared, because we expected 
it. On the other, nothing 
can prepare you for it.

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WILLIAM BRANGHAM: Earlier 
this year, researchers 
confirmed that bird flu

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had spread all the way 
to Antarctica, primarily 
affecting birds known as skuas,

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but, so far, Antarctica's iconic 
penguins haven't been affected.

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DR. RALPH VANSTREELS: But this 
virus can mutate quite quickly,

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right as it did when it started 
infecting mammals. So it could

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mutate again and start 
infecting penguins. So we're 
not quite out of the woods yet.

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WILLIAM BRANGHAM: And it's 
that possibility of mutation,

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where the virus adapts and 
becomes better suited to 
spreading from mammal to mammal,

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that has many on edge, 
particularly now that H5N1 
was discovered spreading 
in all those dairy cows.

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While experts stress it's still 
very unlikely that this outbreak 
will lead to the next human

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pandemic, and government 
officials say pasteurization 
kills the virus in milk

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and dairy products, there are 
real risks for those who work 
in close contact with cows.

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In March, a farmworker 
in Texas was infected, 
but had mild symptoms and 
recovered. The CDC says it's

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monitoring people exposed to 
infected cattle, but admits that 
only 33 people have been tested.

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MICHAEL WOROBEY: What we 
have is a situation where 
the virus, in a sense,

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has more shots on goal 
to jump from a related 
species, a mammal like us.

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

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WILLIAM BRANGHAM: Worobey says 
it's a stark reminder that we

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have not learned as much as we 
hoped from the COVID pandemic.

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MICHAEL WOROBEY: We need to 
be spending billions more 
to do things like routinely

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monitor not the tip of 
the iceberg of cattle 
who are visibly ill,

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but routine monitoring to 
just find, OK, is there 
something that shouldn't

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be spreading in this animal 
species or in humans? And 
we are still not doing that.

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WILLIAM BRANGHAM: And with more 
than nine million dairy cows 
in the U.S. alone, getting eyes

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on where this virus may go next 
remains a monumental challenge 
for animal and human health.

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