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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.