Could the Bird Flu Cause a Human Pandemic?

Since the early 2000s, the H5N1 bird flu has sickened wild and domestic birds around the world -- and sometimes humans and other mammals, too. Learn how this strain of avian influenza works (and why it seems unlikey to cause a pandemic, but researchers are still watching it closely) in today's episode of BrainStuff, based on this article: https://science.howstuffworks.com/life/cellular-microscopic/bird-flu.htm

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:01):
Welcome to BrainStuff, a production of iHeartRadio. Hey BrainStuff, Lauren
Vogelbaum here. Over the past several years, you've probably seen
news stories with dire warnings about the avian flu or
bird flu. This refers to a highly infectious disease that
swept through both wild bird and poultry populations all over

(00:24):
the world. The virus doesn't pass to humans easily, but
when it does, it kills at a rate much higher
than other strains of influenza. But what exactly is the
bird flu, and how is it different from the seasonal
flus that come around every year? Today, let's talk about
how the bird flu works, why it's considered unlikely to

(00:44):
cause an epidemic in humans, and why disease control centers
are nonetheless watching it closely and working to contain it.
But first up, let's talk about how viruses work in general.
A virus particle, or virion, is not a cell. The
cells in our bodies, and bacteria cells, are living entities

(01:06):
with the ability to eat, grow, and reproduce. Viruses are
nothing like that. A virus is a microscopic packet that
contains a set of genetic instructions wrapped up in a
protective layer of protein that sometimes with a protective layer
of lipid around that. Viruses cannot reproduce on their own.

(01:26):
They have to hijack a living cell to do that
work for them. This process destroys the cells and makes
people and other animals sick. Viruses usually enter an animal's
body through the mouth, mucous membranes, or breaks in the skin.
Then they infect specific cells, depending on the type of virus.
For example, cold sores are caused by the herpes virus

(01:49):
attacking cells in the lips. Other viruses, like the measles,
can be widely damaging because they attack cells in the
immune system. A virus particle will attach to the surface
of whatever host cell and inject its genetic instructions into
that cell. The virus's code recruits the cell's enzymes to
make and assemble the parts for many new virus particles.

(02:12):
Once assembled, the new viruses then break out of the
host cell, killing it in the process, and go on
to attack other cells and continue the cycle. Influenza is
a specific type of virus that attacks the respiratory system
of birds and some mammals. It can cause fevers, sore throats,
and congestion as your body works to fight and flush

(02:33):
out the virus. If it attacks muscle cells, it can
also cause muscle aches. There are four basic types, or genera,
of influenza virus, called A, B, C, and D, each
with multiple subtypes, or lineages, and strains. In general, viruses
mutate pretty often. That's why a new flu vaccine comes

(02:54):
out every year, to help protect you from the specific
strains that have developed and spread. All four types of
influenza can infect humans, but influenza A causes the most
serious illnesses and seasonal epidemics, and is the only type
known to have caused pandemics. We identify subtypes of influenza
A based on the specific proteins that they have on

(03:16):
their surfaces. These proteins are antigens, that is, particles that
your immune system uses to tell whether something is a
friend or foe. Influenza A has two kinds of surface proteins,
hemagglutinin and neuraminidase, and each has a bunch of numbered subtypes.
That's why influenza A viruses have names like H1N1, which

(03:39):
was the flu that caused the pandemic of 1918, or H3N2,
which caused the pandemic of 1968. The strain that's commonly
referred to as the bird flu is H5N1. It first
cropped up in 1996, and we'll get more into its
history here in a minute. But, okay, how do new

(04:00):
strains develop in the first place? Like many viruses, influenza
can mutate through antigenic drift or shift. In antigenic drift,
small changes occur to a virus's surface proteins as it reproduces.
This is happening all the time, and these small changes
can add up enough that your immune system eventually won't

(04:22):
be able to recognize the virus anymore, and the virus
will thus be able to infect you again. Iterations of
both H1N1 and H3N2 still circulate as yearly seasonal flus
because of this, and hence new flu vaccines come out
every year. But an antigenic shift sudden, major changes occur

(04:43):
that create a whole new subtype of the virus that
basically no one has immunity to because it's new or novel.
Although of course the coronavirus is not a type of influenza,
this is what happened with COVID-19. It was novel. Being
that these viruses are new, new vaccines have to be
developed in order to help fight them. Antigenic shift is

(05:06):
a lot more rare, but it can happen when two
different strains of influenza encounter each other in a single animal,
and the viruses swap some segments of their DNA. This
is called reassortment, and the novel viruses it can create
can be especially dangerous. For example, the aforementioned flu pandemic
of 68 was caused by a reassortment of a human

(05:29):
strain and an avian strain. And the 2009 swine flu
pandemic was caused by a triple reassortment of swine, human,
and avian strains. Now, birds can carry every known subtype
of influenza A. But when scientists talk about avian strains
of the flu, they usually mean varieties that exist mostly

(05:51):
or entirely in birds, not in people. Most of the time,
birds can't transmit the flu directly to people. They first
infect pigs, horses, and other non-human mammals that can also
contract human flu strains. Reassortment in those other mammals can
then create novel strains capable of infecting humans. Many wild

(06:14):
birds carry avian flu viruses in their intestines but don't
actually get sick from them. However, they can shed the
virus in their droppings, saliva, and other bodily fluids. Domesticated
birds like chickens can get sick when they come into
contact with contaminated water, feed, or soil. Avian flu strains
aren't always dangerous, even to poultry. Low pathogenic strains cause

(06:38):
very mild symptoms, like ruffled feathers and reduced egg production.
High pathogenic strains, however, can be deadly. They can have
a mortality rate in poultry approaching 100%. Birds that do
survive can continue to shed the virus in their droppings
for several days after recovering, which helps the virus continue
to spread. Farmers can protect their birds by frequently disinfecting

(07:03):
their clothing, shoes, and equipment, by keeping domestic birds away
from wild birds as much as possible, and by working
to quarantine birds that may be infected. By doing so,
they're not just protecting their birds or their livelihood, they're
also protecting human health. Because, in rare cases, avian flu

(07:23):
can spread to humans. Since people have no immunity to
these strains, they can be especially deadly. And that's what
happened with H5N1. And we're going to get into the
story of that, but first we're going to get into
a quick break for a word from our sponsor. So,

(07:46):
in 1996, researchers identified a highly pathogenic avian influenza in
domestic waterfowl in southern China. The next year, health officials
in Hong Kong reported that, for the first time, it
appeared that this virus had moved directly from birds to people,
instead of moving through a second species. The virus caused

(08:07):
typical flu symptoms, but could also cause eye infections, pneumonia,
and acute respiratory distress. Tests confirmed that influenza A H5N1
was completely new to humans. And thus, what's now often
simply called the bird flu was born. At the time,
18 people infected with the virus were hospitalized, and six

(08:31):
of them died. The Hong Kong government, alarmed at the
potential threat of an epidemic or even pandemic, took drastic steps.
In about three days, the government destroyed some one and
a half million birds, the country's entire poultry population. Although
this might sound extreme, many health experts think that Hong
Kong's actions prevented an influenza pandemic that year. After that,

(08:54):
the H5N1 strain appeared to lay dormant until 2003. Then,
government officials in Vietnam and Thailand began to report infections
in birds and people. About 100 million birds died in
Asia in 2003-2004, either as a result of the disease
or as part of efforts to stop its spread. In

(09:16):
fall of 2005, the disease spread to Eastern Europe, likely
due to migrating birds. By the following January, several people
in Turkey had contracted the disease, most likely from contact
with dead birds. Health officials found Nigerian birds infected with
the disease in early 2006. Relatively few humans have ever

(09:36):
contracted H5N1, and most who did had extended direct contact
with infected birds. However, of the over 860 people confirmed
to have contracted the virus, over 50% have died, which
is a wildly high fatality rate. Previous influenza pandemics have

(09:56):
killed an estimated 0.1 to 2.5%. percent of the people
they infected. And now, some researchers think that 50 percent
is an overestimate of the bird flu's true fatality rate
in humans, but relatively conservative estimates have put it somewhere
from 14 to 33 percent, which is still wildly high.

(10:18):
Since that first decade, the bird flu has mutated, forming
a few recognized subtypes, and spread around the world. Every
few years, it's caused outbreaks in both wild birds and
poultry in different places, plus occasional infections in humans and
other mammals. Then, in 2021, a new strain of H5N1

(10:40):
caused what's called a panzootic. That's the term for a
pandemic in wild animals, in this case wild birds. This
panzootic continues today. Reports of die-off events involving sometimes tens
of thousands of birds come in every month. And the
bird flu has been observed transmitting from mammal to mammal.

(11:01):
In 2024, researchers found that it was making dairy cattle
in Texas sick. There have since been outbreaks in cattle
in at least 17 U.S. states, causing a lot of
stress and financial loss for dairy farmers. Other mammals that
have contracted H5N1 include farmed mink in Europe, domestic cats,
and wild animals like bears, raccoons, rodents, and even South

(11:24):
American sea lions. Notably, human-to-human transmission has been extremely rare,
which has left some people wondering whether scientists have overreacted
to the H5N1 bird flu. But public health officials have
several concerns about it. There's that high mortality rate and

(11:44):
the fact that in the past, it's killed young adults
in otherwise good health. Also, controlling the spread of it
is difficult because lots of wild and migratory birds can
carry it. And given its recent spillover into different mammals,
the virus is clearly changing. Whether those changes will wind
up making it more dangerous to humans is anybody's guess,

(12:07):
but researchers are closely following the situation. But to cause
a pandemic in humans, the bird flu would have to
mutate in a few ways to become better at traveling
through the air, better at binding to human respiratory cells,
and better at hijacking mammalian host cells in general. Unfortunately,

(12:27):
the more interaction that humans have with the virus through
infected wild and domestic animals, the more opportunities the virus
has to undergo reassortment and antigenic shift. This is one
of the many reasons why it's good to get the
seasonal flu vaccine every year if you can. It can't
protect you against the bird flu, but it will lower
the possibility of human and avian flu viruses interacting with

(12:50):
each other in your body. Speaking of vaccines, there are
vaccines for humans to protect against the bird flu. At
least 50 have been approved by different regulators around the
world for use in different circumstances. And there are plans
in place by bodies like the World Health Organization to
to help make sure that everyone would have access to

(13:10):
vaccines in case of a pandemic. Researchers are also working
on improving bird flu vaccines for non-human animals to help
stop the spread. You can help protect yourself and, you know,
all of humanity by avoiding contact with wild animals, alive
or dead, and by keeping any pet cats indoors. If

(13:31):
you have a backyard flock of chickens, ensure that they're
fully enclosed, away from wild birds and their droppings. If
you work with animals, you probably know more about this
than I do, but always practice good sanitation, like handwashing
and frequent cleaning of equipment, clothing, and shoes. Also, the
spread of H5N1 to farm animals is a great reminder

(13:53):
for us all to practice food safety. Cook meat, and
especially poultry, to a safe temperature, and opt for pasteurized
milk instead of raw milk. Pasteurization inactivates viruses like influenza
and kills bacteria, and thus prevents you from getting sick
from all kinds of potential pathogens. Basically, we're not ready

(14:14):
for a bird flu pandemic. We're never really ready for
a pandemic. But science is working on it, and you
can help. Today's episode is based on the article, How
Bird Flu Works, on HowStuffWorks.com, written by Tracy V. Wilson.
BrainStuff is a production of iHeart Podcasts in partnership with

(14:35):
HowStuffWorks.com and is produced by Tyler Klang. For more shows
from iHeart Podcasts, visit the iHeartRadio app, Apple Podcasts, or
wherever you listen to your favorite shows.