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[INTRO♪]
We often hear about diseases we can get
from animals, and for good reason.
Many of these diseases, collectively called zoonotics,
are serious health threats.
But illnesses are often two-way streets.
Animals can get us sick, and we can pass
pathogenic bacteria, viruses, fungi
and parasites onto them.
These reverse zoonoses are more common
than you might realize.
And they’re a big deal, because they can harm animals
we care a lot about, like the ones we eat,
and the ones we’re desperately trying
to save from extinction.
In June 2009, an outbreak of metapneumovirus
—a kind of respiratory disease—
struck a group of mountain gorillas in Rwanda.
Altogether, eleven of the twelve gorillas
in the group got sick and two
—an infant and its mother—died.
And though that might not seem like a lot,
it’s devastating to a species with just 600
or so individuals left, all of which are roughly
in the same general area.
In fact, infectious disease accounts for 20%
of sudden deaths in mountain gorillas.
20%!
But the worst part was that
that outbreak was caused by people.
You see, although researchers aren’t exactly sure
who passed the virus on to the gorillas,
they know it was a person.
Samples of the deceased gorillas’ lungs
and throats contained genetic sequences
from human viruses.
Researchers also found pneumonia-causing bacteria
in the mother, which turned out to be the cause of death—
basically, it moved in after her body had been
weakened by the virus.
And that confirmed that the apes are
catching human diseases—and those diseases
can kill them.
Which puts local scientists in a tricky spot.
Conservation efforts rely on tourism money.
But our devastating infections can pass
more easily to gorillas than many other
animals because we’re so similar to them.
So the presence of potentially-ill tourists
might do more harm than good.
Right now, the plan is to restrict access
to the gorillas and the area they live,
and in some places, make people wear masks
to help prevent the spread of infectious diseases
like metapneumovirus.
There’s also the possibility of developing
a gorilla-specific vaccine for it.
And that probably wouldn’t be that hard.
Scientists have already successfully given
vaccines to wild gorillas and apes to protect
them from outbreaks of measles, tetanus, and polio.
But whether vaccine development actually happens
ultimately depends on the cost and what governments,
conservation groups, and tourists are willing to do
to save these animals.
Unless you were living under a rock,
you probably heard about the global influenza A
pandemic of 2009, better known as swine flu.
Within six months of the World Health Organization’s
official pandemic declaration, there had been more than
414,000 confirmed cases in people
and nearly 5,000 deaths worldwide.
The first case was thought to have
come from pigs in Mexico—hence the name swine flu—
and then spread among humans from there.
But it turns out we actually started it
by giving them the flu first.
See, the whole reason swine flu was so contagious
and all-around terrible was because its RNA was a mix of
human, pig, and bird influenzas.
And that only happened because sometime around 1998,
we passed our version of the flu onto some pigs.
Pigs can act like tubby genetic mixing bowls
because they’re susceptible to viruses from
different animals, and those viruses can all
hang out together inside them.
The pigs that got our 1998 flu were already harboring
their own pig flu, and then they also managed
to catch a bird flu.
These three viruses combined into what’s called
a triple reassortment—a mix of genetic material
from three sources.
Even then, the virus wasn’t so bad for us at first—
for about a decade, it mostly spread between pigs
and didn’t change much.
Then, around 2008, the triple virus got
even more genetic material from
two other pig viruses.
That’s when it became the pandemic version.
And not only did we basically start this pandemic
with our human virus, we also continued fueling
the evolution of the virus by giving it back to pigs.
As of 2015 there were at least 49 cases from
more than 30 countries where people spread
swine flu back to swine.
In fact, in general, we spread more
influenza viruses to pigs than they do to us.
And that’s bad for the pigs who become infected,
because they often are culled to stop the spread.
But it’s also really bad for us because it can
encourage the viruses to evolve into new strains
which we don’t have vaccines for.
Tuberculosis, or TB, is a disease
where bacteria multiply in the body, usually in the lungs,
causing coughing, weight loss, fever, and sometimes death.
And though you’ve probably heard about it
in the context of people, the bacteria involved
— Mycobacterium tuberculosis and M. bovis—
aren’t all that picky about which lungs they set up shop in.
And thanks in part to us,
the disease has become a huge problem in elephants.
Up to 25% of elephants have TB in some parts of the world.
Though, it’s hard to tell exactly how much of that is our fault.
It’s true that many modern cases involve
zoo or circus elephants—basically, situations
where animals and people are in close contact
a lot of the time.
But there are cases of elephant TB
dating back 2000 years.
And since it’s so contagious
and can infect so many species,
it’s sometimes difficult to know where the disease comes from.
Also, unlike some of the other diseases
we’re talking about in this episode,
the same strains of bacteria can be found
in both people and other animals.
So scientists aren’t always able to tell if
we gave it to an elephant or if it came from
some other animal the elephant came in contact with.
But there have been several cases of
wild elephants dying from TB infections where
the evidence definitely points to a human cause.
For example, a 2017 study looked at the circumstances
surrounding the TB deaths of 3 wild Asian elephants
in a wildlife sanctuary in India.
In that case, there were no recent introductions
of captive elephants, but lots of interaction with
native tribes and tourists who could have carried the bacteria—
and the story is similar for cases from other parks
in India as well as ones from Sri Lanka, Kenya and South Africa.
And elephant TB has become a really big issue in Nepal,
where there are many humans infected with TB,
and the elephants are often lead around
private hotels to entertain tourists.
The elephants don’t even have to come in
direct contact with people.
Wildlife veterinarians think that wild elephants
might be getting TB from human-contaminated food or waste
at rest stops close to where the animals live.
Overall, this is a huge problem because
all elephant species are considered vulnerable to extinction.
Not only is TB probably contributing to that,
but increased contact with people from ecotourism
or even breeding programs could lead
to more sick elephants.
Again, it’s a case of us wanting to help,
but that help could be posing a risk to the animals.
So to keep everyone healthy, we need to
address the human illness, too.
Scientists also want to follow up on cases
where TB is found in the wild to figure out if there are
other animals acting as disease reservoirs
by carrying the bacteria,
or whether it’s basically just us.
This year, we’ve seen a spike in the number
of human measles outbreaks around the world.
And that’s concerning for our species
and our primate cousins.
Measles is a highly transmissible disease
because it spreads through the air.
And there have been a lot of cases of measles in monkeys
—some from other infected monkeys, and some from people.
One of those human-caused outbreaks
happened in 1999 in a primate research facility
at the National Institutes of Health in the US.
In that outbreak, 94 monkeys from several species
became infected with measles.
That’s a lot of rash-covered,
coughing, stuffy-nosed monkeys.
And the thing is, every new monkey
brought in to the facility had undergone a medical exam
which ensured they were measles-free.
That alone is not enough to guarantee that
the initial spread went from person to monkey, though.
So, researchers analyzed samples of the measles virus
taken from the monkeys, and found it was really similar
to the strain going around humans at the time.
And when they tested the people in the facility,
there was one worker who tested positive
for measles antibodies in their blood,
which indicates they had recently fought off the virus.
All and all, the epidemic spread across two buildings;
it lasted 16 days in one and more than a month in the other.
Then many of the monkeys had to spend
two additional months in quarantine.
Though this incident was pretty bad,
it taught scientists a lot about how to prevent
and control measles in primates.
For example, like in humans, vaccinating
the monkeys stopped the outbreak sooner.
You see, the whole scenario served as a kind of
natural experiment because the monkeys in one building
couldn’t be vaccinated because they were
already part of an experiment.
And that’s building where the epidemic lasted twice as long.
Of course, the best way to prevent monkeys
getting measles is to prevent the people
they come in contact with from getting measles—
like, by requiring vaccinations for people who
work with the animals.
But the danger extends beyond captive monkeys.
Like with the gorillas, measles can also spread
to wild populations—in fact, is has, at least four times,
in places like wildlife sanctuaries and temples
where monkeys often get close to tourists.
Such places do have some options
if the situation gets really bad.
Like, in addition to handing out disposable
facemasks and getting tourists to wash their shoes
in disinfectant, these places could decide to
refuse entry to people who aren’t immune
to measles or other contagious illnesses.
Which is another reason to get vaccinated!
As we’ve seen, our germs can have
an impact on species all over the world.
But up until 2018, scientists thought there were
at least some areas that were untouched
by reverse zoonoses.
Like, the continent of Antarctica.
That all changed when paper in
Science of the Total Environment revealed that
Antarctic bird poop contains bacteria from humans.
The researchers found that 24 bird species
from all over Antarctica, including rockhopper penguins,
Atlantic yellow-nosed albatrosses, giant petrels and skuas,
had all been exposed to some kind of bug
from tourists or visiting scientists.
Specifically, they found evidence of
what doctors call enteric bacteria—
ones that live in our guts and sometimes
cause things like diarrhea and food poisoning.
The birds’ feces contained DNA from
known human pathogens like Campylobacter jejuni,
an antibiotic-resistant type of Campylobacter lari,
and several strains of Salmonella.
And while it’s possible some of these were
passed along from their feathered kin,
C. jejuni is almost never seen in wild birds—
just humans and domestic animals.
Now, the Antarctic birds in this study
didn’t appear to be sick,
but it’s hard to say what effects these bacteria
might have in the ecosystem because this is likely
the first time Antarctic animals have
been exposed to them.
Both the Campylobacter and strains of Salmonella
found in the study can give us raging food poisoning.
But they’re mostly harmless in animals
like chickens and cattle,
minus the occasional case of the runs.
Even if these strains aren’t dangerous
to Antarctic animals directly, they indicate that
our pathogens are reaching the most remote corners
of the planet—though, exactly how they’re getting there
remains an open question.
They might be coming from researchers
who come in direct contact with wildlife,
or, scavenging birds might be picking them up
from human waste that’s left near remote research stations.
They could also be coming from inhabited areas
close to Antarctica, like Patagonia, where wild birds
can waddle in places where livestock are kept.
And scientists think that once some Antarctic wildlife
are infected, they could be spreading it
between themselves.
One thing seems clear: as more research stations
and tourist operations pop up in the area,
there’s an increased risk of native animals
getting these bacteria as well as other,
potentially nastier human pathogens.
So the paper’s authors are now urging
governments to do more to protect them,
like requiring stations carry out human waste
when researchers leave.
It’s no secret that our species has conquered the globe,
but now, we’re realizing our pathogens have also
infiltrated the furthest reaches of this planet.
And as human populations grow,
cases of reverse zoonoses are expected to increase.
That means to keep animals healthy,
we have to keep ourselves healthy, too.
And we should be especially mindful of what we
leave behind when we visit natural areas—
from footprints to feces.
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[OUTRO♪]