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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