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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 motherdied.

  • 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 diseasesand 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 Mexicohence 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 elephantsbasically, 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 populationsin 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,