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Usually when we think about animals and disease, we think about illnesses that they transmit to us - like swine flu or Lyme disease. But illness is often a two-way street, and while animals can pass pathogens to us, we can also pass our own pathogens to animals.

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Head over to to learn more about virtual private networks and internet security. [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.

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