How climate change makes the deadly Hendra virus a bigger threat : Goats and Soda It's not often that this pathogen jumps from bats to horses, then humans. When it does, the result is brutal. New research points to a surprising way to stop spillovers.

Hendra virus rarely spills from animals to us. Climate change makes it a bigger threat

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SCOTT SIMON, HOST:

Most scientists believe that COVID spilled over into humans from an animal, maybe a bat. Now a new study shows why these viruses sometimes end up in humans and how we might be able to stop them. NPR's Ari Daniel explains.

ARI DANIELS, BYLINE: Not quite 20 years ago, Raina Plowright stood in a forest in Australia's Northern Territory. It was dusk, and she watched as hundreds of thousands of bats called little red flying foxes launched themselves into the air.

(SOUNDBITE OF BATS SQUEAKING)

DANIELS: This audio is from a videorecording Plowright made at the time.

RAINA PLOWRIGHT: The sky was black with these huge bats taking off in this stream of animals across the landscape looking for nectar.

DANIELS: Plowright, who studies pandemic prevention, was interested in the bats because they carry a virus called Hendra. It's harmless to bats. But horses coming into contact with infected bat feces and urine have contracted Hendra and gotten extremely ill - frothy nasal discharge and odd behaviors like throwing themselves against the wall of a stable.

PLOWRIGHT: When people treat sick horses, that's when people get sick. That's when people get Hendra virus.

DANIELS: Now, Hendra doesn't spread easily among humans. There've only ever been seven cases. But each time a virus jumps from animals to people, it gets another chance at evolving and becoming more infectious. With a Hendra fatality rate around 50%, the possibility of an outbreak could be devastating.

PLOWRIGHT: I mean, we're talking about a catastrophic, civilization-changing event.

DANIELS: Plowright wanted to understand the conditions that make Hendra more likely to jump from bats into horses. And if she knew that, she thought she might be able to halt those spillovers before they ever happened. First, she had to capture bats to look for the virus. So every three months, Plowright and her team strung up nets along the riverbanks, keeping an eye out for crocodiles.

PLOWRIGHT: They actually learn your behavior, so we'd have to move our nets every day so that the crocodiles wouldn't learn where we were.

DANIELS: Plowright sampled the blood of the bats, and the results were surprising.

PLOWRIGHT: We often couldn't find any virus at all within the population.

DANIELS: Then, a few years later, Plowright was out testing the bats again for Hendra. She'd even convinced a team from National Geographic to come and document the dramatic swarms of bats. Except this time, she couldn't find any.

PLOWRIGHT: So we arrived, and all of these hundreds of thousands of bats had disappeared. There'd been a cyclone off the coast of Australia, and there was no food for the bats.

DANIELS: When Plowright and her crew finally did find bats, it was an unusually small group. They were emaciated and starving. And when they sampled the bats' blood, it was teeming with Hendra virus.

PLOWRIGHT: And so that cued us that maybe nutritional stress is leading to infection and shedding of this virus in these bats.

DANIELS: Plowright and her colleagues wanted further confirmation, so they looked back at 25 years' worth of data of another species of bat - black flying foxes in Eastern Australia. And they found the same pattern. When these animals went without food...

PLOWRIGHT: They just don't have enough energy to maintain an immune response to keep these viruses in check.

DANIELS: This means they have to look farther afield for food. So they splinter into small groups and move into urban and agricultural areas, which brings them into contact with horses and people. But...

PLOWRIGHT: Whenever there was winter flowering, there was never a spillover case. We were just stunned.

DANIELS: As long as the bats had enough food, the virus didn't show up in people. But due to climate change and habitat loss, these pulses of wintertime food are becoming less common. And this suggests an elegant intervention.

PLOWRIGHT: What we really need to be doing are planting those winter forests back. And it's not a really difficult thing.

DANIELS: These findings from Plowright, now at Cornell, and her colleagues are published in the journal Nature. And she says the same scenario's playing out in Asia and Africa as well.

PLOWRIGHT: So the world is just being transformed at this alarming rate, and animals are responding by changing their distribution to survive. Often, that leads to virus shedding.

DANIELS: So to give bat-borne viruses fewer chances of spreading, Plowright advises pay attention to their habitat, plant the right kind of trees, and keep those massive colonies healthy. Ari Daniel, NPR News.

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