Stressed, hungry bat populations are linked to growing cases of an emerging zoonotic disease in Australia, new research finds. The bats have learned to adapt to more persistent food shortages by roosting closer to humans. That raises the risk of the potentially fatal Hendra virus jumping from bats to horses to people, according to a study published today in the journal Nature.
“We want to be able to keep the reservoir hosts [aka bats] happy”
Nevertheless, the research tells a story about why it’s important to protect bats that have gained a somewhat unfair bad rap for exposing humans to new kinds of viruses. At the end of the day, we actually influence each other’s ability to live in a healthy environment. It’s part of a concept called “One Health” that looks at how the well-being of plants and animals is important for public health — as we saw last year in the US when an outbreak of Salmonella in people was linked to sick wild songbirds. The new research published today shows how One Health plays out with bats in Australia. And that could help pinpoint ways to prevent disease from spreading through a phenomenon called “spillover,” when viruses hop from one species to another.
“We want to be able to keep the reservoir hosts [aka bats] happy,” says Raina Plowright, a professor at Cornell University and one of the authors of the study, “with enough food to eat, protected from stress, [to] ensure that we don’t see events like Hendra virus spillover.”
Luckily, Hendra virus infection is still rare. In humans, it can look like the flu but can also become fatal as it affects the body’s respiratory and neurological systems. Since the virus was first detected in humans in Australia in 1994, four of seven people who’ve contracted it have died. They were exposed to the virus by treating or handling sick or dead horses, which in turn got the virus from bats. There have been more than 80 documented cases of horses dying from Hendra, although there’s now a vaccine for them. And the World Health Organization says that treatment with monoclonal antibodies “is being investigated” for humans.
That’s all to say that there’s no need to panic about the Hendra virus now. The research gives us a bigger picture of how bats, horses, and humans interact with each other so that we can potentially preempt a more worrisome virus from following similar pathways as Hendra from animals to people. Hendra doesn’t spread easily from human to human, but if there was a related virus with a high fatality rate and the ability to spread efficiently among people — “And that was driven out of nature into humans, then that would be a catastrophe,” Plowright says.
To conduct their research, Plowright and her colleagues analyzed 25 years of data between 1996 and 2020 on bat behavior, environmental changes, and cases of Hendra spilling over from bats to horses in Australia. They studied fruit bats called flying foxes that have an extremely important job to do for the continent. Like bees, they’re pollinators who feed on nectar from flowers. “Their whole face is immersed in these flowers, and it gets covered in pollen, and it’s wonderful,” Plowright tells The Verge, describing the bright yellow faces of the bats after mealtime.
The bats can travel hundreds of kilometers in a night, spreading that pollen over great distances. That’s crucial for sustaining fragmented forests that have been cut up by human development. “[They’re] ensuring that these forests remain genetically diverse and resilient, particularly as we’re changing the climate,” Plowright says. “They’re really the only animals that can maintain this resilience over big landscapes.”
The bats’ resilience, however, has been increasingly tested — and that has consequences for human settlements. Hendra virus doesn’t lead to discernible disease in bats and is believed to have circulated in their populations for much longer than Europeans have even occupied Australia. It wasn’t until the 2000s that Hendra virus spillovers increased quickly, coinciding with more deforestation and worsening food shortages for the bats. Those food shortages drove clusters of Hendra virus spillovers, particularly in the wintertime.
It started with habitat loss and climate change making the bats’ preferred food — flowers — more scarce. The new research finds that those forces have pushed bats to become less nomadic, choosing instead to persistently roost near agriculture for a more consistent food source. When the bats feed in places where horses graze, it gives the virus the opportunity to jump from bats’ feces and spit to horses. Documenting that behavior builds on mounting evidence that deforestation and climate change raise the risk of virus spillover.
Still, there was a surprise in their findings. The researchers found that during rare winter flowering events, spillover all but disappeared as bats left their roosts near humans and horses to find their favorite food. “If there was winter flowering, then we had a nearly zero chance of there being spillover,” Plowright says. “When we looked at the data systematically, it was stunning.”
That’s a big ray of hope for Plowright. Provide the bats a habitat so that they always have food, and we won’t have to worry so much about them — or any viruses they might carry — becoming our pesky neighbors. That might look like planting species of trees that produce the flowers bats prefer much more than what they might find in agricultural fields or urban areas. “We have this potentially quite simple solution,” she says. “It’s achievable, it’s scalable, it’s not that expensive to implement. People are enthusiastic to do it.”