Almost every pandemic we've seen over the last century has come from a virus that's spilled over into humans from an animal. "Generally, pandemics are seen as a biomedical problem," says Raina Plowright, an infectious disease ecologist at Cornell University. "Certainly, once the pandemic is underway, it is a biomedical problem because you need to have vaccines, you need therapeutics, you need testing," she says.
"But the genesis of the pandemic is actually an ecological problem," says Plowright. That is, it's due to the complex interactions between wildlife, habitat, climate and people.
But there's been relatively little discussion about a spillover's ecological origins and how to stop it from happening in the first place. Plowright found only four publications on how the coronavirus circulates in natural bat populations. That's compared to the tens of thousands of research articles she's pulled up on the coronavirus spike protein, which has been the focus of vaccines and efforts to understand how the virus enters and infects our cells.
Now, in a new perspective paper in Nature Communications, Plowright and a team of 24 ecologists, infectious disease scientists and policy experts have distilled their collective observations into three recommendations to prevent spillovers and halt epidemics and pandemics before they even start.
José Chies, an immuno-geneticist at Federal University of Rio Grande do Sul in Brazil who wasn't involved in the research, says the paper demonstrates the importance of proposing solutions based on the whole ecosystem at once. It's something public health researchers call the One Health approach.
Human health is not something that should be considered in isolation, says Chies. "You should consider animals, microorganisms and environment altogether. It's ecology in a broad approach."
How flowers can stop spillovers
For Plowright, these ideas grew out of her work on black flying foxes in eastern Australia, which she's studied for over two decades. These bats are a sight to behold.
"They have a wingspan of a bald eagle," says Plowright, "and they have these big beady eyes to see in the dark. Little pointy noses."
Black flying foxes feed on nectar. And they'll fly hundreds of miles to find it. "They plunge their heads into the flowers and become covered in pollen," she says. "And then they move that pollen from forest to forest, so they're actually really important pollinators in the Australian forest system."
When black flying foxes are well-fed, they tend to be healthy. "But over time," says Plowright, "the trees that produce flowers in winter were selectively cleared" for agriculture and property development. "And when this happened, the bats then had to move into human-dominated environments — so farmlands and cities — to find alternative food."
A lack of food stressed the bats — and stress causes them to shed, or release, viruses into the environment. In eastern Australia, this included the deadly Hendra virus, which the flying foxes shed in their urine and feces.
When horses have become infected with Hendra from bat excretions, the virus has a fatality rate of 80%. Only seven people have fallen ill from infected horses, but Hendra killed four of them.
"It's a very scary virus," says Plowright. It's especially concerning because each time the virus finds its way into humans, it gets another opportunity to evolve and become more infectious.
(Hendra and a variety of other pathogens don't harm bats, however. The prevailing explanation, Plowright says, is that bats "are very good at controlling viral replication while limiting the inflammation that would usually be associated with feeling sick.")
When hungry and stressed bats roam into human settlements, it creates the perfect storm for a Hendra spillover from bat into horse and occasionally from horse into human.
But here was the crucial observation: Whenever the remaining trees that hadn't been cut down produced a big pulse of nectar in the winter, "we found the bats actually emptied out of these human areas and went back to native forests and started feeding back in these trees," says Plowright. "And when that happened, the spillover stopped."
In other words — the ecological problem of spillover had an ecological solution. "And if spillover doesn't happen," she says, "then a biomedical intervention isn't needed."
Easier to reach the bat cave
Plowright's work with the flying foxes suggested the tantalizing possibility that spillovers could be brought to a standstill simply by restoring these forests. And she figured this approach was likely to work elsewhere in the world. When she spoke with other researchers during a series of workshops and meetings she convened in 2022, she heard echoes of the flying fox story playing out all over the world — all pointing to a planet in which humans and wildlife are bumping into each other more often as natural habitats are being destroyed.
A virologist at Rocky Mountain Laboratories who's studied bats in the Congo for a decade explained the trek to their field site used to take days and involve motorbikes and dugout canoes. Now it's a short few-hour drive down a two-lane highway. A researcher from the Pasteur Institute in Cambodia described how they once had to machete their way through the jungle to study a particular bat cave. Now they can drive there. A colleague from Uganda said they used to research bats in the forest, but now the bats live in the cities. That's because as habitat is cleared and native food disappears, the bats seek alternatives like "mango trees in backyards, fig trees for shade, flowering trees for ornamentation," says Plowright.
"And so what I saw was this picture of this environmental degradation happening worldwide at a really rapid pace," she recalls. "So of course, spillover must be becoming more likely."
It was this set of collective observations, gathered over decades of research, that formed the basis for the three recommendations in the new paper. These strategies, the authors argue, should work for any animal harboring viruses with spillover potential, including bats, birds, rodents and primates.
The three recommendations
First, they suggest protecting where and what animals eat.
"If animals have enough to eat," explains Plowright, then they don't have to move to new environments "to find food, which often is what brings them into higher encounter rates with humans and domestic animals."
In addition, sufficient amounts of food mean less stress, which means "they're more likely to be able to maintain a strong immune system and keep viruses in check."
Second, the authors of the paper advise protecting where animals aggregate — where they "sleep, shelter, mate, socialize and raise their young," as they write in the paper. Bats, for instance, can roost by the tens of thousands. By safeguarding the caves and canopies and other habitats where animals congregate — or introducing buffers around those spots to further minimize human disturbance — that reduces stress considerably too.
Finally, they propose keeping people at risk safe, which they admit isn't an ecological solution but is just as crucial. "There are many communities who have contact with wild animals as part of their vocation or part of their culture," says Plowright. And so the idea is to protect these individuals by providing adequate gear, like masks to guard against aerosolized viruses and PPE to prevent contact with animal fluids and excretions. Plowright says this effort also involves education and training, since some of her colleagues found that "when communities were aware of bat ecology and how important bats are to pollination, seed dispersal and insect consumption, they were less likely to harm or harass the bats, creating conditions where the bats were less stressed."
The proposed solutions are simple enough to articulate. But Plowright acknowledges that implementing them won't be easy.
"Land is extremely valuable," she says. "It's valuable for agriculture. It's valuable for mining, for development, and nature doesn't generate dollars."
Still, for Plowright, the answer is clear. "Nature doesn't stand up and say you have to fight," she says. "It's up to us to figure out a mechanism whereby we are protecting our future and the common interest of all."
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