What if a deadly disease outbreak could be prevented by something as seemingly unrelated as protecting flowering trees?
It might seem like a small price to pay, given what can happen when a virus jumps from wild animals to humans. Witness the cost of the COVID-19 pandemic, thought to have originated in bats. The death toll today is more than 6.5 million people worldwide.
Now, scientists for the first time have uncovered a strong link between habitat destruction and eruptions of a bat-borne disease that can kill both horses and humans. Their meticulous detective work traces how climate and habitat loss can conspire to spark a “spillover” event – when a disease hops from one species to another.
The work offers the possibility of predicting and even preventing similar outbreaks in the future. Among key steps: Improving wild habitat rich with bat food.
“Right now, the world is focused on how we can stop the next pandemic,” said Raina Plowright, a Cornell University ecologist who led the research. “We’re hoping that this paper will bring prevention and nature-based solutions to the forefront of the conversation.”
For years it has been thought that habitat destruction could help fuel spillovers by blurring boundaries between the places where humans and wildlife live. Past research found correlations between habitat loss and disease outbreaks. But scientists lacked a clear sequence of events that showed exactly how it happened.
Plowright teamed up with Australian researchers to see if they could uncover what was driving sporadic spillovers of Hendra virus in the Queensland area of eastern Australia. Fruit bats in the region, also known as flying foxes, carry the disease but aren’t sickened by it. However, horses can get infected when they are exposed to tainted bat feces and urine. The virus can then move from horses to people. Though rare, infections are fatal 75% of the time in horses (84 documented deaths) and more than half the time in humans (4 deaths). More than 60% of the 63 known spillover events have happened in clusters in eastern Australia, chiefly in the winter.
To figure out why such outbreaks happened in certain places and certain years, the researchers collected an ocean of data from eastern Australia spanning 25 years, 1996 to 2020. Focusing on black flying foxes, they tracked habitat: the amount of prime bat winter feeding grounds and how much forests bloomed in the winter with nectar-bearing flowers. They monitored climate-related variables: whether or not it was an El Niño year—a warming in the tropical part of the eastern Pacific Ocean that’s associated with drier winters in eastern Australia. They watched the bat population: where it was found throughout the year, the number of bats grouped in roosts, and how many bats were taken into wildlife rehabilitation centers. Finally, they tracked where the disease showed up in horses and people.
When the different pieces were funneled into a computer model, patterns stood out. Over the course of 25 years, bats increasingly spent winters in farmland and cities. There, they fractured into more groups. Early in the study, bats traded forests for human dominated landscapes only for weeks at a time, when it was a bad year for flowering eucalyptus trees, a favorite source of nectar during through the winter. There, the animals could survive on domesticated plants such as mango and figs, even though that is less nourishing than the forest nectar.
But as time went on, the amount of natural forest dwindled, shrinking by nearly a third from 1996 to 2018. At the same time, the bat’s visits to agricultural and urban land became longer and more regular, increasing the likelihood they and the Hendra virus would come into contact with horses.
Climate fluctuations heaped on top of this steady change in habitat and bat behavior to create the worst clusters of spillover events, the scientists reported Wednesday in the journal Nature.
Their analysis illuminated how a cascading series of events stretching over years conspired to fuel the most intense outbreaks. First came a strong El Niño, followed by a flower shortage during the winter and spring. A bust in the crop of flowering trees the following winter exacerbated things further. That sequence coincided with clusters of 3 or more outbreaks, the researchers found. The pattern was strong enough that when the scientists looked back over the 25 years, the computer model could reliably predict in which years such outbreak clusters would happen.
In addition to increasing contact, food shortages could also exacerbating the spillover risk. The researchers found that bats who set up new roosts outside their usual range shed more of the Hendra virus into the environment, particularly when there was a food shortage. A lack of food is tied to increasing viral loads, potentially because stress affects a bat’s immune system.
The results underscore the role people played in exacerbating the problem—and, potentially, what they could do to help tackle it. While people can’t reverse El Niño’s, that alone wasn’t enough to trigger more frequent outbreaks. If there was a surge in flowering trees the following winter, the bigger spillovers didn’t materialize. But as forests shrank, booms in winter flowering became less common.
“When remaining habitat produces food, spillover stops,” said Plowright. “Therefore a sustainable way to stop these events could be to preserve and restore critical habitat.”
Eby, et. al. “Pathogen spillover driven by rapid changes in bat ecology.” Nature. Nov. 16, 2022.
Photo: Grey headed flying fox (Pteropus poliocephalus). ©Vivien Jones
