Deforestation brings home bat-borne viruses

In September 1994, a mysterious interspecies epidemic broke out in Hendra, a suburb of Brisbane, Australia.

First, a pregnant mare fell ill and died.

Soon other horses became ill, running a high fever and expelling a frothy discharge from their snouts.

Two middle-aged men, an ostler and a horse trainer who had reportedly tried to hand-feed the dying mare also developed flu-like symptoms.

Although the groom recovered, the trainer was ultimately dead, as were more than a dozen horses.

Scientists eventually traced the outbreak to a virus brought by fruit batsalso known as flying foxes.

Bats shed the pathogen, called Hendra virus, in their feces and saliva, passing it on to horses, which can then pass it on to humans.

In the years since then, there have been dozens of gems additional cases in horses and many more cases in humans.

A new study, based on 25 years of data from Australia, suggests that environmental changes have driven these side effects radically alter the ecology of black flying foxes.

The deforestationcoupled with weather-related food shortages, it has pushed bats into human-dominated habitats like farms, where food is readily available but may be of lower quality, scientists reported in Nature on Wednesday.

In many of these new refuges, the bats are not only in closer contact with the horses, but are also able to spread higher levels of the virus, possibly due to the nutritional stressaccording to a second study conducted by many of the same researchers and published in Tasting Ecology last month.

“We’re transforming the planet in this way by pushing animals to really at the edge, at the edge of their ability to cope,” said Raina Plowright, an infectious disease ecologist at Cornell University and lead author of both studies.

“And that’s creating stress that’s also more likely to bring pathogens into human populations.”

The idea that deforestation may increase the risk of disease spread is not new, and scientists have repeatedly documented the connections between forest fragmentation and disease outbreaks as varied as ebola, malaria and Lyme disease.

But the new research is a remarkably detailed case study, experts said, revealing exactly how environmental changes can drive disease risk and how and where experts might intervene.

“It’s just an extremely impressive feat,” said Dr. Aaron Bernstein, interim director of the Center for Climate, Health, and the Global Environment at the Harvard TH Chan School of Public Health, who was not involved in the research.

“These scientists have essentially drawn the dots across a number of factors that we know can drive emerging infections.”

He added: “I think it indicates how critically important it is to focus on upstream prevention to really prevent spillover.”

No more nomads

The new study is the result of a decade-long collaboration between Plowright and Peggy Eby, a wildlife ecologist with an additional position at the University of New South Wales who has spent 30 years studying flying foxes that live in a subtropical region. of eastern Australia.

They worked with an interdisciplinary group of colleagues to analyze a wide range of ecological data, including bat roosts, bat fitness, weather patterns, nectar scarcity, habitat loss and viral transmission to horses, collected in the region between 1996 and 2020.

Historically, the local black flying foxes, which feed largely on the nectar of eucalyptus flowers, have lived in huge nomadic groups, making their way through native forests in search of flowering trees.

While flowers are plentiful in the summer, the supply is much more limited during the winter and spring.

And every few years, a weather fluctuation, such as a strong El Niño event, stops flowering winter or spring, creating food shortages.

Bats have generally been content to break up into smaller groups and establish temporary roosts near more readily available food sources, such as farms or urban gardens.

When the nectar shortage subsided, the bats would return to the forest.

“As soon as the nectar started flowing again, they banded together in large aggregations and started becoming nomadic and again foraging in native forests,” Plowright said.

This pattern persisted into the early years of the study period, from about 1996 to 2002, the researchers reported.

And during these years no outbreaks of Hendra have been detected in the region.

But around 2003, the pattern changed, the scientists found.

When severe food shortages arose, new groups of bats would still break away from their compatriots and settle near farms and towns.

But now bats have created these new habitats permanentabandoning their nomadic lifestyle in the forest.

Between 2003 and 2020, the total number of perches in the region has tripled, while the size of each bat group has decreased.

Additionally, the roosts moved closer together and the bats foraged for food in smaller areas.

The researchers theorized that this change in behavior was because the forests upon which the bats depended, mainly for their meager supply of nectar in winter, were rapidly disappearing.

In southeast Queensland, almost one third of bat winter feeding habitat disappeared between 1996 and 2018.

“We think what’s happened is that it no longer makes sense for these animals to support these large nomadic populations,” Plowright said.

“It’s very difficult to find food.”

Instead, he said, bats may find it easier to survive by settling near the available supply of lower-quality food provided by farms and gardens.

“You don’t have to spend a lot of energy finding it,” he said.

“It’s easier to make a living. alive next to McDonald’sSt.

In their second study, Plowright, Eby and their colleagues reported that bats also live in these new habitats they expelled more viruses in winter than those left in the forest, perhaps because they were not sufficiently fed to maintain a robust immune response.

(Virus shedding also tended to be higher in both bat populations after food shortages.)

Together, these findings suggest that more bats are now shedding more virus while living closer to horses, increasing the frequency of Hendra transmission.

Between 2003 and 2020, at least 40 indirect bat-to-horse events occurred in the region, the scientists found.

These events were especially common in agricultural areas in the winters following food shortages.

Cara Brook, a disease ecologist at the University of Chicago, praised the study.

“This is outstanding work,” he said.

“Indeed, we have a rigorous and quantitative example of how land use change affects the habitat of a wildlife species and accelerates zoonotic risk. And I think it is really important for conservation issues.

Similar dynamics may be at play in other ecosystems, he added:

“There are wild bat populations that are reservoirs of emerging diseases around the world.”

The study also points to possible solutions.

Within a few years, the remaining eucalyptus trees bloomed in winter, attracting more than 100,000 flying foxes to the forest.

These increasingly rare winter ‘flowering legumes’ reduced the risk of spreading Hendra virus, the scientists found.

In this particular ecosystem, Plowright said, replant species that flowering in winter could help keep bats off farms, while protecting horses and humans from the virus.

“The Hendra virus case study,” Eby said in an email, “shows that with the collaborative efforts of wildlife ecologists, disease ecologists, data scientists, and veterinary health specialists, threat mitigation can be added. side effects to the available tools. reduce the risk of a pandemic”.

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