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Australian foxes carry a virus called Hendra, which has spread to humans and can cause a rare but deadly respiratory infection
.
"Hey, guys, can you spread your wings and show me?" Peggy Eby said
, looking up at the habitat of a group of flying foxes in the Sydney Botanic Gardens.
"I talked to them a lot
.
"
Eby, a wildlife ecologist at the University of New South Wales in Sydney, is looking for lactating females and their newborn pups, but cloudy weather leaves them snuggling under
their mother's wings.
Eby has been studying the flying fox, a species of bat, for 25 years
.
Using binoculars, she counted the number of lactating female bats that were about to be weaned — a proxy for whether bats are experiencing nutritional stress and therefore may be more likely to spread viruses
that can make people sick.
Australian foxes are of interest because they carry a virus called Hendra, which causes a very rare but deadly respiratory infection that kills one in every two infected people
.
Hendravirus, like Nipah virus, SARS-CoV-1 virus and SARS-CoV-2 virus (the virus that causes the COVID-19 pandemic), is a bat virus that has spread to humans
.
These viruses are usually transmitted to humans through intermediate animals, sometimes with fatal consequences
.
Scientists know spillover effects are linked to habitat loss, but until now it was difficult to pinpoint the specific conditions
that triggered the event.
After a detailed investigation, Eby and her colleagues can now predict — in up to two years — when the Hendra virus might overflow
in clusters.
Emily Gurley, an infectious disease epidemiologist at Johns Hopkins University in Baltimore, Maryland, said, "They have identified environmental drivers of
spillovers.
" They have determined how to prevent these incidents
.
The findings were published Nov.
16 in the journal Nature
.
Specifically, the researchers found that Hendra virus underwent a cluster spillover
years after bats experienced food stress.
These food shortages are usually accompanied by years of strong El Niño, a climatic phenomenon in the tropical Pacific Ocean often associated with
drought in eastern Australia.
But if the trees that bats depend on for their livelihood in the winter have a big flowering event the second year of food shortages, there is no spillover effect
.
Unfortunately, the problem is that "there is almost no winter habitat left," says
Raina Plowright, a disease ecologist and study co-author at Cornell University in Ithaca, N.
Y.
Sarah Cleaveland, a veterinary and infectious disease ecologist at the University of Glasgow in the UK, said the research was "absolutely remarkable and exciting and it leads directly to the solution
.
" Cleaveland said the study, which looks at climate, environment, nutrient stress and the impact of bat ecology, could bring new insights to the study of other pathogens, including Nipah and Ebola viruses and their virus families
.
Alice Hughes, a conservation biologist at the University of Hong Kong, said the study provided "a clearer understanding of the drivers of this problem, with broad correlations
to epidemics elsewhere.
" With climate change and habitat loss on the rise, she said, "this paper highlights the increased
risks we may see.
" ”
In 1994, an outbreak of Hendra virus
occurred in horses and people at a thoroughbred horse training facility in Brisbane.
Later studies confirmed that the virus spread to horses
from its bat host, most likely a black fox, through the fox's droppings, urine and chewed pulp splashes on grass.
Infected horses spread the virus to people
.
Infections usually occur during the Australian winter, and it may be a few years before another cluster appears in horses, but cases have been increasing
since the beginning of the 21st century.
To study the spillover mechanism, Plowright, Eby and their colleagues collected data
on the location and timing of about 300,000 square kilometers of bat habitat in southeastern Australia between 1996 and 2020, the location of bat habitats and their health, climate, nectar shortages and habitat loss.
They then used the model to determine which factors were associated with
spillover effects.
"I'm just in awe
of the valuable ecological datasets they have," Cleaveland said.
Over the course of the study, the team noticed significant changes
in the bat's behavior.
Foxes have moved from a predominantly nomadic lifestyle — moving in groups from one primeval forest to another in search of nectar — to settling in small groups in urban and agricultural areas, bringing bats closer to where
horses and people live.
The overall number of bats has tripled since the beginning of the 21st century, reaching around
320 by 2020.
Another study by the team found that newly established habitats spread Hendra virus every winter, but bats spread more in
the years following food shortages.
Daniel Becker, a co-author of the study, said there was a "very significant surge in infections during the winter months.
"
Becker is an ecologist
at the University of Oklahoma in Norman who focuses on infectious diseases.
The study also linked
increased viral shedding in bats to an increase in virus infections in horses.
The models in Plowright and Eby's recent paper published in Nature show that when food is scarce, fox populations split into small groups and migrate to agricultural areas close to horses, with food shortages immediately following strong El Niño events, possibly because the buds of native eucalyptus trees are sensitive
to climate change.
To save energy, bats have only flown short distances over the years, searching for food
in nearby agricultural areas.
The spillover effect on horses is most likely to occur after
winter food shortages.
Their model was able to accurately predict these years
.
Then the unexpected happened
.
El Niño in 2018, followed by drought in 2019, suggests that 2020 should also be an overflow year
.
But it happened only once in May, and it hasn't been detected since
.
"We threw all the cards back into the air, taking a closer look at all the other elements we hypothesized about," Eby said
.
Ultimately, they found that when native forests experienced large flowering events after winter food shortages, this helped avoid spillover effects
.
In 2020, a forest of red eucalyptus near the town of Kinpi bloomed, attracting about 240,000 bats
.
Similar winter flowering events
occurred in other regions in 2021 and 2022.
Researchers believe that these large-scale migrations keep bats away from horses
.
They propose that by restoring the habitat of those few species that bloom in winter, there will be fewer spillovers to horses and possibly spillovers to humans
.
By restoring the habitat of other animals carrying dangerous pathogens, "maybe we can prevent the next pandemic
.
" ”
