TUESDAY 22 Nov. 2022 (HealthDay News) — Research in wild bats reinforces an idea crucial to stopping future pandemics: When wildlife populations remain healthy, crossover viruses are less likely to infect humans.
In Australia, deforestation has allowed a deadly respiratory virus to pass from fruit bats to humans, forcing the two species into closer contact, a new study reports.
Deprived of their winter habitats, large populations of “flying foxes” have begun to break up over the past quarter century and nest in smaller groups closer to human agricultural and urban areas in subtropical Australia, the study authors explained.
These bats are the natural reservoir of Hendra viruswho jumped from bats to horses and then from horses to humans, according to the report published Nov. 16 in the journal Nature.
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The Hendra virus causes a serious respiratory infection that has been proven to be 75% lethal in horses and 57% in humans.
The case study offers a glimpse into the process that causes infectious diseases such as Ebola to jump from animals to humans, a process called “pathogenic spillover,” the researchers noted.
“We collected and collected 25 years of data and saw this amazing pattern. We have captured this rapid transition from bats feeding on large populations as nomadic animals to bats living in small populations, in areas where there are people,” said senior researcher Raina Plowrighta professor of public health and ecosystem health at Cornell University in Ithaca, NY
For the study, the researchers tracked the locations and sizes of fruit bat populations in subtropical Australia from 1996 to 2020.
Typically, the fruit bats stayed together in large groups, living in forest areas and feeding on the nectar of flowering trees.
But during El Niño weather events, which occur in cycles, the buds of trees that bats depend on for nectar would not produce flowers in winter, leading to a food shortage.
When that happened, the bats split into smaller groups and moved into agricultural and urban areas, where they fed on weeds and fig, mango and shade trees, Plowright said. These food sources are more reliable, but provide less nutrition than the nectar of flowering trees.
“It makes a lot more sense to save energy to split up into small groups, so as not to have to compete against each other too much,” she said.
As it turns out, the winter food shortages caused by El Niño were a sign of what would happen as a result of deforestation, the researchers noted.
Human destruction of forest habitat for farmland and urban development has reduced the number of places that could produce enough tree nectar to support large and nomadic bat populations, Plowright said.
“That behavior that we previously saw as a response to a food shortage that was very short, we are now seeing behaviors become persistent throughout the year in the bat population,” she said.
The smaller groups of hungry bats also tend to shed more viruses, the researchers found — possibly because their starving bodies don’t have the energy to mount an effective immune response.
In agricultural areas, Hendra viruses spread when bat urine and feces fall on the ground where horses graze, leading to infections. Occasionally, the horses’ own feces spread the virus to humans.
“We selectively removed the trees that provide food in the winter, so the bats had to die or adapt,” Plowright said. “They have been looking for small populations in agricultural areas to find new food sources. Essentially, we changed their food source, so they had to come to us.
The removal of the bats has also jeopardized the survival of the few remaining areas of forest, Plowright said. That’s because the bats that feed on nectar behave similarly to bees, spreading pollen from tree to tree.
The study “precisely illustrates what happened in this particular ecological condition, reinforcing the idea that human intrusion into the environment alters the natural balance,” said Dr. William Schaffner, medical director of the Bethesda, Md.-based National Foundation for Infectious Diseases. “Then the animals that are the source of the virus have to change their behavior to survive, bringing them into closer contact with humans and giving them a chance for a species jump.”
Interestingly, the researchers found that when the remaining eucalyptus trees were allowed to flower in the winter, large numbers of bats flocked to those areas and the virus spill stopped.
This shows that humans can prevent the future epidemics caused by spillover viruses by preserving existing natural habitats and restoring some of the forests that have been cut down unnecessarily, Plowright said. Wild animals can return to their former patterns, given the chance.
“We think if we restore that winter habitat, within 10 or 20 years we could have a healthy population of bats moving nomadic across the landscape, restoring that pollination they provide to forests,” Plowright said. “It is a solvable problem. It’s actually not even very expensive, not that difficult. It doesn’t require any kind of technology. Just replant these trees.”
Schaffner is a bit more skeptical that replanting forests will restore the natural patterns that typically keep virus-carrying critters away from humans and livestock.
“We have to try to refrain from entering the habitat and degrading the natural environment, because that is what puts us in contact with the flora and fauna of the wild, and that creates opportunities for the transmission of these viruses from wild species to humans,” he said. “That story has been told over and over now.”
The World Health Organization has more on it Hendra virus.
SOURCES: Raina Plowright, PhD, professor, public and ecosystem health, Cornell University, Ithaca, NY; William Schaffner, MD, medical director, National Foundation for Infectious Diseases, Bethesda, Md.; NatureNovember 16, 2022