Global land-use changes — including forest fragmentation, agricultural expansion and concentrated livestock production — are creating “hotspots” favorable for bats that carry coronaviruses and where conditions are ripe for the diseases to jump from bats to humans, finds a new analysis by researchers at the Politecnico di Milano, University of California, Berkeley and Massey University.
While the exact origins of the SARS-CoV-2 virus remain unclear, many scientists believe the disease likely emerged when a virus that infects horseshoe bats was able to jump to humans, either directly through wildlife-human contact, or indirectly by first infecting an intermediate animal host such as the pangolin. Horseshoe bats are known to carry a variety of coronaviruses, including strains genetically similar to ones that cause Covid-19 and severe acute respiratory syndrome (SARS).
The new study used remote sensing to analyze land use patterns throughout the horseshoe bat’s range, which extends from Western Europe through Southeast Asia. By identifying areas of forest fragmentation, human settlement, and agricultural and livestock production, and comparing these to known horseshoe bat habitat, they identified potential “hotspots” where habitat is favorable for these bat species, and where these so-called zoonotic viruses could potentially jump from bats to humans. The analysis also identified locations that could become hotspots with changes in land use.
“The analyses aimed to identify the possible emergence of new hotspots in response to an increase in one of three land use attributes, highlighting both the areas that could become suitable for spillover and the type of land-use change that could induce hotspot activation,” said study co-author Maria Cristina Rulli, a professor in hydrology and water and food security at the Politecnico di Milano.
“We hope these results could be useful for identifying region-specific targeted interventions needed to increase resilience to coronavirus spillovers.”
“Land use changes can have an important impact on human health, both because we are modifying the environment, but also because they can increase our exposure to zoonotic disease,” said study co-author Paolo D’Odorico, a professor of environmental science, policy and management at UC Berkeley.
“Every formal land use change should be evaluated not only for the environmental and social impacts on resources such as carbon stocks, microclimate and water availability, but also for the potential chain reactions that could impact human health.”
Most of the current hotspots are clustered in China, where a growing demand for meat has driven the expansion of large-scale, industrial livestock farming. Concentrated livestock production is particularly concerning because the practice brings together large populations of genetically similar, often immune-suppressed animals that are highly vulnerable to disease outbreaks, the researchers said.
The analysis also found parts of Japan, the north Philippines, and China south of Shanghai are at risk of becoming hotspots with further forest fragmentation, while parts of Indochina and Thailand may transition into hotspots with increases in livestock production.
Human encroachment into natural habitat can also can indirectly increase exposure to zoonotic disease by reducing valuable biodiversity. When forest lands become fragmented and natural habitats are destroyed, species requiring very specific habitat to survive, called “specialists,” may dwindle or even go extinct. Without competition from specialists, “generalist” species, which are less picky about their habitat, can take over.
Horseshoe bats are a generalist species, and have often been observed in areas characterized by human disturbance. Earlier work by Rulli, D’Odorico and study co-author David Hayman has also linked forest fragmentation and habitat destruction in Africa to outbreaks of the Ebola virus.
“By creating conditions that are disadvantageous to specialist species, generalist species are able to thrive,” D’Odorico said.
“While we are unable to directly trace the transmission of SARS-CoV-2 from wildlife to humans, we do know that the type of land-use change that brings humans into the picture is typically associated with the presence of these bats who are known to carry the virus.”
While China has been a leader in tree-planting or “greening efforts” over the past two decades, many of the trees have been planted in discontinuous land areas or forest fragments. To tilt the ecological balance back in favor of specialist species, creating continuous areas of forest cover and wildlife corridors are more important than increasing total tree cover.
“Human health is intertwined with environmental health and also animal health,” D’Odorico said.
“Our study is one of the first to connect the dots and really drill down into the geographic data on land use to see how humans are coming into contact with species that might be carriers.”