- A new review study traces the complex links between biodiversity loss and emerging infectious diseases — though one doesn’t necessarily lead to the other.
- Instead, complex interactions between factors (including climate change, habitat loss, agricultural practices, and closer contact between wildlife, livestock and people) can contribute to emergent infectious diseases and new pandemics.
- It’s now well understood that human actions are causing a major increase in pandemics. To stave off future global outbreaks, researchers say we need to better understand the shared upstream drivers of both biodiversity loss and emerging disease.
- The study highlights significant gaps in the monitoring and surveillance of wildlife pathogens worldwide. It suggests that prevention and early interventions targeting locales and situations where emergent disease spillover is likely are important to avoiding future human pandemics.
Five years ago, the COVID-19 pandemic served as a wake-up call. Since then, health experts worldwide have sat on tenterhooks, hyperaware that pathogens can jump from animals to humans at any moment, bringing the next disruptive global health crisis that kills millions.
But COVID-19 didn’t happen in a vacuum. Researchers have long warned that human activities make pandemics far more probable and more frequent.
“We know that spillovers from non-human animals are more common,” says Daniel Becker, assistant professor of biology at the University of Oklahoma. “We know that biodiversity loss is increasing. We know that the climate is getting warmer and harder to predict, and that these things are all happening at the same time. So we’ve … entered what we call this polycrisis,” where many problems intersect and influence each other in complex ways.
In a recent review study published in Nature Reviews Biodiversity, Becker and co-authors explore connections between emerging infectious disease and biodiversity loss, identify data gaps, and suggest ways forward to better monitor and prepare for emerging diseases.
“We’re not only in the middle of human pandemics, but also … wildlife pandemics,” Becker explains. In the new review, Becker’s team examines “the common drivers of disease emergence and biodiversity loss.”
The review study adds to the growing evidence that human, animal and planetary health are inextricably entwined, says Neil Vora, senior director of One Health at Conservation International, an NGO.
The new research “shows that humanity’s broken relationship with nature is driving global changes that have severe consequences for our own health,” Nora, who wasn’t involved in the study, wrote in an email to Mongabay. The review “underscores that only by mending this broken relationship can we address climate change, biodiversity loss, and the overlapping health crises that increasingly afflict us.”
Virtually all human infectious disease has its roots in animal pathogens at some point in the evolutionary past. What’s changing now is the frequency with which cross-species transmission, called spillover, occurs. This can lead to new diseases in humans, or rare diseases becoming more common, or well-known diseases moving into new regions.
A 2023 study that surveyed data from 1963-2019 found that spillover events of a subset of well-reported zoonotic viruses have been rising at a rate of nearly 5% per year — and that was before COVID-19 burst upon us.
The world is also simultaneously experiencing a biodiversity crisis, with monitored wildlife populations declining by 73% in the past 50 years, according to WWF’s 2024 Living Planet report.
Generally, biodiversity loss is followed by higher pathogen transmission, according to various studies. But the relationship between the two is nuanced.
“Current pop science thinking is, one thing drives another, so biodiversity loss always causes more disease,” says Cole Brookson, a Ph.D. candidate at the Yale School of Medicine and one of the study co-authors. “We prefer to think these are both problems that are increasing in their severity through time, and they interact with each other. [I]t’s really important to try to not think of one in the absence of the other, because they are so related in so many cases.”
Habitat loss, agricultural practices, climate change and the global wildlife trade are widely recognized as drivers of biodiversity loss. These human-caused disruptions also increase the risk of emerging infectious diseases — providing pathogens with opportunities to move into new species, or aiding known diseases in expanding their geographic range. Spillover can happen due to greater overlap and/or greater interaction between wildlife and humans, or due to changes in wildlife communities or their health.
Take the example of Hendra virus in Australia. The pathogen is carried by flying foxes, the Pteropus genus of bats, for which it’s largely harmless. But today, habitat loss is pushing flying foxes onto agricultural lands, where they roost in horse corrals, putting them in close proximity to livestock and people. In addition, the splintering of previously connected populations can decrease the immunity of flying foxes to the virus, as disconnected populations may not be continuously exposed in the same way Lastly, when flying foxes are nutritionally stressed (due to habitat loss or climatic changes, for example), they shed greater viral loads. The combination of all these factors increases the likelihood of spillover, and in recent decades has likely contributed to the rising number of serious and often fatal cases of Hendra virus in horses, and subsequently humans.
Land-use change is likewise implicated in the spread of Lyme disease in North America. Lyme disease was first recognized in the eastern United States in 1975 and is still spreading rapidly. It’s caused by the Borrelia burgdorferi bacteria and passed to humans via tick bites. But ticks are merely a bacterial carrier; the real reservoir is mammals, particularly mice. It’s theorized the trigger for the emergence of Lyme in humans was forest clearing and fragmentation, along with suburb development in the U.S. state of Connecticut, altering the balance of mammal species. These changes favored mice, which are frequently infected with the bacteria, with biting ticks acting as the vector between mice and people. Climate change is also facilitating Lyme’s spread, as tick populations survive milder winters and move northward.
These two examples show how complex interactions between climate, land-use change, ecosystems, wildlife and human drivers can trigger infectious disease emergence via widely varying pathways, Becker says. But more case studies are urgently needed to better understand these and other spillover patterns.
The review highlights significant data gaps in the surveillance and monitoring of wildlife pathogens and emerging disease. To see where the gaps are, the researchers looked for records of mammal viruses in a standard database of vertebrate viral interactions.
“It turns out that what we don’t know is probably more than what we know,” Becker says.
The study team found that the majority of mammal species in the database lacked virus records. Some of the largest data gaps were in bats and rodents, which are known virus reservoirs. Gaps were especially pronounced outside North America, Europe and East Asia, leaving major blank spots on the map regarding emerging disease. This is concerning since many of these blind spots are in Africa and Latin America, where land use is rapidly changing and where habitat loss is stressing wildlife, bringing wild animals more frequently into contact with livestock and people.
The researchers suggest a partial solution: Develop techniques to locate virus data that’s locked away in manuscripts or spreadsheets, then transfer it into large online databases for better access and monitoring, Becker says.
But even with that data, predicting which of the thousands of existing zoonotic viruses are of most concern remains very challenging, says Jonathan Davies, a professor at the University of British Columbia’s Biodiversity Research Centre, who wasn’t involved in the study. Identifying the most likely ecological scenarios for spillover, and pinpointing the pathogens that have the highest probability of being virulent and easily spread once they infect humans, would help researchers decide which parts of the world need more surveillance, he adds.
There also needs to be a shift in thinking regarding prevention and response being an either/or proposition, says Brookson. In this, we can take a lesson from climate change action, where policymakers now regularly pursue mitigation and adaptation in tandem, he says. “Approaching it with this sort of two pillar mindset is always going to give you a stronger structure” for safeguarding human health, he says.
A broader focus that includes both emerging and existing infectious diseases would also be beneficial to public health. “We think a lot about pathogen X [causing a new illness] … But the vast majority of people that contract some form of disease from nature? It’s things we already know about,” Brookson says, such as malaria and dengue fever.
Anticipating and preparing for the next pandemic is important, but so is considering the ongoing health burden of other kinds of zoonotic disease, and how that’s impacted by biodiversity loss, he adds. “There’s two sides of the coin, right? What is X? And then what are we actually dealing with now?”
Banner image: A black flying fox (Pteropus alecto) feeding in a palm tree in Brisbane, Australia. Flying foxes can carry Hendra virus, a rare emerging zoonotic disease that can infect horses and subsequently people. Image by Andrew Mercer via Wikimedia Commons (CC BY-SA 4.0).
‘Polycrisis’ threatens planetary health; UN calls for innovative solutions
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