The COVID-19 pandemic brought zoonotic diseases into the global spotlight in a way nothing has done for a century, even though zoonoses — diseases passed between humans and animals — have always posed a public health threat. The coronavirus pandemic, however, has forced us to confront the connection between human and environmental health and the painful consequences of that relationship breaking down.
Research has already linked deforestation to increased zoonotic disease risk, finding that as habitat is lost, ecological dynamics are no longer as adept at regulating disease. But can restoring forests perhaps protect us from zoonotic diseases? Possibly, says a recent study led by Paula Ribeiro Prist, a researcher at EcoHealth Alliance, a nonprofit based in New York.
Simulations run by Prist and colleagues found that restoring Brazil’s Atlantic Forest could lower the prevalence of hantavirus cardiopulmonary syndrome, a highly lethal zoonotic disease, by reducing populations of the small mammals that harbor the disease. The team concluded that, if forests are just restored to levels mandated by Brazilian law, hantavirus transmission could be reduced by as much as 45% in Brazil’s most populous region.
To see how forest restoration might impact zoonotic disease prevalence, Prist and colleagues took advantage of rodent abundance data from forested and deforested sites across São Paulo state. The team used these numbers to estimate hantavirus prevalence under a business-as-usual scenario versus restoration levels mandated by the Brazilian Native Vegetation Protection Law.
“This was the first study that addressed [zoonotic diseases and restoration] in tropical areas,” Prist said, “and at least as far as we know, the third that makes the link of zoonotic diseases and restoration in the whole world.”
Hantavirus cardiopulmonary syndrome, or hantavirus, is spread through the urine, feces and saliva of infected rodents. Transmission rises when carrier rodent populations are large and population density is high; rates become even higher when temperatures increase locally, which happens when forests are logged or cleared. Rising hantavirus prevalence in rodents can lead to outbreaks in nearby human populations. While hantavirus is highly lethal to humans, it rarely spreads between people. This means that while it’s unlikely to start a global COVID-19-style pandemic, it can still be devastating in local regions.
The scientists’ findings add to a growing list of incentives to restore forests, including the Atlantic Forest.
Forest restoration is a major international goal intended to fight climate change, save species, and improve the lives of forest-dependent communities, said Matthew Fagan, a professor at the University of Maryland, and colleagues in an article in Conservation Letters. Most notably, the Bonn Challenge, started by Germany and the IUCN in 2011, aims to restore 350 million hectares (865 million acres) of forest by 2030 through voluntary commitments by participating countries.
Two of the primary animal reservoirs for hantavirus cardiopulmonary syndrome, the black-footed pygmy rice rat (Oligoryzomys nigripes; above) and the hairy-tailed bolo mouse (Necromys lasiurus). Images courtesy of Thomas Puttker and Pablo Gonçalves.
Brazil’s Atlantic Forest is a globally important ecosystem, with remarkably high biodiversity and great carbon storage potential. Growing human pressure over centuries, however, has taken a toll. Prior to the arrival of Europeans 500 years ago, the Atlantic Forest covered more than 150 million hectares (370 million acres) of what is today eastern Brazil — an area roughly the size of Mongolia. Today, the once massive Atlantic Forest supports more than two-thirds of Brazil’s population, including the mega cities of São Paulo and Rio de Janeiro, and has lost more than 90% of its native forest cover.
While research by Prist and colleagues finds that forest restoration could help mitigate hantavirus, conservationist Niall McCann says it’s more complicated in practice.
“Intact ecosystems with established predator-prey dynamics naturally regulate the numbers of rodents and other species that carry diseases with high zoonotic potential,” said McCann, the director of conservation at National Park Rescue and a rotating chair at the new organization EndPandemics. “It is the complexity of these systems that needs to be protected and restored, not just tree cover.”
But even as researchers call for more restoration and protection to combat disease, Brazil’s leaders have been stripping away forest protections.
“The ‘Old Forest Code’ … ensured the maintenance of forests and other forms of natural vegetation in areas considered fundamental, such as around springs and riverbanks,” Prist told Mongabay, referring to groundbreaking legislation first passed in 1965 that required landowners to preserve a certain percentage of forest.
In 2012, however, shifts in political power led to the introduction of a new forest code, the Brazilian Native Vegetation Protection Law (BNVPL). While the BNVPL still mandates a minimum area of native vegetation on private lands, it dramatically weakened the guidelines of the old forest code, allowing for increased deforestation.
Restoration projects like this one are mandated under the 2012 Brazilian Native Vegetation Protection Law to recover native forest on private lands in the Atlantic Forest Region. Image courtesy of Anazélia M. Tedesco.
Forest loss isn’t the only potential driver of increased hantavirus risk; climate change may already be worsening the situation. A 2017 study led by Prist and published in PLOS Neglected Tropical Diseases found that small increases in temperature, even without forest clearing, could put more than 30% more people at risk of contracting the deadly disease.
Prist said they aim to validate their findings with field studies at restoration sites. He added the team also hopes to develop a more nuanced understanding of how animal communities reassemble as forests are restored.
“We also want to evaluate whether restoration would have the same benefit with other zoonotic diseases here in Brazil, such as spotted fever and yellow fever,” Prist told Mongabay, “and what would be the most appropriate configuration for restored landscapes to provide this service.”
The COVID-19 pandemic has shown how much is at stake when diseases jump from animals to humans due to impacts on forests and wildlife. While the risk of zoonotic disease spread is never zero, the case of potential hantavirus reduction under forest restoration in Brazil shows that communities and governments have agency in reducing the likelihood of spillover.
Fagan, M. E., Reid, J. L., Holland, M. B., Drew, J. G., & Zahawi, R. A. (2020). How feasible are global forest restoration commitments? Conservation Letters, 13(3), e12700. doi:10.1111/conl.12700
Prist, P. R., Uriarte, M., Fernandes, K., & Metzger, J. P. (2017). Climate change and sugarcane expansion increase Hantavirus infection risk. PLOS Neglected Tropical Diseases, 11(7), e0005705. doi:10.1371/journal.pntd.0005705
Prist, P. R., Prado, A., Tambosi, L. R., Umetsu, F., de Arruda Bueno, A., Pardini, R., & Metzger, J. P. (2021). Moving to healthier landscapes: Forest restoration decreases the abundance of Hantavirus reservoir rodents in tropical forests. Science of the Total Environment, 752, 141967. doi:10.1016/j.scitotenv.2020.141967