Could deforestation trigger the next pandemic?

By James Ashworth

Forests aren’t just the lungs of the world – they’re also its immune system.

The world’s forests are vanishing, due to human activity. We’re not just losing a vital habitat, but also our first line of defence against pandemics. Large areas of trees have been felled for their timber, causing forests to be degraded (meaning they don’t function efficiently) and entire forests have been cleared (deforestation) for grazing cattle, growing soy, and producing palm oil.1 

Healthy forests keep diseases locked away, but this damage we’re causing is allowing them to escape. HIV, Zika, Sars, mpox (formerly known as monkeypox), and Ebola are just some of a long line of diseases that have emerged from tropical forests. Though it’s too late to stop the escape of these pathogens, restoring forests could prevent even more following in their wake.

Botanical barriers

Our planet’s forests are hubs of biodiversity, from large trees to tiny insects. But where there’s life, there’s disease. Viruses, bacteria, and other pathogens have been evolving here for millions of years, with some having gained the ability to infect multiple species, including humans. These are the zoonotic diseases, and they’re the main source of new human illnesses. Between 1940 and 2004, 335 diseases emerged in humans, with an estimated 72% coming from wild animals.2

Bats soar across a forest sky.
The species divide will become smaller with deforestation. © Getty
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Each encounter between humans and wildlife, alive or dead, is an opportunity for a zoonotic disease to jump the species divide. Anything from a chance encounter in the wild to eating an infected animal could allow this first infection to occur.

Fortunately, the same diversity that makes forests such vibrant ecosystems reduce the chances of this happening. As animals fight for food, water and other resources, no one species can become too common, keeping any diseases they carry at bay.3

With their hosts living in low numbers and many miles from any people, these diseases have been safely sealed away in the world’s forests. But as trees are lost, the protective barrier is starting to break down – and the diseases are getting out.

Damaged forests and disease

While damage to any forest could potentially encourage diseases to spread, the greatest risk is in the tropics. For instance, more than 60% of the risk of encountering a new disease in Africa is found in the Democratic Republic of the Congo, Cameroon and Gabon – all of which lie within the Congo Basin rainforests.4

Diseases such as Ebola5, mpox6 and HIV7 are all believed to have had their origins in the region’s wildlife, before spreading into humans and around the world. While many factors, such as the wildlife trade, have had their part to play in the emergence of these diseases, the risk of new ones following them has only increased as deforestation has accelerated. The DRC now loses around three times as much forest each year as it did 20 years ago8, with zoonotic disease outbreaks jumping by 63% in the same period.9

Cut down trees.
The protective barrier provided by trees is starting to break down. © Getty

Partly, this is just a matter of shape.10 As forests are cut down, the combined edges of all the smaller fragments that remain are longer than that of the original untouched forest, making a larger area where humans and wildlife can come into contact. Smaller fragments are also likely to contain less food, forcing wildlife to look for it outside the forest. Research in Uganda’s Kibale National Park suggests it’s more likely that primates and people will encounter each other as forests get more fragmented.

As forests are reduced to smaller fragments, their capacity to sustain a diverse range of life diminishes – raising the risk of diseases getting out. Dr Paula Prist, a senior research scientist at EcoHealth Alliance, focuses on the connections between forest health and disease. She notes that when forest coverage declines to less than 30% of its original extent, animal movement is severely hindered. “As they become limited to smaller and less connected fragments, these animals are increasingly vulnerable to external threats. Habitat specialists are normally the first to disappear, with the resulting loss of diversity lowering the forest’s ability to control disease-spreading species", she explains.

The risk of infection continues to rise as the species which are better adapted for degraded forests become more common. In larger numbers, it’s more likely they’ll encounter humans in woodland and pass on their diseases. This only gets worse as logging continues. In the rainforests of West and Central Africa, it’s predicted that deforestation could accelerate the spread of Ebola, as human-bat interactions increase.11 As these species grow in number and the Earth gets hotter, they will be more likely to meet other wildlife as their ranges expand and increasingly overlap. Some climate change models predict that over 15,000 viruses will cross between species for the first time in the next 50 years, any one of which could cause a future pandemic.12

Though these new encounters will occur everywhere in the world, the majority will be focused in the tropical forests of Africa and southeast Asia – making it vital to start restoring them now.

Potential solutions

While the idea that restoring forests can tackle disease is relatively new, what’s been found so far has been promising.  

Hantaviruses are passed on to humans by rodents, causing severe respiratory illness or haemorrhagic fever with kidney complications, in extreme cases.13 Prist’s models suggest that restoring Brazil’s Atlantic forest could decrease the chance of Hantavirus transmission across almost half of its area, with the number of rats known to carry the virus dropping by as much as 89%.14

To fight the pandemics of the future, this restoration needs to start now.  However, forest recovery will take time and things could get worse before they get better. “We are currently testing the idea that infection risk increases at the start of forest restoration,” Prist explains. “When the structure of a forest becomes highly degraded, it will take at least 20 to 30 years to recover to a point where a diverse group of species can colonise it.

Forest recovery will take time. © Getty

“We think this means that the abundance of habitat generalists, the species that can thrive in degraded areas, will increase at first. To survive, these species tend to focus more on rapid reproduction than their immune defences, making them more likely to catch and spread disease.15  This means that, until a more complex forest structure and a more diverse community can form, the risk of disease is higher.”

This restoration can be accelerated with careful planning, Prist adds. By planting trees to reconnect forest fragments, and providing food plants for herbivores, wildlife can be drawn more quickly back into degraded woodland.

Global initiatives like the Bonn Challenge and the UN Decade on Ecosystem Restoration echo the importance of long-term efforts in restoring ecological balance. These programmes aim not only to revive forest landscapes but also to create sustainable environments that will mitigate ecological challenges, including disease spread.

The health of our ecosystems is inextricably linked with our own. With our world’s natural defences against disease failing as forests are cut down, it’s vital that we restore them to protect not just human life, but all wildlife from the threat of new diseases. While the risk of a new pandemic will never go away, healthy forests remain one of our most important allies in our battle to reduce the likelihood of that happening.

This article was commissioned as part of 'Our Planet Earth’. This is a digital initiative from BBC Earth, co-produced with Wellcome, bringing you compelling stories of our changing climate and its direct impacts on both wildlife and human health. Discover more here .  #OurPlanetEarth.

Cover image © Getty

1. Deforestation and degradation, 2. Source of zoonotic diseases, 3. Diversity limits disease 4. Novel disease risk in Africa, 5. Ebola virus emerged in 1976, 6. Mpox discovered in 1958, 7. HIV is a mutated version of a chimpanzee virus, 8. Rates of deforestation in the DRC, 9. Increase in zoonotic outbreaks in Africa, 10. Effect of forest fragmentation on disease, 11. Deforestation increases Ebola, 12. Climate change and the risk of disease, 13. Hantaviruses, 14. Forest restoration reduces disease risk, 15. Habitat generalists are better at spreading disease