by Piyush Nanda
figures by Corena Loeb
In an area devastated by deforestation, an 18-month-old toddler from the nearest settlement, Meliandou in Guinea, was seen playing around a fallen tree swarming with bats. The child then contracted a mysterious illness, which spread to many who came in contact. After it had already killed 30 people, the illness was identified as Ebola. Comprehensive studies have since connected 25 of the 27 Ebola outbreaks in Africa, like the 2014 outbreak that originated in Guinea, to regions with intense deforestation. Scientists set out to find the connecting links between deforestation and the then-disparate phenomenon of emerging diseases. Through years of collaborative efforts, they were able to show that human-induced damages inflicted on nature are an important driver of human diseases emerging from the wild.
Several animals, including human beings, are hosts to a myriad of viruses that do not cause any disease and remain in their latent state within the host. This harmony is shaped by millions of years of evolution that enable the co-existence of the virus and their host. Nevertheless, evolution also allows these viruses to leap from their original host into newer hosts with a few simple changes in their genetic material, without any guarantee that the virus will remain latent in the new hosts. The United Nations reports that 75% of the emerging diseases have their origins in other animal species. Viruses like Ebola, SARS, MERS, and SARS-CoV-2 originated from mammals like wild bats and subsequently jumped to humans through animal-human contact. The pandemics resulting from these disease outbreaks resulted in the loss of hundreds of thousands of lives and billions of dollars to the global economy. It is imperative to understand the routes through which human activities are triggering these epidemics.
How could deforestation have caused the Ebola outbreaks?
Human encroachment on forest land is often the first step to expanding urban settlements. Cleared forests leave the resident animals homeless, forcing them to travel to nearby locations to settle. This migration may bring them, and the viruses they host, in close proximity to humans in the neighbouring settlements. As with the toddler in Guinea, increased contact with wild animals increases the odds of transmitting diseases from animals to humans, which then triggers human-to-human transmission.
In a Scientific Reports article published in October 2017, scientists showed outbreaks of Ebola around the rainforest were associated with forest loss within the preceding two years. Using a combination of geographical data and modelling, researchers found a link between deforestation and Ebola outbreaks, showing significant correlation between these events.
The association between deforestation and disease incidence was not just limited to Ebola. In 1997, hundreds of thousands of square-kilometres of land were burnt down in Indonesia to make way for agriculture. The remaining trees in the rainforest area could not produce sufficient fruit for the fruit bat population. Consequently, the fruit bats, which were hosts to potentially deadly viruses, had to fly to nearby locations in search of food. As soon as they settled on the Malaysian orchards nearby, the pigs and local farmers started falling ill. This was one of the first reported cases of the Nipah virus.
Similarly, work published in the Proceedings of the National Academy of Sciences demonstrated an association of malaria transmission with deforestation across the Amazon basin. They report an annual 10% increase in forest loss could have resulted in the 3.3% rise in malaria cases between 2003 and 2015.
Can we predict the transmission of diseases from the wild?
The intimate and intricate relationship between human activities like deforestation and the spillover of diseases from the wild is being appreciated now more than ever before. Global alliances like EcoHealth Alliance and Planetary Health Alliance are now in action deciphering the connections between human and environmental health (Figure 1). Given the high cost of lives and economic burden due to unprecedented pandemic events, the efforts to identify global hotspots of emerging diseases from the wild have been drastically accelerated.
The PREDICT program funded by the United States Agency for International Development (USAID) was aimed at identifying potential zoonotic viruses that can spill over into human settlements. Between 2009 and 2019, researchers under this program collected samples from 10,000 bats and 2,000 other mammals while identifying more than 1,200 viruses including 160 coronaviruses, which have the potential to cause human disease. EcoHealth Alliance (a partner in the PREDICT program) continues to operate projects that aim to identify events of virus transmission between wild animals and humans.
The current portability and decreasing costs of DNA sequencing are also making it possible to catalogue viruses in the wild at a massive scale. This enables scientists to use bioinformatics tools to predict the viruses that can jump from animals to humans. The Global Virome Project (GVP), a multi-billion dollar project founded in 2018, plans to comprehensively sequence and identify viruses in the wild.
While identifying the potential viruses that can spill over to humans, it is also imperative to locate hotspots from where these viruses can emerge. A comprehensive 2017 study identified global hotspots for infectious disease outbreaks (Figure 2). Researchers identified characteristics of the hotspots in terms of human population density, animal population density, number of trees, and other factors. They found forested tropical regions experiencing a change in land usage, e.g., conversion of forest land to agricultural land or urban areas, and with high wildlife biodiversity to be most at risk. They also reported global hotspots from which infectious disease could possibly emerge. Notably, the region containing Wuhan, the suspected origin of the COVID-19 pandemic, was named as a top high-risk area.
Collectively, these efforts are enabling us to understand the close connection between human health and the environment. Increased pressure on land due to human activities has the potential to lead to the disturbance of the delicate balance in nature, which can backfire in the form of global pandemics (Figure 3).
As the current COVID-19 pandemic continues to hamper lives and put the world economy in the hotbed of uncertainty, discerning the emergence of pandemics before they affect us is becoming an absolute necessity. A report drafted jointly by the United Nations Environment Programme and the International Livestock Research Institute highlights 10 recommendations, such as raising awareness of, and expanding scientific inquiry into, zoonotic diseases, and encouraging sustainable land management practices to curb future pandemic events. Scientific evidence increasingly supports the need for sustainable practices in development projects in order to prevent future global pandemics.
Piyush Nanda is a first-year PhD student in the Biological and Biomedical Sciences program of Harvard Medical School. He is interested in how cells make decisions through biological mechanisms and how evolution shapes them.
Corena Loeb is a first-year Ph.D. student in the Harvard-MIT program in Speech, Hearing, Bioscience and Technology
For More Information:
- Learn more about Planetary Health Alliance and its activities in brief here.
- Learn more about how EcoHealth Alliance hunts for the viruses that could trigger the next pandemic here.
- Here is a story report from United Nations Environment Program on preventing future pandemics.
- Interested about how bats carry viruses without getting infected? Learn more here.