The Economist explains
How environmental damage can lead to new diseases
Deforestation and intensive farming are linked to outbreaks of zoonotic pathogens
THE WORLD’S monitored populations of wild animals have decreased by an average of 68% in the past 50 years, according to the Worldwide Fund for Nature. Deforestation, intensive farming and the changing use of land are largely to blame. But nature can recover, provided it is given a chance. COP15, a UN biodiversity summit that took place last week, sought to do just that. More than 100 countries recognised the need to reverse species decline by 2030 and acknowledged the consequences of harmful environmental practices and climate change for biodiversity. These efforts are long overdue—and not just for the sake of wildlife. The same actions that threaten ecosystems endanger human health too.
Growing evidence points to a connection between destructive environmental practices and emerging diseases. Exactly how one leads to the other is not yet fully understood, as the struggle to establish the origin of covid-19 shows (the virus may have leaked from a lab, or “spilled over” from bats into humans, via an intermediary species). Why are changes in ecosystems linked to the spread of disease, and what increases the risk of outbreaks?
Of more than 330 diseases which emerged between 1940 and 2004, nearly two-thirds were zoonotic, meaning they were transmitted from animals to humans, as with, for example, HIV/AIDS and probably covid-19. Of those over 70% originated in wildlife, as opposed to domesticated animals. And although many factors are involved in disease transmission, including population growth, migration and climate change, scientists are increasingly turning their attention to how altering land interferes with a pathogen’s journey from animals to humans. A study published in March this year by researchers at Montpellier University and Aix-Marseille University found a link between changes in global forest cover between 1990 and 2016 and an increase in reported epidemics, even accounting for the fact that deforestation usually means more humans living nearby. As forest cover shrank (from 31.6% to 30.7%) occurrences of diseases rose, particularly in tropical, biodiverse areas.
One probable reason for the increase in pathogens is that felling trees increases contact between humans and disease-carrying animals. Scientists found a correlation between the loss of forests in west and central Africa and outbreaks of Ebola between 2004 and 2014. The Ebola virus is thought to be transmitted by infected bats and primates, although exactly how is not yet fully understood. And interactions with other mammals are not the only concern. Cutting down trees may also increase the threat to humans posed by viral infections transmitted through mosquito bites, such as Zika, dengue and chikungunya. Researchers at the University of Florida analysed studies of 87 mosquito species in 12 countries. Around half of the species were associated with deforested environments. Of these, more than half are known to carry diseases.
Replacing old-growth forests with a single crop, such as oil palm, can also lead to the transmission of disease. If predators’ habitats are destroyed and their populations dwindle, other creatures such as rodents, mosquitoes, bats and some primates can proliferate. These harbour potentially zoonotic pathogens and tend to cluster in places where they will be more frequently exposed to humans and livestock. Rodents, for example, often inhabit the border areas between newly created pastures and forests. A study published last year by disease ecologists in Science described the edges of tropical forests as “a major launch-pad” for new viruses. Wildlife may also move towards human settlements in search of food. Mango trees planted on pig farms in Malaysia probably attracted fruit bats carrying nipah, a virus that infected local pig farmers in 1999 and still breaks out yearly in Bangladesh.
Ultimately, more work is needed to understand how people’s interactions with nature spread disease. But the emergence of new pathogens, such as the virus that causes covid-19, gives efforts to preserve the planet’s biodiversity a new importance.
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