Outbreaks of infectious diseases are more likely in areas of deforestation and monoculture plantations, according to a study that suggests epidemics are likely to increase as biodiversity declines. Land use change is a significant factor in the emergence of zoonotic viruses such as Covid-19 and vector-borne ailments such as malaria, says the paper, published on Wednesday in Frontiers in Veterinary Science. Even tree-planting can increase health risks to local human populations if it focuses too narrowly on a small number of species, as is often the case in commercial forests, the research found. The authors said this was because diseases are filtered and blocked by a range of predators and habitats in a healthy, biodiverse forest. When this is replaced by a palm oil plantation, soy fields or blocks of eucalyptus, the specialist species die off, leaving generalists such as rats and mosquitoes to thrive and spread pathogens across human and non-human habitats. The net result is a loss of natural disease regulation. “I was surprised by how clear the pattern was,” said one of the authors, Serge Morand, of the French National Centre for Scientific Research. “We must give more consideration to the role of the forest in human health, animal health and environmental health. The message from this study is ‘don’t forget the forest’.” The researchers examined the correlation between trends for forest cover, plantations, population and disease around the globe using statistics from international institutions such as the World Health Organization, the World Bank, the Food and Agricultural Organization and the Gideon epidemic database. Over the period of study from 1990 to 2016, this covered 3,884 outbreaks of 116 zoonotic diseases that crossed the species barrier and 1,996 outbreaks of 69 vector-borne infectious diseases, mostly carried by mosquitoes, ticks or flies. The paper shows outbreaks increased over time, while plantations expanded rapidly and overall forest cover declined gradually. By itself, a correlation is not proof of causality because other factors may be involved, such as climate disruption. The authors bolster their argument with multiple references to individual case studies that highlight the links between epidemics and land use change. In Brazil, scientists have demonstrated that deforestation increases the risks of outbreaks of malaria. In south-east Asia, studies have shown how forest clearing favours the mosquito Anopheles darlingi, which is a vector for several diseases. Loss of primary forests has also been identified as a factor in the emergence of Ebola in west Africa and the re-emergence of arthropod-borne leishmaniasis. The new study adds to a growing body of evidence that viruses are more likely to transfer to humans or animals if they live in or near human-disturbed ecosystems, such as recently cleared forests or swamps drained for farmland, mining projects or residential projects. This is shaped by trade patterns and consumer behaviour. A quarter of global forest loss is driven by the production of commodities such as beef, soy, palm oil and wood fibre. Mining adds to this problem by contaminating rivers and streams that are vital for a resilient ecosystem, carbon sequestration and soil quality. Morand said his study showed that disease risks needs to be added to risk-benefit analysis of new projects. “We should take the costs of public health into account when considering new plantations or mines. The risks are first to local people, but then worldwide because we have seen with Covid how quickly diseases can spread.” He is particularly concerned about the deteriorating environmental health of the Amazon rainforest. Under the Brazilian president, Jair Bolsonaro, deforestation has surged to levels not seen in more than a decade and public health systems have been so mismanaged that the country now has the worst Covid death rate in the world. “Everyone in the field of planetary health is worried about what is happening to biodiversity, climate and public health in Brazil,” Morand said. “The stress there is growing. The Amazon is near a tipping point due to climate change, which is not good at all for the world ecosystem. If we reach the tipping point, the outcomes will be very bad in terms of drought, fires and for sure in terms of disease.” Other areas of concern include the rainforests of the Congo basin and south-east Asia, and monoculture afforestation projects in China, Europe and the US. “Our results clearly suggest that it is not only forest clearance that is responsible for outbreaks of infectious diseases, but also reforestation or afforestation, particularly in countries outside the tropical zone,” the paper notes. Morand is now working on a more detailed study that will use satellite analysis of forest cover to examine links with disease. With more information, he believes it may be possible to predict future outbreaks and to work with local communities to build ecologically diverse and economically productive landscapes that reduce the risks. As the author of a 2016 book called The Next Plague, he says it is only a matter of time until the next pandemic. “The risks are very high. It’s just a case of when and where. We need to prepare.”