Deforestation opens paths for disease spread from animals to humans

Six years after Malaysia first announced it had halted malaria spread by mosquitoes from person to person, the country is grappling with another big problem: a rapid rise in cases of the disease that stem from animals. And scientists say deforestation is a driving factor in this.

Around the world, deforestation — often to clear land for agriculture — has been linked to outbreaks of diseases that have jumped from animal to human. HIV, Zika, Sars, mpox and Ebola are some of the diseases that have emerged from tropical forests.

However, 20 years ago, Malaysia’s malaria problem was caused by parasites spread by mosquitoes from human to human. Then, between 2008 and 2017, there was a 861 per cent increase in cases of a type of malaria — zoonotic Plasmodium knowlesi — initially found in animals, according to academic research.

Zoonotic diseases are illnesses that can be transmitted directly or indirectly between animals and humans, and they include many vector-borne diseases, where an arthropod, such as a mosquito, plays a role in transmission.

Thousands of people a year are now infected with malaria carried by mosquitoes that first bit macaque monkeys, notes Kimberly Fornace, who heads the Climate, Environment and Health Programme at the National University of Singapore’s Saw Swee Hock School of Public Health and has spent years studying malaria in Malaysia.

“The human risk to this type of malaria is really quite closely linked with deforestation,” she says. Malaysia lost about a third of its total tree cover in the first two decades of this millennium.

Deforestation “is a really big problem” for human health worldwide, Fornace stresses. “It has been identified as a major driver of human infections and diseases.”

Three-quarters of new or emerging diseases that infect humans originate in animals, according to the US Centers for Disease Control and Prevention. So, when humans destroy natural habitats and infringe on biodiversity-rich forests, diseases are able to spill over to them from wildlife more easily.

In Malaysia, for example, deforestation means some monkeys are living in much closer proximity to people. Similarly, the outbreak of Ebola has been linked to deforestation in Guinea. And, earlier this month in Uganda, scientists looking at virus spillovers that can cause pandemics found the disappearance of an important food source due to deforestation had left chimpanzees, monkeys and antelope eating bat excrement containing a range of viruses.

Neil Ward, vice-president of PacBio Emea, a biotechnology company, says biodiversity loss linked to deforestation increases the likelihood of pandemics. “Disease escape is a major risk, considering that many pandemics have zoonotic origins,” he explains.

Deforestation has long prompted concerns about the loss of biodiversity and vital carbon dioxide storage, but many people are only now waking up to the risks to human health, reckons Serge Morand, senior scientist at the French National Centre for Scientific Research, the state research organisation.

Disease escape is a major risk, considering that many pandemics have zoonotic origins

Morand was the lead author of a 2020 study of the links between deforestation and human health, taking into account population growth. The research found that increases in outbreaks of zoonotic and vector-borne diseases from 1990 to 2016 were heavily associated with deforestation. This was especially the case in the southern hemisphere, where deforestation was “really clearly linked in zoonotic and vector-borne disease”, says Morand.

Companies and investors are also realising the risks of deforestation, says Tim Steinweg, head of stewardship for nature at the PRI, a group focused on responsible investment — with many considering it as more than just a climate issue in which trees act as carbon sinks.

Mercedes Bustamante, a biologist and a professor at the University of Brasília, says that after Covid-19 and other outbreaks, people are “more aware of these risks” posed by deforestation. “But, globally, I don’t think we are trying to do a good job to change the situation.”

She adds that, while every effort must be made to prevent deforestation and to ensure existing forests are healthy, more work also needs to be done to share information and resources within different organisations and departments, both nationally and globally.

“We live in a much more connected world — people move around the world in aeroplanes,” says Bustamante. “There are now ways where disease can spread much faster than in the past. That makes it much harder to control. That is why we need to increase international co-operation.”

Morand believes a global treaty on forests is needed, akin to those agreed for desertification and the seas. “We need governments to step up,” he says, and calls for a proper framework that takes into account the regulation of disease transmission.

But he warns that simply planting trees is not enough to halt the risk of disease spreading from animals to humans. Morand’s study found that plantations focused heavily on one type of tree, as well as badly executed reforestation or afforestation (new forests), often came with their own risks to human health, as well. In temperate countries, reforestation was linked to disease, as were oil palm plantations.

In Malaysia, Fornace says a rise in so-called forest edges — the transition zones between woodland and other open spaces, which can be increased by deforestation — as well as patchy forest cover have both been linked to the rise in malaria cases.

“It’s a very important public health issue,” she says. “It’s important not just for Malaysia, but actually for the region.” She adds that it shows the challenges countries face in eradicating malaria and other illnesses.

Whether the world can halt the spread of further vector-borne and zoonotic disease now, after years of deforestation, remains to be seen, she says. “Hopefully, [the world is] not messed up too much, but there’s a need for more research. [We] need to think long term about how to develop sustainable landscapes.”

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