- From Ebola in Africa to malaria in Brazil to tick-borne illnesses in the U.S., there is a common thread linking outbreaks of vector-borne and zoonotic diseases: fluctuating forest cover, according to a recent study.
- Deforestation and even increases in green cover can fuel the rise of diseases, the research shows, citing the link between oil palm plantations and outbreaks of malaria and dengue as a case in point.
- The study examined linkages between forest cover changes with vector-borne and zoonotic diseases between 1990 and 2016 and found that outbreaks of both types of infections have increased.
- The evidence that human health can suffer even as a result of misguided afforestation or plantation shows that more research is needed on the role of forests in modulating diseases.
The COVID-19 pandemic has raised plenty of questions about whether the planet’s declining health is what ails humans. The answer is yes, and it’s not a metaphorical response.
Distorting the blue planet’s green cover, hollowing out dense forests and tacking tree plantations at will fuels the rise of diseases, a growing body of research shows. From Ebola in Africa to malaria outbreaks in Brazil to tick-borne illnesses in the U.S., there is a common thread: fluctuating forest cover, a recent study in Frontiers in Veterinary Science has found.
The study authors spotlight an overlooked hazard: oil palm plantations, which they call a “threat to global health.”
COVID-19 is a zoonotic disease, one where the pathogen emerges in animals and spills over into humans. How the SARS-CoV-2 virus made this jump is still under investigation, but one theory posits that a bat coronavirus entered human populations via an intermediary host. This scenario has raised concerns about the health risks related to wildlife trade and human incursions into natural habitats.
The new study examined linkages between deforestation and reforestation with vector-borne and zoonotic diseases between 1990 and 2016. Vector-borne diseases including malaria, dengue and Lyme disease are transmitted by insects like mosquitoes, ticks and fleas. Both vector-borne and zoonotic diseases occurred more frequently in this period, with the former seeing a marked surge.
Greener tropical countries like Brazil and Indonesia that report high deforestation rates suffer more frequent vector-borne disease outbreaks, the analysis found. Data from other nations like Peru and Bolivia in South America; the Democratic Republic of Congo and Cameroon in Africa; and Myanmar and Malaysia in Southeast Asia, support this conclusion.
A quarter of all forest lost between 2001 and 2015 was cleared for the production of commodities like beef, soy, palm oil, and wood fiber, according to a 2018 study. Plantations of these crops have metastasized in some of the most forest-rich countries in the world. Indonesia is the world’s largest palm oil producer, and Brazil churns out most of the global supply of soybeans, and a large chunk of its beef.
In Southeast Asia, replacing forests with oil palm plantations was found to boost the abundance of Anopheles mosquitoes that transmit malaria. In Malaysian Borneo, agricultural land replacing forests led to shrinking populations of Aedes niveus but the swelling of A. albopictus, both dengue-carrying mosquitoes.
Though changes in forest cover create conditions favorable for maladies, the relationship is not straightforward. Malaria experts found that outbreaks are linked to forest clearing, especially at deforestation frontiers. But in some cases, the rise in malaria cases also leads to a dip in deforestation as those who log forests lose labor days owing to illness, creating a convoluted feedback loop.
Mapping of Ebola outbreaks in western and central Africa revealed that index cases were more likely to occur in forest fragmentation hotspots. When closed forests are pried open, amplifying people’s exposure to wild animals and pathogens, it raises the risk of zoonotic infection spillover.
The harmful impacts don’t just arise from deforestation, the research found; disease spread is also associated with unsuitable afforestation. Though such efforts can promote biodiversity and benefit carbon sequestration, they might have unintended consequences. In the U.S., for example, planting trees and growing green cover supported the resurrection of deer populations but also led to an increase in tick populations.
This was true of temperate-zone countries, which had lower forest cover. But the researchers also saw this pattern in places like China and Vietnam, which have embarked on vigorous reforestation drives. However, the authors caution that their data on the increase in green cover don’t differentiate between commodity plantations, afforestation, or abandoned agricultural lands.
The limitations of satellite imagery are only part of the problem. There is no consensus about what makes a forest.
There is no global treaty dedicated to forests. The International Tropical Timber Agreement is the only global pact that deals directly with forests, but it focuses on the sustainable timber trade. The Bonn Challenge, which aims to restore 350 million hectares (865 million acres) by 2030, and the New York Declaration for stopping deforestation are both voluntary commitments that encompass restoration of degraded land.
Adherence to these goals has encouraged countries to launch massive tree-planting drives and led to the conversion of grasslands, important ecosystems in their own right, into forests. The authors are calling for an update to the FAO’s definition of a forest so that it does not promote the expansion of forests in areas that were historically not forested.
The evidence that human health can suffer even as a result of misguided afforestation or plantation shows that more research is needed about the role of forests in modulating diseases.
Morand, S., & Lajaunie, C. (2021). Outbreaks of vector-borne and zoonotic diseases are associated with changes in forest cover and oil palm expansion at global scale. Frontiers in Veterinary Science, 8. doi:10.3389/fvets.2021.661063
Curtis, P. G., Slay, C. M., Harris, N. L., Tyukavina, A., & Hansen, M. C. (2018). Classifying drivers of global forest loss. Science, 361(6407), 1108-1111. doi:10.1126/science.aau3445
MacDonald, A. J., & Mordecai, E. A. (2019). Amazon deforestation drives malaria transmission, and malaria burden reduces forest clearing: A retrospective study. The Lancet Planetary Health, 3. doi:10.1016/s2542-5196(19)30156-1
(Banner Image: An oil palm plantation in Indonesia. Image by Rhett A. Butler/ Mongabay.)
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