The Connection between Increased Rates of Infectious Disease and Deforestation in Tropical Rainforests
In recent history, dangerous pandemics of emerging infectious diseases, such as the severe acute respiratory syndrome (SARS) outbreak of 2003 and the Ebola outbreaks of 2013 and 2018, have had devastating effects, especially in lower-income countries. Not only did the 2013 outbreak of Ebola take over 10,000 lives in West Africa, but the gross domestic product (GDP) of Guinea, Sierra Leone, and Liberia was estimated to have lost $2.2 billion. During the 2003 SARS outbreak in China, the fatality rate was estimated to be as high as 15%. In the last decade alone, the World Health Organization (WHO) estimated that the cost of zoonotic diseases globally is over $20 billion.
“Emerging infectious diseases” refers to diseases that have significantly increased in prevalence over the past 20 years – generally from a new strain of a known disease or in a previously unaffected population. Among recent outbreaks of emerging infectious diseases, some of the most lethal have been zoonotic, meaning transmitted from animals to humans. The WHO reports that 75% of emerging pathogens in the past decade have been zoonotic. The case fatality rates of many zoonotic infectious diseases are extremely high – Ebola is estimated at 50% – as humans have little natural protection and outbreaks commonly occur in lower-income countries.
Are we bringing this danger upon ourselves through mass deforestation? Several studies published over the last two decades highlight associations between deforestation, primarily in tropical rainforests, and increased risk of infectious diseases. The potential consequences of the aggressive deforestation occurring in the world’s rainforests on human health are daunting. Research shows that the conditions resulting from the actions of deforestation (i.e. road construction, land clearing) create optimal circumstances for zoonotic disease vectors to propagate and infect humans in adjacent communities. In fact, half of global emerging zoonotic infectious diseases prevalent in 1940-2005 are thought to have been be due to changes in land usage and food production practices, typically involving areas cleared for agriculture, urban expansion, dam building, and more.
The Amazon rainforest and tropical rainforests in general are the locations with the highest biodiversity on Earth. Included in this biodiversity are many types of zoonotic diseases and pathogenic species. These pathogens and viruses that humans have never been exposed to have been quietly evolving alongside their animal hosts and vectors for centuries, to the point where the animals have little to no symptoms when infected. Humans have limited natural protection against these pathogens as historically there has been low exposure to them.
Deforestation in the Amazon rainforest has occurred at an unprecedented rate since the mid-20th century and is likely to continue into the foreseeable future. Predictions project that by 2050, 40% of the Amazon rainforest will be eliminated by deforestation. In South American communities that live close to the rainforest and adjacent to such deforestation, malaria has re-emerged to be a leading cause of morbidity and mortality even though it was thought to have been eradicated in many places by the mid-20th century.
Research has revealed multiple explanations for deforestation and land-use change in or near tropical rainforests resulting in massive disease outbreaks in the communities neighbouring the forest edges. The first reason is that deforestation results in habitat-loss and increased exposure of disease-carrying vectors to the human populations living nearby, especially if humans colonize the deforested areas. When forest is cleared, areas that were previously untouched become exposed and thus likely in closer contact with humans particularly if land is used for agriculture. This brings the vectors closer to the humans than before and increases the likelihood of transmission and infection. In one area of the Peruvian Amazon, the prevalence of malaria increased from 2.1 cases per 1000 people in 1992 to 343 cases per 1000 people in 1997. There have been similar cases of malaria outbreaks following deforestation in other tropical rainforests globally. In one state in Borneo, the incidence of malaria infections rose from 2% in 2004 to 62% in 2013 following rainforest deforestation for oil palm plantations. The research demonstrated that higher forest loss in an area and higher levels of surrounding forest was associated with higher levels of Plasmodium knowlesi, a common malaria parasite.
Researchers hypothesized that the increase in malaria prevalence and incidence is linked to a reintroduction of Anopheles darlingi – the mosquito species that is the primary malaria vector in the Peruvian Amazon and South America as a whole. A. darlingi was previously thought to have been eradicated in the 1990s. This area had been heavily deforested in the decade before reintroduction of A. darlingi, with peak deforestation between 1983 and 1995. The results of the study clearly indicated that areas with higher levels of A. darlingi larvae were associated with lower levels of forest cover. In one region of Brazil in which malaria is also primarily caused by A. darlingi, a 2010 study found that malaria risk increased 50% in districts with just 4% deforestation from 1997-2006.
When forest is cleared and land is inhabited by humans, what often follows is construction of access roads. Roads and construction in general can create stagnant water, from culverts along the roadside that fill with water to streams that entirely prevent water flow. Lack of trees increases sunlight exposure and reduces leaf litter, lowering the acidity in the water. Nutrients from agricultural runoff fill the water with algal blooms, ideal food for the mosquito larvae. All of these factors result in ideal mosquito breeding grounds. It is a vicious cycle; roads bring increased migration and even more human exposure to mosquito vectors. In one region of Peru in which a road had been built through previously untouched forest, malaria cases caused by A. darlingi jumped from 600 to 120,000 per year.
Deforestation can and will increase the spread of vector-borne diseases if the vectors live in the forest that is being cleared. It is critical to highlight the fact that the communities affected by deforestation, and therefore by disease, are often in low- and middle-income countries. They typically impact vulnerable people in rural areas the most because they have less access to the healthcare and resources needed to tackle these challenges. Anthropogenic climate change will do nothing but exacerbate this major issue. While the future of emerging infectious diseases is daunting in the face of environmental adversity, measures to tackle the aggressive levels of tropical deforestation are urgently needed to control emerging infectious diseases.