The Impact of Deforestation on Transmission of Disease to Humans

The Impact of Deforestation on Transmission of Disease to Humans

By: Brenna Daly

The environmental changes caused by deforestation have significant effects on wildlife communities and alter how humans and wildlife interact. These changes in the environment contribute to increased human health risk, specifically from zoonotic spillover potential (the transmission of disease from wildlife to humans). The process of reforestation, when trees begin to regrow in an area, is also a major contributor to spillover risk depending upon the rate of forest regeneration and how developed the forestry becomes. Researchers have been studying these factors that can increase spillover risk and examining how spillover risk varies between habitat types.

A CEID research team, including John Vinson, Nicole Gottdenker, RajReni Kaul, Andrew Kramer, John Drake, and Richard Hall, developed a mathematical model to predict spillover risk over time based on land-use history. This history included the maturity of a landscape and examined if landscapes were highly developed forests or if they had been partially or completely deforested. 

The team investigated the relationship between land-use history and zoonotic spillover risk in four habitat types: mature habitat (a forest that has been established for a long period of time with a large variety of species present and advanced ecological diversity), cleared habitat (a landscape that has been partially or fully deforested for human activities), a settled habitat (dense human population, less wildlife, and an altered landscape), and regenerating habitat (landscape that is reverting back towards a mature forest). 

The results of this modeling showed that land-use history significantly impacts spillover risk over time, and the spillover risk of a landscape is determined by these factors: the rate of forest regeneration, the relative spillover risk of deforested land compared to regenerating forest, and how quickly regenerating forests develop characteristics of a mature forest. Findings reveal that when the original landscape is a mature forest that then suffers deforestation, the spillover risk will initially peak when the land is cleared, and then the risk starts decreasing as the cleared land becomes urbanized or reforested. Cleared land poses the greatest spillover risk, likely due to a loss of competitors and predators for wildlife disease reservoir species. Conversely, if the initial landscape was largely cleared of forest, the risk of spillover is high initially, and then declines. 

The research found that forestry reversion will always reduce spillover potential when the regenerating habitat has low spillover risk. The most rapid decline in zoonotic spillover potential occurs when the land reversion occurs quickly and there is a slow regeneration transition to a mature forest. The evidence also revealed that if both the cleared and regenerating habitats are equal contributors to spillover risk, then reversion of the land will increase spillover potential. Overall, a high spillover risk is apparent in cleared and regenerating habitats that display forest regrowth similar to a mature forest, compared to spillover risk of deforested land. 

The research team highlights that these findings can be utilized to reduce zoonotic spillover risk by supporting the reversion and regeneration of land and by preventing human exposure to regeneration areas of potentially high risk. Future research can build upon this land-use model by further exploring transmission potential in certain habitats, integrating parasite transmission models, and examining the relationship between exposure and human behavior. 

For more information on this study, click here.