BES-Net Op-Ed Series on COVID-19 (#5): Linkage between land degradation and zoonotic disease outbreak - Hypotheses underpinning the linkage

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While zoonotic diseases are considered as one of the most pressing global issues of our era as a result of 2019 novel coronavirus (COVID-19) outbreaks, a renewed focus is also being placed on the issue of land degradation[1]. Land degradation refers to a process in which the value of the biophysical environment is affected by a combination of human-induced processes acting upon the land[2]. It is associated with the loss of life-supporting land resource through soil erosion, desertification, salinization, acidification, etc.[3] Considerable interest in the relationship between land degradation – and associated biodiversity loss - and zoonotic diseases outbreaks has recently captured the attention of the research community, with important public policy implications. BES-Net Op-Ed series on COVID-19 seeks to contribute to this dynamic by providing insights into concrete examples of linkage between land degradation and the rise of zoonotic diseases in order to help identify more integrated, cross-sectoral, and inclusive policy options that build the health and resilience of people and the planet.

This Op-Ed series #5 specifically compiles the available evidences concerning the interface between land degradation and zoonotic disease with a focus on the main hypotheses underpinning this linkage. 

Hypotheses underpinning the linkage between land degradation and zoonotic disease outbreaks
There are several hypotheses put forward by scholars to explain the association between environmental degradation in general and land degradation in particular and emerging infectious diseases. The most notables include the ‘coevolution effect’ and the ‘dilution effect’.
The ‘coevolution effect’ is rooted in ecology and evolutionary biology. It suggests that, as humans alter landscapes and former intact habitats are lost, forest fragments serve as islands harboring wildlife hosts of pathogens that undergo rapid diversification. This change leads to greater probability that one of these pathogens will spill over into human populations, and will cause new disease outbreaks. Accordingly, maintaining healthy and well-connected ecosystems should help reduce the prevalence of infectious diseases[4].


Figure 1. Coevolution effect describing the mechanisms underlying increased zoonotic pathogen spillover with habitat loss[5]

The ‘dilution effect’ suggests that more virus transmission events occur within a single species in communities that have low species diversity than in communities that have greater species diversity. The dilution effect occurs because communities with more species dilute transmission events by reducing the number of susceptible animals. For example, in communities of higher biodiversity, disease-transmitting vectors feed on a larger variety of hosts that are poor reservoirs for the pathogen (e.g., West Nile virus and tick-transmitted Lyme disease)[6]. While more biodiversity means greater viral richness, the risk of pathogen spillover stems from increased exposure, for example as more humans visit environments where pathogens are present[7].

Table 1. Comparisons of predictions for factors affecting the emergence of human disease based on the dilution effect and the coevolution effect[8]

Effect on emergence of human diseases Dilution effect Coevolution effect
Habitat fragmentation Increases risk Increases risk
Biodiversity Loss of biodiversity
increases risk
Increase in pathogen biodiversity increases risk
Host population structure N/A Increases risk
Coevolution of host and obligate vector N/A Increases risk
Lack of coevolution of host and
bridge vector (mosquito)
N/A Increases risk
Loss of predators and hosts (dilution of vectors) Increases risk N/A
Higher host–pathogen encounter rates (more edge) Increases risk Increases risk if true for bridge vectors
Enhanced abiotic conditions for bridge vectors (higher population size for mosquitoes) Increases risk Increases risk

 

[1] O'Riordan 2000.
[2] Conacher, Arthur; Conacher, Jeanette (1995). Rural Land Degradation in Australia. South Melbourne, Victoria: Oxford University Press Australia. p. 2. ISBN 0-19-553436-0.
[3] IPCC, 2019
[5] (A) The left (purple) transmission cycle of pathogens (indicated by black viral particles) between wildlife host (e.g., rodent) and obligate parasite (e.g., louse). Right (red) transmission cycle of pathogens through a bridge vector (e.g., mosquito) among hosts causing the spillover of the pathogens that diversified in a species-specific cycle (purple). (B) Loss of habitat connectivity leads to small forest fragments (green shapes) that act as separate coevolutionary engines, with genetic drift and selection causing divergence in pathogens among fragments indicated by the colored viral particles in each fragment. Across a landscape, an accelerated rate of pathogen diversification and increased probability that a new variant would have zoonotic potential in other species (e.g., humans). With transmission through the bridge vector maintaining exposure to the pathogens (spillover), this process would lead to an increased probability of disease emergence in a fragmented habitat.
[6] Ostfeld, 2009
[7] Allen et al. 2017
[8] Zohdy et al., 2019
Article By: 
Bertrand Tessa & Yuko Kurauchi