The forest detectives: Searching for proof about forests and disease

Research suggest forest fragmentation increases the likelihoods of Emerging Infectious Diseases being transmitted to humans.
Gambian Epauletted Bats (Epomophorus gambianus)
Bats are thought to be the source of the Ebola virus – but it’s unclear if habitat loss is directly linked to the recent outbreaks. Stephen C. Smith.

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Editor’s note: Robert Nasi and John Fa will be speaking on the role of bushmeat in food security and nutrition at the Wildlife Forum at the XIV World Forestry Congress next week. To see all of CIFOR’s side events, visit our event webpage.

An increasing number of studies on Emerging Infectious Diseases (EIDs) point to land-use changes as a major cause of infectious outbreak.

Yet, indisputable proof – the ‘smoking gun’ – of the association between and land-use change and EIDs has failed to surface.

When intact forests are opened, increased human contact with unknown forest pathogens may result. But finding causal pathways, even good correlational evidence, is still difficult.

Three quarters of recognized EIDs are zoonotic in origin, i.e. transmitted from animals to humans. Comprehending the nature and extent of animal-human contact under different socio-ecological circumstances seems an urgent priority.

Understanding factors responsible for outbreaks and the re-emergence of infectious diseases remains a most difficult scientific problem.

Significant knowledge gaps still exist.

But, perhaps more worrying, as Bruce Wilcox and Rita Colwell argue, this dearth of information is likely because of how we view pathogens. They have proposed a new paradigm of interdisciplinary research, which considers pathogens not in isolation, since disease, environmental issues as well economic development, land use and governance are intertwined requiring cross-sectoral solutions.

So, to be able to predict where EIDs are likely to emerge, we must understand how natural reservoirs and transmission rates of EIDs are affected by environmental, physical, social and economic factors and their interactions.

For those of us preoccupied with ensuring the sustainable use of forests, a great concern is whether we can establish if certain rates of deforestation or fragmentation of tropical forests increase the chances of an EID appearing in an area.

Studies that contemplate emerging infectious diseases in a more holistic fashion, the notion of biocomplexity of Wilcox and Colwell, can help us discover if disease emergence processes are affected by changing demography, consumption and waste generation of human populations and their effects – urbanization, agricultural expansion and intensification, and forest habitat alteration.

So, where do we go from here? Is there any potential relation between deforestation or habitat fragmentation and the appearance of EIDs?


Two recent sources of information can help start piecing the puzzle together: our knowledge of forest fragmentation worldwide, and the global distribution of EIDs.

The recently published high-resolution map of global tree cover by Joseph Sexton and others in 2013 and by Nick Haddad’s team in 2015 has revealed that nearly 20 percent of the world’s remaining forests are just within 100 meters of a forest edge, and over 70 percent of the world’s forests are within 1 kilometer of an edge.

This puts most forests well within the range where human activities, altered microclimate, and non-forest species may influence the natural ecosystems.

Kate Jones and others in their 2013 Nature paper, drawing from a database of 335 EID ‘events’ between 1940 and 2004, demonstrated clear global patterns in the distribution of EID events, the majority of which (71.8 percent) originated in wildlife.

But, are EID events actually enabled by forest fragmentation? The study does demonstrate that EIDs are largely a product of anthropogenic and demographic changes, but wildlife host species richness was a significant predictor for the emergence of zoonotic EIDs, with no role for human population growth, latitude or rainfall.


The issue of EIDs, humans, the state of forests and animals (essentially bushmeat) has never been more in the limelight than since the emergence of the Ebola virus disease (EVD).

EVD is transmitted to people from wild animals and spread in human populations through human-to-human transmission. Explanations for EVD outbreaks abound, but none of them are certain.

Linkages between the EVD outbreaks and deforestation have been implied, with entities such as the World Health Organization (WHO) suggesting (albeit implicitly) that forest loss may have brought potentially infected wild animals and humans in greater contact.

The clock is ticking and there is urgency in corroborating if indeed there is a link between Ebola virus outbreaks and deforestation...

John E. Fa

Although direct contact with diverse mammal species – e.g. non-human primates, gorillas, chimpanzees and duikers (transmission has only been associated with this taxonomic group) – has been demonstrated as the main cause of the disease jumping to humans, bats in particular are often named as the most probable reservoir for the Ebola virus.

Recently, some researchers have proposed, though it’s unproven, that in the Gueckedou district – where the Guinea outbreak began – contact with an insectivorous free-tailed bat colony may have been responsible for the outbreak because of forest loss.

Yet others contend that because the upper Guinea forests have been a dynamic mosaic of forest, savannah and farmland for centuries, with people in this region having long co-habited with bats, it is unlikely that deforestation/fragmentation is the cause of the EVD outbreak.

Since humans and great apes have lived alongside bats for millennia, advocating habitat disturbance as the main driver of the Ebola virus emergence in these species may be overly simplistic and could ignore possible underlying causes.

Thus, the narrative claiming that rapid and unprecedented deforestation may have led to EVD outbreaks could be true.

But, how does it work?

Research by the Environmental Resources Management (ERM) Foundation suggests that forest fragmentation changes the movement dynamics of wildlife in fragmented forests; fragmented forests are often areas with more people, so contact between humans and potential reservoir or carrier species may increase.

This study, which also compared forest fragmentation patterns in six EVD outbreak sites with a sample of randomly selected sites outside these, found that forest fragmentation was higher in the EVD sites. The inference the authors of the ERM report make from these results, is that forest fragmentation, in affecting the foraging and roosting habits of bats, may be indirectly responsible for the expansion of the bat–human interface, and in so doing increase the risk of EVD.

The ERM report also makes a passing suggestion that habitat fragmentation could also promote the abundance of smaller prey (linked to the loss of large-bodied fauna), which if hunted would intensify contact with wild animals.

The study is arguably the first to empirically examine possible connections between the state of the forest and EVD outbreaks. But more work needs to be done, as the authors themselves argue.

We must not only increase the sample of EVD outbreak sites to be assessed, but also test a array of alternative hypotheses to investigate whether single or multiple factors are linked to the EVD outbreaks.

The clock is ticking and there is urgency in corroborating if indeed there is a link between Ebola virus outbreaks and deforestation/forest fragmentation and wildlife consumption and handling of wild meat.

If this association is satisfactorily validated, such a finding could allow a clear entry point to further understand the conditions that increase the risk of Ebola outbreaks and propose adequate mitigation strategies.

John Fa is a Senior Research Associate at CIFOR and Professor of Biodiversity and Human Development at Manchester Metropolitan University. Contact John at

Robert Nasi is CIFOR’s Deputy Director General-Research. Contact Robert at

Come to the Wildlife Forum at the XIV World Forestry Congress on Wednesday 9 September 2015.



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Topic(s) :   Wildlife Food & diets Bushmeat