Guest Post: Does Biodiversity Protect Humans Against Infectious Disease?

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The prevailing view among scientists is that a healthy ecosystem reduces the transmission of infectious diseases in humans. But is this really true for all environments? 

Conserving nature can improve human lives. From forest watersheds that perform natural filtration of drinking water to coral reefs that break tsunami waves before they flatten seaside villages, intact ecosystems provide innumerable services to human society. Might nature also protect people against infectious diseases?

The prevailing view among disease ecologists is that habitat destruction, climate change, and biodiversity loss will lead to an inexorable increase in the transmission of infectious diseases. There are several case studies to support this perspective: forest fragmentation can increase the prevalence of Lyme disease, dam construction facilitates the transmission of schistosomiasis, and loss of rodent species diversity may permit Hantavirus to thrive. These cases are exciting because they suggest common goals for conservation and human health.

But this entrenched perspective has recently been questioned. The USGS, in collaboration with Stanford University, the University of California, Santa Barbara, and Pennsylvania State University, recently published a comprehensive review of the effects of biodiversity on infectious disease, in the journal Ecology.

Although they found several examples in which biodiversity loss seems likely to lead to an increased risk of disease for humans, these examples were eclipsed by the many instances in which disease risk is likely to decline as biodiversity is lost.

For example, in the Yucatan Peninsula, Belize, and Guatemala, locals have long had a nickname for the symptoms of cutaneous leishmaniasis, caused by the sandfly-vectored protozoan parasite Leishmania mexicana: “chiclero’s ulcer”. Chicle is the latex produced by sapodilla trees, and chicleros are the men who spend months at a time in the forest collecting it.

Contact with intact forest ecosystems can lead to cutaneous leishmaniasis, also known as "chiclero's ulcer"

Contact with intact forest ecosystems can lead to cutaneous leishmaniasis, also known as "chiclero's ulcer" (inset). (Forest photo courtesy of Peter Hudson. Leishmaniasis photo copyright Current Medicine LLC. All rights reserved. Images reproduced with permission from Springer Science+Business Media B.V.)

Several other species of Leishmania have the same positive association with biodiversity: they primarily infect people who spend time in intact forests. This relationship is not uncommon among “zoonotic diseases” --- those that pass from animals to humans. Because wildlife hosts represent both habitat and resources for infectious disease agents, if biodiversity loss causes a reduction in host abundance, these disease agents may decline alongside their hosts.

The idea that biodiversity can protect people against infectious disease suggests that conservation and disease control go hand-in-hand. But this new research suggests that conservation initiatives might, in many cases, exacerbate human disease risk --- for as long as humans continue to interact with, live near, and utilize wild habitats and species. Armed with this knowledge, officials charged with improving public health or managing wildlife can design appropriate interventions to offset these collateral impacts of conservation on public health.

Guest post by Dr. Chelsea L. Wood, Department of Ecology and Evolutionary Biology, University of Michigan (currently at the University of Washington). Wood collaborated with USGS WERC principal investigator Kevin Lafferty on this review paper. Visit Wood's research website and Lafferty's research website.

This study was funded by a National Science Foundation Graduate Research Fellowship, an Alyce B. and Henry J. Ramey, Jr. Stanford Graduate Fellowship, the Italian Ministry of Research (PRIN2008), an NSF–NIH Ecology and Evolution of Infectious Diseases Program grant (OCE-1115965), and the NIH RAPIDD program.