Land Use and Disease Emergence

Zoonoses, diseases transmitted from animals to humans, are currently enjoying a moment in the sun—at least by disease standards.

There are now popular depictions of spillover events, as in the movie Contagion, reflecting real-world examples where disease is transmitted from animals to humans. The most recent and significant series of zoonoses events points to this trend: HIV/AIDS coming originally from apes, SARS from horseshoe bats, and the Ebola virus transmitted from great apes (though thought to be harbored in bat species). Animal and human disease, once viewed as more or less unrelated provinces of veterinarians on one hand and MDs on the other, are now understood to overlap substantially. In fact, more than half of the pathogens infectious to humans originated in animals, with the majority of recently-emerging diseases coming from wildlife.

Flying foxes provide critical ecosystem services. While they serve as reservoirs for Nipah virus, it is human activities that typically drive risk for disease emergence in humans. Photo taken by J.H. Epstein/EcoHealth Alliance.

It’s easy to accept that animals can make humans sick, but maybe less so that humans have a hand in the process. In public discussions of zoonoses, there is often the sense that the animal-to-human route is the whole narrative. In this story line, disease in wildlife is seen as an unmoved mover—a system undergoing self-contained, spontaneous cycles of disease and cure that occasionally spins out of control as to bruise human bystanders. Wildlife, in our minds, is wild, and as such probably exists far beyond human influence in a disease-ridden state of nature.

Taming wild areas may be a primary trigger of zoonoses events

In actuality, the probability that a disease will make the leap to zoonosis is often very closely related to what humans do. Which makes sense: nature is, in general, poorly buffered from humanity. Anthropogenic activities, including land use and agricultural production changes, are among the leading drivers of disease emergence in humans.

On the most basic level, human expansion into wildlife habitat means more opportunities for exposure to pathogens. Take Nipah virus, the previously unknown pathogen behind a deadly form of encephalitis that emerged in Malaysia in the late ’90s. The virus is thought to be carried by Pteropus spp. fruit bats, apparently without causing them any ill effect. When agriculture encroached on bat habitat, Nipah virus made the leap to commercially raised (densely-stocked) pigs, and then to pig farmers.

We live in a world where continued land-use change is not only inevitable, but arguably necessary for human survival.

Human development may also alter landscapes in ways that favor certain disease vectors or hosts. The establishment of large-scale agriculture in northern Argentina and Bolivia led to rodent population explosions that likely drove outbreaks of the never-before-seen Junín and Machupo viruses, which cause highly virulent hemorrhagic fevers. Irrigation and dam-building may have major effects on the transmission of mosquito and snail-borne disease. This is not to say that humans are entirely to blame for the emergence of disease through animal proxies (as wildlife epidemics are a problem in their own right), but there are many examples of disease transmissions potentially being triggered by human activities.

EcoHealth Alliance and its partners are conducting pathogen surveillance in wildlife in Brazil to shed light on how changes in land use affect viral diversity and ecological dynamics. Photo taken by EcoHealth Alliance.

The double-edged sword of agricultural development

We live in a world where continued land-use change is not only inevitable, but arguably necessary for human survival. The FAO has estimated that developing countries will have to convert an additional 110 million hectares to arable land by 2050. With some estimates reporting only 25% of current global land cover still existing as wild lands, it will be difficult to avoid the effects of dramatic habitat fragmentation. The topic of fragmentation lies behind many of the complexities of disease ecology. The “edge effects” that stem from new interfaces between wildlands and agricultural fields mean not only the increased exposure of humans to vectors of wildlife disease as discussed above, but also the generation of new interactions between pathogens and nonhuman hosts. The reduced connectivity of isolated wildlife reserves may concentrate new outbreaks among densely packed host species, while human settlements increasingly link up and effectively aid the rapid spread of disease once the spillover occurs.

EcoHealth Alliance employs a One Health approach that considers the connections between human, animal and environmental health, helping to better understand integral links and find integrated solutions. Graphic provided by EcoHealth Alliance.

The integral links between agriculture, nutrition, land use policy, and health outcomes call for a multidisciplinary approach to find sustainable and healthy solutions. The One Health approach, which considers connections between human, animal, and environmental health, can help us more fully understand risks and identify prevention opportunities (or at the least, balance risks). Integrated surveillance remains critical for early detection, especially without yet knowing all existing pathogen diversity and their risk to humans. We may also be able to target pathogens in wildlife long before the disease takes prevalence for detection: i.e., by identifying, and mitigating the risk of, human activities, policies, engineered landscapes, and so on that are likely to generate zoonotic disease spillover.

For an example of such an attempt, see EcoHealth Alliance’s current partnership under USAID’s Infectious Disease Emergence and Economics of Altered Landscapes (IDEEAL) Program. Working with the Sabah Wildlife Department, the University of Malaysia and community groups, IDEEAL aims to identify relationships between land-use change, disease emergence, and economic costs of disease risk in the Malaysian state of Sabah. The ultimate goal is to inform local land use policy, considering the health value of not disturbing a forest. These combined efforts can make zoonotic events preventative, as they mark a way of curbing development decisions that might otherwise lead to a new Nipah virus.

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More on the Ecology of Zoonoses about human-animal links in disease can be found at EcoHealth Alliance.