Turning aquatic vegetation near agricultural land into compost simultaneously eradicates habitat for disease-carrying snails while improving agricultural output and increasing incomes in northern Senegal, Cornell researchers have found.
Combining highly detailed biological and microeconomic modeling, the team's finding has the potential to aid rural residents of the west African nation, who are often caught in a vicious cycle of poverty and disease.
"This is about really thinking hard about the microeconomics in the cycle of poverty and disease, really thinking hard about households making decisions and the tradeoffs that they're facing," said Molly Doruska, a doctoral student in the field of applied economics and management, and first author of "Modeling How and Why Aquatic Vegetation Removal Can Free Rural Households From Poverty-disease Traps," which published Dec. 17 in Proceedings of the National Academy of Sciences.
Chris Barrett, the Stephen B. and Janice G. Ashley Professor of Applied Economics and Management in the Cornell SC Johnson College of Business and professor in the Cornell Jeb E. Brooks School of Public Policy, is the paper's senior author. Jason Rohr, professor and chair of the Department of Biological Sciences at the University of Notre Dame, is a co-author.
The researchers show analytically, using data from a previous study, that removal of aquatic vegetation reduces habitat for snails, which carry the infectious helminth (a parasitic worm), while also returning soil nutrients that leach into surface water via runoff to agricultural land. The result, the researchers wrote, is "healthier people, more productive labor, cleaner water, more productive agriculture and higher incomes."
The helminth schistosomiasis, also known as bilharzia, infects hundreds of millions of people worldwide and has been termed the second-most socioeconomically devastating parasitic disease, after malaria, by the World Health Organization.
"Humans do things that perturb the environment, and those perturbations have a feedback effect upon humans, which influences human behavioral response, impacting nature and starting the cycle over again," said Barrett, also a senior faculty fellow at the Cornell Atkinson Center for Sustainability.
"We know human behavior is changing with climate, but what we don't really understand much is how those intersect," Barrett said. "That's what Molly's work offers - a nice foundation for other people to attempt similar sorts of modeling to help us think through candidate interventions to help with planetary health, to help improve the health of both humans and the natural environment on which rural people, in particular, depend."
Barrett said Doruska's modeling of both the economics and the disease ecology was painstaking, but produced valuable information.
"These sorts of models are very sensitive," he said. "There's so much feedback that they can blow up very quickly if you don't calibrate them right. That's one of the reasons why people commonly don't attempt this level of granular interactions between the biology and the social science: It's hard to get it right."
The group's bioeconomic model has two sub-models: a disease ecology model that describes how the schistosome, aquatic vegetation and snail populations interact; and an agricultural household model that describes how households make decisions about how to allocate their land, labor and income. The models are linked dynamically to study patterns over time, and assumed that the households in rural northern Senegal, where the original study was conducted, engage in subsistence farming and do not rent land to or from third parties.
Doruska ran a total of 28,000 20-year simulations, using three land sizes (0.5, 2.0 and 5.5 hectares), with and without aquatic vegetation removal, and with numerous other variables factored in. The bottom line: Clearing aquatic vegetation from bodies of water adjacent to farmland succeeds in dramatically reducing schistosomiasis infection rates while boosting agricultural productivity.
Barrett said this work can be adapted to other diseases and vectors. And with a changing climate, where and how people become infected will change, too.
"Dengue fever, malaria - these are diseases that are very clearly affected by how humans manage natural landscapes," he said. "We should not assume that the range of these diseases is going to stay static, and we're going to have to think carefully about how and where to intervene in ways that don't upset the stability of the system."
This research was supported by grants from the National Science Foundation and the Indiana Clinical and Translational Sciences Institute.
