Cut down tropical forests to prevent the spread of disease? Such drastic measures actually have been proposed in light of an apparent connection between forest cover and certain insect-borne, infectious diseases.
But a new analysis by U-M researchers suggests that socioeconomic factors, rather than landscape, best explain patterns of at least one disease, American cutaneous Leishmaniasis (ACL), and that deforestation may make socially marginalized human populations more, not less, vulnerable to infection.
“The classical idea has been that people working or living close to the forest were at risk for the disease, but that view failed to consider such factors as quality of life and general level of health,” says the paper’s first author Luis Fernando Chaves, a graduate student in the Department of Ecology and Evolutionary Biology.
The study results are published in the journal PLoS Neglected Tropical Diseases.
Social connection
Researchers examined county-level ACL case data from 1996 through 2000 for Costa Rica, a country in which approximately 20,000 acres of land are annually deforested to make way for cattle ranching and banana, mango and citrus fruit plantations.
In addition to looking at factors such as percent forest cover, monthly rainfall, elevation and percent of the population living less than about three miles from the forest edge, the U-M team also used scores on an index of social marginalization. This index, which takes into account income, literacy, level of education, average distance to health centers, health insurance coverage and other indicators of life at the margins of mainstream society, provides a single measure of quality of life.
“When we looked just at factors such as climate and the physical environment, we found no specific patterns with respect to the disease,” Chaves says. “But when we looked at the social data, we found clear patterns according to marginality.”
El niño
Putting everything together, the researchers discovered that there is a relationship between ACL and deforestation, but it’s not the simple, “less forest, less disease” relationship that previously was believed to exist. Instead, there’s a complex connection with El Niño Southern Oscillation (ENSO), a periodic ocean-atmosphere fluctuation in the Pacific Ocean that is an important cause of inter-annual climate variability around the world and also influences disease cycles. In highly deforested counties, socially marginalized human populations are more vulnerable to ENSO’s effects, and disease incidence actually is higher, the analysis suggests.
“Contrary to what was previously believed, the more forest you have, even in a marginal population, the more protected you are against the disease,” Chaves says.
This result makes sense when the natural history of ACL is taken into account. Humans get the disease from sand flies, but sand flies pick it up from small mammals such as rodents, marsupials and sloths. Forest fragmentation increases the density of these mammal species, because the large predators that normally keep their numbers in check are the first animals to be lost from small, isolated forest fragments. With the large predators gone, the small mammals burgeon.
The researchers now plan to do similar analyses for other diseases, such as malaria. They’d also like to look more closely at how patterns of mammal diversity relate to human disease patterns.
In addition to Chaves, the paper’s authors are Mercedes Pascual, associate professor of ecology and evolutionary biology; Mark L. Wilson, professor of epidemiology and of ecology and evolutionary biology; and Justin Cohen, who was a graduate student at U-M when the research was done and now is a senior research associate with the Clinton Foundation HIV/AIDS Initiative.
The researchers received funding from Fundación Polar of Caracas, Venezuela, and U-M funds from the International Institute, Rackham Graduate School, the Global Health Program of the School of Public Health and the Graham Environmental Institute.
