Don't blame the trees: Social factors, not forests, dictate disease patterns

February 6, 2008

A new study published February 6 in the open access journal PLoS Neglected Tropical Diseases suggests that socioeconomic factors best explain patterns of the infectious disease American Cutaneous Leishmaniasis (ACL) in Costa Rica. Contrary to the established belief that deforestation reduces the risk of infection, the research shows that deforestation may actually make socially marginalized human populations more 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," said co-author Luis Fernando Chaves of the University of Michigan. "Contrary to what was previously believed, the more forest you have, even in a marginal population, the more protected you are against the disease."

The 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 deforested annually to make way for cattle ranching and banana, mango and citrus fruit plantations. In addition to examining such factors as forest cover, rainfall, elevation, and percent of the population living less than five kilometers from the forest edge, the researchers also incorporated an index of social marginalization into their analysis. 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.

The researchers found a strong geographic overlap between disease incidence and social marginalization that was not found between disease incidence and the other ecological variables.

"When we looked just at factors such as climate and the physical environment, we found no specific patterns with respect to the disease," Chaves said. "But when we looked at the social data, we found clear patterns according to marginality."

Putting everything together, the researchers discovered that in fact 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.

Dr. Chaves concluded that the “study calls for control efforts targeted to socially excluded populations and for more localized ecological studies of transmission in vectors and reservoirs in order to understand the role of biodiversity changes in driving the emergence of this disease.”

The researchers are now planning on conducting similar analyses for other diseases, such as malaria, paying close attention to how climatic fluctuations, ecological factors, and patterns of biodiversity relate to human disease patterns.

Citation: Chaves LF, Cohen JM, Pascual M, Wilson ML (2008) Social Exclusion Modifies Climate and Deforestation Impacts on a Vector-Borne Disease. PLoS Negl Trop Dis 2(2): e176. doi:10.1371/journal.pntd.0000176 (

Source: Public Library of Science

Explore further: Time travel with the molecular clock

Related Stories

Time travel with the molecular clock

November 23, 2015

Migration isn't a new phenomenon, but new insights suggest that modern-day Europeans actually have at least three ancestral populations. This finding was published by Johannes Krause and prominently featured on the cover ...

New study rings alarm for sugar maple in Adirondacks

October 21, 2015

The iconic sugar maple, one of the most economically and ecologically important trees in the eastern United States and Canada, shows signs of being in a significant decline, according to research results published today (Oct. ...

Bacteria in ancient flea may be ancestor of the Black Death

September 28, 2015

About 20 million years ago a single flea became entombed in amber with tiny bacteria attached to it, providing what researchers believe may be the oldest evidence on Earth of a dreaded and historic killer - an ancient strain ...

Greening cities reduces air pollution

September 28, 2015

A UTS research collaboration has provided the first experimental evidence that higher levels of urban forestry are associated with lower levels of air pollution, specifically the dangerous fine airborne particles, or particulate ...

Genetic mutants alter entire biological communities

September 10, 2015

Scientists from Trinity College Dublin have discovered that one gene mutation in a single species can trigger dramatic changes in whole biological communities; changes can be as great as those caused by the extinction of ...

Recommended for you

How the finch changes its tune

August 3, 2015

Like top musicians, songbirds train from a young age to weed out errors and trim variability from their songs, ultimately becoming consistent and reliable performers. But as with human musicians, even the best are not machines. ...

Machine Translates Thoughts into Speech in Real Time

December 21, 2009

( -- By implanting an electrode into the brain of a person with locked-in syndrome, scientists have demonstrated how to wirelessly transmit neural signals to a speech synthesizer. The "thought-to-speech" process ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.