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Social factors and changes in biodiversity caused by deforestation are critical to the spread of American cutaneous leishmaniasis (ACL), according to a study in Costa Rica.

Prior research had indicated that forest areas increased the risk of infection, and that deforestation could help reduce transmission.

But a study published in PLoS Neglected Tropical Diseases this month (6 February) finds that socioeconomic factors are also crucial in explaining spatial disease patterns.

Luis Fernando Chaves from the US-based University of Michigan and lead author of the study says previous studies tended to emphasise the physical environment, without considering social factors. This led to generalised conclusions such as "the more forest you have, the more disease you have."

Chaves' study analyses Costa Rican data on ACL incidence from 1996–2000, along with information on peoples' quality of life. Factors considered include literacy, access to water, access to health centres and health insurance cover.

The researchers found that areas of social exclusion correspond to a high incidence of disease, highlighting the need to apply disease-control measures to these parts of the population.

"It doesn't matter that much if you have the 'ideal' environmental conditions for leishmaniasis. You are more likely to have the disease on a broad scale where there are people who are socially excluded in comparison to the rest of the population."

The researchers also found that deforestation can actually make humans more vulnerable to ACL rather than less. This is because the disease's major reservoirs — small mammals such as rodents and marsupials — thrive in fragmented forests.

Chaves says there are fewer such animals in continuous forest because larger predators regulate the population. "In patchy, fragmented forests, you lose this regulation."

The El Niño weather phenomenon also plays a role. In countries with more deforestation, occurrences of ACL tend to increase with El Niño.

According to Chaves, this is because El Niño changes the environmental conditions, which can lead to better resources — such as food — for small mammals, allowing their populations to grow.

"The combination of deforestation and El Niño improves the living conditions for [disease-carrying mammals in] these disease reservoirs."

Link to full paper in PLoS Neglected Tropical Diseases

Reference: PLoS Neglected Tropical Diseases doi:10.1371.journal.pntd.0000176 (2008)

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