Friday, September 25, 2015

Snakebite, Economics, and El Nino in Costa Rica

Snakes and snakebites in Costa Rica. (A) The terciopelo 
B. asper. (B) Average annual snakebite incidence, by 
canton, from 2005 to 2013. County color indicates snakebite
incidence rate, county boundary color indicates relative risk, 
and a marking described in the map legend indicates the 
primary cluster. From Chaves et al. 2015
Snake envenomation is frequently considered a neglected medical problem in rural, tropical or subtropical populations. In a new paper Chaves et al. (2015) examine how climate change's impact on snake ecology could influence the incidence of snakebites. They asked whether snakebites reported in Costa Rica between 2005 and 2013 were associated with meteorological fluctuations. The El Niño Southern Oscillation (ENSO) is a climatic phenomenon associated with cycles of other neglected tropical diseases. They examine how spatial heterogeneity in snakebites and poverty are associated, given the importance of poverty in other neglected tropical diseases.

The authors found that periodicity in snakebites reflects snake reproductive phenology and is associated with ENSO. Snakebites are more likely to occur at high temperatures and may be significantly reduced after the rainy season. Nevertheless, snakebites cluster in Costa Rican areas with the heaviest rainfall, increase with poverty indicators, and decrease with altitude. Altogether, our results suggest that snakebites might vary as a result of climate change.

Chaves et al. found 6424 snakebites were reported in Costa Rica from 2005 to 2013. The 9-year average incidence rate was 15.24 per 100,000, ranging from 10.63 per 100,000 to 22.98 per 100,000 when the entire population was assumed to be at risk. However, those statistics underestimate the incidence rate in the at-risk population which is mainly rural. The average rate jumps to 41.27 per 100,000, ranging from 30.53 per 100,000 to 58.94 per 100,000 with a steadily decreasing at-risk population. The highest incidence of bites occurred in southern Costa Rica and in the northern portion of La Cruz, bordering Nicaragua. Snakebites usually occurred in suburban or rural regions.
The study shows that snakebites are associated with changes in temperature and rainfall across time, and that unusually high numbers of snakebites occur during the cold and hot phases of ENSO. Spatially, snakebites cluster in the most humid lowland areas of Costa Rica, where terciopelos (Bothrops asper)  are common, and are more frequent in the economically poorest areas with similar weather patterns. This combination of patterns highlights the fact that snakebites follow meteorological changes, and these patterns  reflect the impact of meteorological fluctuations on snake biology.

Snakebites follow a common pattern seen in other tropical diseases in the region and reflecting the general vulnerability of impoverished human populations to the adverse effects of climate change and neglected diseases. The latter is a pattern that might be extrapolated to other areas where snakebites are a major health problem. The findings highlight the need for increased research on the eco-epidemiology of snakebites, a neglected tropical disease that should be included in the list of diseases or health hazards that are sensitive to environmental changes.

Chaves, L. F., Chuang, T. W., Sasa, M., & Gutiérrez, J. M. (2015). Snakebites are associated with poverty, weather fluctuations, and El Niño. Science Advances, 1(8), e1500249.

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