Snakebites are associated with poverty, weather fluctuations, and El Niño

Abstract

Snakebites are environmental and occupational health hazards that mainly affect rural populations worldwide. The ectothermic nature of snakes raises the issue of how climate change's impact on snake ecology could influence the incidence of snakebites in humans in ways that echo the increased predation pressure of snakes on their prey. We thus ask whether snakebites reported in Costa Rica from 2005 to 2013 were associated with meteorological fluctuations. We emphasize El Niño Southern Oscillation (ENSO), a climatic phenomenon associated with cycles of other neglected tropical diseases (NTDs) in the region and elsewhere. We ask how spatial heterogeneity in snakebites and poverty are associated, given the importance of the latter for NTDs. We 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.

Keywords: Bothrops asper; Climate change; antivenoms; ectotherm; population cycles.

Fig. 1. Snakes and snakebites in CR.

(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.

Fig. 2. Variables spatially associated with snakebites in CR.

(A) Altitude. (B) Rainfall. (C) Poverty gap index. (D) Destitute housing. Coefficients are shown (in the legend of each panel) only when pseudo t values are significant (P < 0.05).

Fig. 3. Temporal snakebite incidence patterns in CR.

(A) Monthly time series from 2005 to 2013; colors indicate the phases of ENSO as explained in the legend (inset). (B) Seasonality in snakebite incidence; monthly box plot shows the log-transformed number of snakebites, and colors indicate the different phases of ENSO. (C) ACF of monthly snakebites. (D) CCF between snakebites and temperature. (E) CCF between snakebites and rainfall. (C to E) Dashed lines represent the 95% confidence interval for correlations expected to arise randomly. (F) Cross-wavelet coherence analysis of snakebites and ENSO. We used SST4 as an ENSO index. In the analysis, a 6-month smoothing window was used. The cross-wavelet coherence scale is from 0 (blue) to 1 (red). Red regions in the plots indicate frequencies and times for which the two series share variability (or power). The cone of influence (in which results are not influenced by the edges of data) and the significantly coherent time-frequency regions (P < 0.05) are indicated by solid black lines.