Coupling between Annual and ENSO Timescales in the Malaria-Climate Association in Colombia

Beginning of article

We present evidence that the El Nino phenomenon intensifies the annual cycle of malaria cases for Plasmodium vivax and Plasmodium falciparum in endemic areas of Colombia as a consequence of concomitant anomalies in the normal annual cycle of temperature and precipitation. We used simultaneous analyses of both variables at both timescales, as well as correlation and power spectral analyses of detailed spatial (municipal) and temporal (monthly) records. During "normal years," endemic malaria in rural Colombia exhibits a dear-cut "normal" annual cycle, which is tightly associated with prevalent climatic conditions, mainly mean temperature, precipitation, dew point, and river discharges. During historical El Nino events (interannual time scale), the timing of malaria outbreaks does not change from the annual cycle, but the number of cases intensifies. Such anomalies are associated with a consistent pattern of hydrological and climatic anomalies: increase in mean temperature, decrease in precipitation, increase in dew point, and decrease in river discharges, all of which favor malaria transmission. Such coupling explains why the effect appears stronger and more persistent during the second half of El Nino's year (0), and during the first half of the year (+1). We illustrate this finding with data for diverse localities in Buenaventura (on the Pacific coast) and Caucasia (along the Cauca river floodplain), but conclusions have been found valid for multiple localities throughout endemic regions of Colombia. The identified coupling between annual and interannual timescales in the climate-malaria system shed new light toward understanding the exact linkages between environmental, entomological, and epidemiological factors conductive to malaria outbreaks, and also imposes the coupling of those timescales in public health intervention programs. Key word: climate variability, Colombia, El Nino/Southern Oscillation, ENSO, human health, malaria, tropical medicine, vectorborne diseases. Environ Health Perspect 109:489--493 (2001). [Online 4 May 2001]

http://ehpnet1.niehs.nih.gov/docs/2001 /109p489-493poveda/abstract. html

More than five million people in Colombia live in endemic malaria regions. During 1996, transmission of malaria reached 42 cases per 1,000 inhabitants in high-risk areas (1). In the province of Choco (along the Pacific coast), there were [is greater than] 80,000 cases during 1998, when the population at risk was 380,000. The most important malaria vectors in the country are Anopheles albimanus, Anopheles darlingi, and Anopheles nuneztovari (2), transmitting Plasmodium falciparum (46.5%) and Plasmodium vivax (53.5%), and rare cases (8-10 per year) of Plasmodium malariae (3). The geographical distribution of malaria in Colombia is associated with prevalent climatic conditions. Mean annual temperature and precipitation are related to diverse factors such as elevation over the Andes, the distance to the Caribbean Sea and the Pacific Ocean, and the influence of the circulation, vegetation, and land-surface feedbacks of the Amazon basin and the tropical Andes, which vary at annual and interannual timescales.

El Nino refers to the unusual warming of sea surface temperatures (SSTs) in the eastern and central tropical Pacific. The accompanying Southern Oscillation, the "seesaw" of the atmospheric mass that produces a pressure gradient between the western and the eastern equatorial Pacific, is quantified by the Southern Oscillation index (SOI), defined as the standardized difference between Tahiti and Darwin sea level atmospheric pressures. Negative values of the SOI are associated with warm events (El Nino), whereas positive values accompany cold events (La Nina). El Nino/Southern Oscillation (ENSO) is an aperiodic oscillation that occurs approximately every 3-7 years, with an average of about every 3-4 years (4). The onset of El Nino events occurs during spring in the Northern Hemisphere, exhibiting a strong phase-locking with the annual cycle (5). El Nino events, which continue through 2 calendar years, are generally characterized by positive …