There is good evidence that the ENSO cycle is associated with increased risk of certain diseases (PAHO, 1998b). ENSO events may affect the distribution (reproduction and mortality) of disease vectors (Epstein et al., 1998). El Niño events raise SST over the tropical Pacific, affecting some pathogenic agents. McMichael and Kovats (1998b), Patz (1998), and Instituto Nacional de Salud (1998a,b) have speculated that the last cholera outbreak in Peru, beginning in 1991, was linked with an El Niño phenomenon (1990-1995). The outbreak spread to most of the South American subcontinent, including places as far away as Buenos Aires (OPS, 1999). de Garín et al. (2000) have reported that El Niño has important impacts in andean population; they found large amounts of the insect, as well as eggs for the next period, in the northern part of Argentina.
Higher temperatures over coastal Peru associated with ENSO may have an impact on gastrointestinal infections. Salmonella infections increased after a flood in Bolivia, which resulted from the El Niño event of 1983 (Valencia Tellería, 1986). Salazar-Lindo et al. (1997) report that the number of patients with diarrhea and dehydration admitted to a rehydration unit in Lima, Peru, was 25% higher than usual during 1997, when temperatures where higher than normal as a result of the emerging El Niño. Increases in the incidence of acute diarrheas and acute respiratory diseases were recorded in Bolivia (Valencia Tellería, 1986) and Peru (Gueri et al., 1986).
The effects of natural events vary by region, and the same weather condition may have the opposite effect in different areas for the same disease (e.g., a dry year may induce malaria epidemics in humid regions but cause malaria decreases in arid regions) (McMichael and Kovats, 1998b). For example, ENSO has been associated with severe drought in Iquitos, Peru, and the state of Roraima, Brazil, where malaria cases have drastically decreased (OPS, 1998; Confalonieri and Costa-Díaz, 2000). In Venezuela, malaria mortality has been shown to be more strongly related to the occurrence of drought in the year preceding outbreaks than to rainfall during epidemic years (Bouma and Dye, 1997). The El Niño phenomenon appears to be responsible in particular for serious epidemics in Peru, including one of malaria in 1983 (Moreira, 1986; Russac, 1986; Valencia Tellería, 1986), as well as cutaneous diseases, leptospirosis, and respiratory infections in 1998 (Instituto Nacional de Salud, 1998b). Compared with other years, malaria cases in Colombia increased 17.3% during an El Niño year and 35.1% in the post-El Niño year. Upsurges of malaria in Colombia during El Niño events are associated with its hydrometeorological variablesin particular, the increase in air temperature that enhances reproductive and biting rates and decreases the extrinsic incubation period, as well as changes in precipitation rates that favor formation of ponds and stagnant pools and thus create more mosquito breeding sites (Poveda and Rojas, 1997; Poveda et al., 1999a,b).
Global analyses have shown no association between ENSO and the number of flood disasters (Dilley and Heyman, 1995) or between ENSO and the numbers of persons affected by floods and landslides (Bouma et al., 1997). However, the number of persons affected by landslides, particularly in South America, increases in the year after the onset of El Niño (Bouma et al., 1997). In 1983, the impacts of El Niño in Peru increased total mortality by nearly 40% and infant mortality by 103% (Toledo-Tito, 1997).
Predicting malaria risk associated with ENSO and related climate variables may serve as a short-term analog for predicting longer term effects posed by global climate change. The ability to predict years of high and low risk for malaria can be used to improve preventive measures (Bouma et al., 1997).
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