Analysis of climatic data and forecast indices for human fascioliasis at very high altitude

Human infection with Fasciola hepatica has recently been recognized as an i mportant health problem worldwide, and particularly at very high altitudes in South America. The highest prevalences and intensities of human fascioli asis known are those of the northern Bolivian Altiplano, where infected Lym naea truncatula occur at altitudes of 3800-4100 m. In the present study, th e climatic data for this area of the Altiplano, which differ markedly from those of endemic areas in the lowlands, were analysed. There is no marked s easonality in temperature but there are large variations in temperature wit hin a daily, 24-h period. Rainfall is seasonal, with a long dry season, coi nciding with the lowest minimum temperatures, and a long wet season. The ra te of evapotranspiration is very high, and temporary water bodies dry out v ery quickly. Solar radiation at ground level is intense, not only because o f the altitude but also because of the lack of trees and shrubs. Two climat ic indices for forecasting fascioliasis, Mt and Wb-bs, were calculated. Mod ifications in these forecast indices are proposed, to reflect the environme nt at high altitude and low latitude. Estimates, based on climadiagrammes, of the durations of the wet and dry seasons were greatly effected by the in clusion of an aridity-index modification. The usefulness of the modified in dices was examined using prevalence data for human and cattle fascioliasis collected in the neighbourhoods of the stations providing the meteorologica l data. Values for both indices indicated that conditions were optimum for transmission between December and March. The results were statistically sig nificant for the modified Wb-bs index when the data for a meteorological st ation in which no lymnaeids were found were excluded. The modified hit inde x did not appear sufficiently accurate to be useful. The values for the mod ified Wb-bs index permitted the study areas to be designated low-, moderate - or high-risk areas for the transmission of fascioliasis to man and domest ic animals.