El Niño and the dynamics of vectorborne disease transmission.

The objective of the study was to investigate the relationship between reported incidence of dengue fever and El Niño southern oscillation (ENSO) in 14 island nations of the South Pacific. Using a mixed ecological study design, we calculated correlations between annual averages of the southern oscillation index (SOI), local temperature and rainfall, and dengue fever. We also calculated temporal correlations between monthly reports of dengue fever cases on different islands. There were positive correlations between SOI and dengue in 10 countries. In five of these (including all of the larger islands) there were also positive correlations between SOI and estimates of local temperature and/or rainfall. There were temporal correlations between monthly reports of dengue cases within two groups of countries. Climate changes associated with ENSO may trigger an increase in dengue fever transmission in larger, more populated islands where the disease is endemic. There was also evidence of propagation of infection from larger islands to smaller neighbors. Unlike the initiation of epidemics, this transfer between islands appears to be independent of interannual climate variations, pointing to the importance of modulating factors in dengue transmission such as population density and travel. In the future, models of the impact of climate change must attempt to account for these factors.