Australia: Climate-Ecosystem Variability and Impacts on Disease

Climate variability in Australia is largely driven by an atmospheric phenomenon called the Southern Oscillation (SO), which involves a see-saw like behavior between low and high pressure systems within the equatorial Pacific regions. The interaction of SO with abnormally high sea surface temperatures (SSTs) - El Niño - or abnormally low SSTs - La Niña ("anti-El Niño") - creates extreme drought or extreme flooding respectively throughout the Australian continent. These El Niño-Southern Oscillation (ENSO) events have significant impacts on Australia's landscape, ecosystems, agriculture production, and, as this report show, human health. The teleconnection between ENSO and human health is straight forward but not obvious. During La Niña years, when ENSO events are characterized by increased rainfall and consequential flooding, Australia's tropical, warm climate in addition to an associated increase in vegetation growth from the increased rainfall creates an ideal habitat for mosquito population increase. Certain species of Australian mosquitoes [Culux annulirostris] are carriers of Murray Valley Encephalitis (MVE) virus which is a rare but potentially fatal infection that attacks neurological and muscular functioning. It is hypothesized that a widespread increase in vegetation indicates an expansion of ideal mosquito production habitats and will translate to an increased risk of MVE contraction. The objective of this research is to show if a correlation exists between the ENSO-driven climate- and consequential ecosystem- changes and MVE outbreaks throughout Australia. To do so, this study makes use of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor operating on NASA's Terra satellite to obtain monthly Normalized Difference Vegetation Index (NDVI) data. It is assumed in this research that an anomalous increase in NDVI values - indicative of vegetation growth - occurs as a result of increased rainfall. Due to Australia's tropical positioning and therefore high exposure to warm, sunny conditions, the NDVI data serves as an adequate proxy for the combined influence of rainfall and temperature on mosquito ecology. We compiled MVE outbreak data from two online databases that reported incidents: (1) Pro-MED mail and (2) National Notifiable Disease Surveillance System (NNDSS). NDVI monthly anomalies were calculated with respect to the long-term averages of plant growth to assess the departure patterns. The outbreak sites were geo-coded and mapped on NDVI anomaly maps and statistical analysis was run on four case study sites where human infection was most prevalent. The research shows a general correlation between outbreak occurrences and a prior period of positive NDVI anomalies or "greening". The correlation and timing of these results support the hypothesis that increased rainfall, as a result of ENSO events, stimulates mosquito population growth, raising the potential for transmitting MVE to humans. However, further research is needed to confirm this hypothesis. It is expected that through the use of actual rainfall data and time-series Southern Oscillation Index data, a comprehensive, robust link can be shown between ENSO events and vector-borne disease outbreaks. Additionally, added geographically accurate outbreak data will provide a more scientifically sound spatial correlation.