Contribution of Environmental Drivers to Seasonality of Waterborne Infections: Novel Methodologies for Assessment

Diseases which are climate and environment sensitive, such as waterborne or food-borne enteric infections, alternate periods of low endemic levels with outbreaks, forming a seasonal pattern. For example, in temperate climates illness caused by termo-sensitive bacteria (i.e., salmonella, campylobacter) rises in the summer and declines in the winter where as enteric infections caused by protozoa (i.e., cryptosporidium, giardia) exhibit a peak in the fall. Temporal patterns of diseases may be governed by factors related to increases in the probability of pathogen spread, transmission, and viability. Consistent temporal fluctuations for diseases with similar sources of exposure or routes of transmission suggest the presence of environmental factors that synchronize seasonal variation.

Although seasonality is a well-known phenomenon in the epidemiology of enteric infections, analytical tools for examination, evaluation, and comparison of seasonal patterns are insufficient limiting analysis of factors associated with variations. We offer a framework for seasonality assessment, and a parametric approach for evaluating peak timing and intensity. We will discuss the importance of proper aggregation for the time series and determination of the calendar year for each health outcome. We contrast our approach with non-parametric modeling and demonstrate this methodology on hospitalization data for environmentally driven diseases using examples related to the effect of extreme temperature and precipitation. This requires an appropriate definition of seasonality and analytic tools suitable for routine use. An understanding of how environmental factors influence human disease can improve disease forecasting, improve the design of integrated warning systems, and advance development of efficient outbreak detection algorithms.