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Title: Dissecting the Influences of Climate and Demography on the Dynamics of Leptospirosis in California Sea Lions

item LLOYD-SMITH, J - University Of California
item PEPIN, K - Pennsylvania State University
item GREIG, D - Marine Mammal Center
item MELIN, S - National Oceanic & Atmospheric Administration (NOAA)
item LAAKE, J - National Oceanic & Atmospheric Administration (NOAA)
item LOWRY, M - National Marine Fisheries
item NUTTER, F - Marine Mammal Center
item Zuerner, Richard
item DELONG, R - National Oceanic & Atmospheric Administration (NOAA)
item GULLAND, FRANCES - Marine Mammal Center
item GRENFELL, B - Pennsylvania State University

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 8/1/2009
Publication Date: 12/2/2009
Citation: Lloyd-Smith, J.O., Pepin, K., Greig, D., Melin, S., Laake, J., Lowry, M., Nutter, F., Zuerner, R.L., Delong, R.D., Gulland, F., Grenfell, B.T. 2009. Dissecting the Influences of Climate and Demography on the Dynamics of Leptospirosis in California Sea Lions [abstract]. Poster No. 6B.2.

Interpretive Summary:

Technical Abstract: Leptospirosis is a zoonotic infection of global importance, yet its population dynamics remain poorly understood. We present the first empirically-motivated study of the dynamics of leptospirosis, drawing on a unique 24-year time series of disease in California sea lions (CSLs). Since the early 1980s CSLs have suffered from annual outbreaks of leptospirosis, with great variation in intensity among years and shifting patterns of cyclicity. Proposed explanations for severe outbreaks have included terrestrial spillover via agricultural runoff, El Nino events, and the exponential growth of the CSL population over recent decades. We conducted a cross-disciplinary analysis to determine the support for these hypotheses and explore the interplay between intrinsic and extrinsic drivers of the observed patterns. Molecular and serological evidence indicate that outbreaks are not due to spillover from exogenous sources, but instead the infection appears to be persistent in the CSL population. By analyzing the dynamics of outbreaks within years we found that transmission rates are independent of population abundance, so population growth does not account for the observed shifts in dynamics. To assess the influence of intrinsic drivers, variation in susceptible population size was reconstructed based on historical recruitment and incidence data, and was found to explain roughly half of the variance in outbreak intensity. Multivariate statistical models, using cross-validation so that predictors were tested against out-of-sample data, showed that outbreak intensity is further influenced by environmental factors acting at global and local scales. Our analysis discriminates between two pathways by which climate influences leptospirosis dynamics: indirectly via impacts on demography, and directly via impacts on trophic conditions or contact patterns.