Leptospirosis, the disease caused by pathogenic bacteria of the genus Leptospira, is a major health burden for humans and animals worldwide and a recognized risk for military personnel. Leptospira circulated endemically for decades in California sea lions (CSL: Zalophus californianus), then disappeared from 2013 to 2017 during a period of severe oceanographic anomalies. This project studied the ecology of Leptospira in CSL and their surrounding ecosystem, to understand how non-stationary environmental conditions contributed to the fadeout and re-emergence of the pathogen in this system. The objectives were to learn: (1) what factors gave rise to the fadeout of Leptospira in the CSL population in 2013, (2) what conditions enabled the re-emergence of Leptospira in 2017, and (3) whether these perturbations had lasting impacts or whether CSL/Leptospira system dynamics returned to pre-fadeout patterns.

With the partners, the project team extended long-term studies of host demography, movement and Leptospira spread in CSL. Spatiotemporal patterns and genomic sequencing data to confirm and characterize the fadeout event were analyzed. The project team compiled and analyzed time series of putative drivers of transmission that could mediate the impact of environmental perturbations. The project team searched for possible outside reservoirs that could have reintroduced the pathogen to CSL to enable re-emergence. Throughout, mechanistic and statistical models were developed to reveal underlying processes and integrate data streams.

It was confirmed that Leptospira ceased circulating in CSL from 2013-2017, using data from diagnostic testing of ~3800 samples collected during this period, and analysis of pathogen genomic sequences. The fadeout event in 2013 was associated with unprecedented reductions in yearling CSL recruitment and normal migratory behavior, both driven by anomalously poor feeding conditions, which eliminated the susceptible ‘fuel’ needed to sustain the transmission chain. The fadeout period lasted for four years until the confluence of two events: oceanographic conditions returned to normal, and the pathogen was reintroduced to CSL. The resulting outbreak in 2017 was small, but 2018 brought one of the largest outbreaks on record; these patterns are explained by severe demographic impacts of a marine heat wave in 2015-2016, which decimated the juvenile cohorts needed to drive an outbreak in 2017. Field surveillance and genomic sequencing revealed that terrestrial mesocarnivores in mainland California could be the reservoir that reintroduced Leptospira to CSL, since they carry a closely related strain of the pathogen.

This project generated new knowledge and tools to support management of two wildlife species of concern in the California coastal ecosystem. By extending long-term field studies and developing mathematical and statistical models to analyze the data, the causes of an unprecedented fadeout event was uncovered, discovered a new community of hosts carrying Leptospira in proximity to humans and sensitive wildlife species, and generated evidence-based guidance for species managers.

(Project Completion-2022)