Project: Leptospirosis in California sea lions: Population impacts and persistence in a long-term study of infectious disease in marine mammals

Description from NSF award abstract:
Despite increasingly stark evidence that infectious diseases have dramatic effects on marine ecosystems, research on disease ecology in marine systems has lagged behind that in terrestrial systems. There is a particular need for long-term case studies, with quantitatively accurate data and well-characterized biology, to enable research into how oceanographic processes affect pathogen spread and how disease impacts will be altered by climate change. This unique project will carry out an integrated field, clinical, laboratory and modeling research approach to establish a case study for the dynamics of Leptospira interrogans in California sea lions (CSL; Zalophus californianus). Leptospirosis has caused recurrent seasonal outbreaks in CSL since at least 1984, resulting in major disease outbreaks every 3-4 years. The PIs will investigate the quantitative impact of leptospirosis on CSL population dynamics and the mechanism that has enabled L. interrogans to persist in the CSL population over the past three decades. The project will answer the following hypotheses:
1. Data from stranded CSL provide quantitative information on long-term dynamics of L. interrogans prevalence in the free-ranging population,
2. L. interrogans impacts CSL population dynamics by infecting more than 20% of the CSL population during disease outbreaks, and killing 1-2% of infected CSL. The project will use this data to develop a mathematical model that integrates immunological and stranding data, which will then allow for predictions of the fraction of the CSL population that is infected each year,
3. L. interrogans is persistent in the CSL population, and the mechanism of persistence involves a chronic asymptomatic carrier state and subsequent shedding of L. interrogans by individual CSL. A longstanding puzzle concerns how the pathogen persists in the 6-8 month interval between outbreaks. The PIs will conduct a pathology study to determine whether CSL can become chronic carriers of L. interrogans, and will also sample free-ranging CSL individuals when disease outbreaks are not occurring to search for asymptomatic shedders. This project will establish the L. interrogans/CSL system as a unique and powerful case study for infectious disease in marine mammals, enabling subsequent research into impacts of climate change and connections to the broader coastal ecosystem.