Research article

Cyclical changes in seroprevalence of leptospirosis in California sea lions: endemic and epidemic disease in one host species?

James O Lloyd-Smith¹ , Denise J Greig² , Sharon Hietala³ , George S Ghneim⁴ , Lauren Palmer⁵ , Judy St Leger⁶ , Bryan T Grenfell¹ and Frances MD Gulland²

¹  Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, USA

²  The Marine Mammal Center, Marin Headlands, 1065 Fort Cronkhite, Sausalito, USA

³  California Animal Health and Food Safety Laboratory, University of California, Davis, USA

⁴  Research Triangle Institute International, Research Triangle Park, USA

⁵  Marine Mammal Care Center, Fort Macarthur, San Pedro, USA

⁶  SeaWorld of California, San Diego, USA

author email corresponding author email

BMC Infectious Diseases 2007, 7:125doi:10.1186/1471-2334-7-125

Published:	6 November 2007

Abstract

Background

Leptospirosis is a zoonotic disease infecting a broad range of mammalian hosts, and is re-emerging globally. California sea lions (Zalophus californianus) have experienced recurrent outbreaks of leptospirosis since 1970, but it is unknown whether the pathogen persists in the sea lion population or is introduced repeatedly from external reservoirs.

Methods

We analyzed serum samples collected over an 11-year period from 1344 California sea lions that stranded alive on the California coast, using the microscopic agglutination test (MAT) for antibodies to Leptospira interrogans serovar Pomona. We evaluated seroprevalence among yearlings as a measure of incidence in the population, and characterized antibody persistence times based on temporal changes in the distribution of titer scores. We conducted multinomial logistic regression to determine individual risk factors for seropositivity with high and low titers.

Results

The serosurvey revealed cyclical patterns in seroprevalence to L. interrogans serovar Pomona, with 4–5 year periodicity and peak seroprevalence above 50%. Seroprevalence in yearling sea lions was an accurate index of exposure among all age classses, and indicated on-going exposure to leptospires in non-outbreak years. Analysis of titer decay rates showed that some individuals probably maintain high titers for more than a year following exposure.

Conclusion

This study presents results of an unprecedented long-term serosurveillance program in marine mammals. Our results suggest that leptospirosis is endemic in California sea lions, but also causes periodic epidemics of acute disease. The findings call into question the classical dichotomy between maintenance hosts of leptospirosis, which experience chronic but largely asymptomatic infections, and accidental hosts, which suffer acute illness or death as a result of disease spillover from reservoir species.