Abstract
BackgroundLeptospirosis 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.MethodsWe 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.ResultsThe 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.ConclusionThis 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.
Highlights
Leptospirosis is a zoonotic disease infecting a broad range of mammalian hosts, and is re-emerging globally
As the first step to elucidate the epidemiology of recurring leptospirosis outbreaks in California sea lions, and to identify future research directions, this study investigated changes in seroprevalence in an unusually detailed longitudinal dataset from 1995–2005
Because of the consistently higher titer scores for serovar Pomona, combined with the fact that all Leptospira isolates from wild California sea lions have been serovar Pomona [15,18,28], we restrict the remainder of our analysis to serovar Pomona and attribute the positive titers to other serovars to the known cross-reactivity of the microscopic agglutination test (MAT) [10,35]
Summary
Leptospirosis is a zoonotic disease infecting a broad range of mammalian hosts, and is re-emerging globally. A central tenet of the epidemiology of leptospirosis is the distinction between maintenance hosts and accidental hosts for a given serovar, or, equivalently, between host-adapted and non-host-adapted serovars [1,2,3]. In this framework, maintenance hosts develop a chronic, largely asymptomatic infection of their proximal renal tubules, and may shed leptospires in their urine for months or years. Accidental hosts experience acute infections, with symptoms ranging from malaise to multi-organ failure and death
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