Abstract

Leptospirosis is a zoonotic disease infecting a broad range of mammalian hosts, and is re-emerging globally in humans and domestic dogs. Disease outbreaks have occurred periodically in California sea lions (Zalophus californianus) off the central and northern coasts of California, with hundreds of animals dying in each outbreak since 1970.^2,3,7,12 Repeated epidemics have occurred at approximately three to five year intervals since 1984.^2,3,4 Leptospira interrogans serovar Pomona has been isolated from infected sea lions, but it is unknown whether the pathogen persists in the sea lion population or is introduced repeatedly from external reservoirs. The potential for transmission to and from both domestic animals and humans, coupled with the large numbers of sick and dying sea lions lying on beaches during epidemics, makes the question of the source of the infection in these free-living marine mammals one of public as well as veterinary interest. Here we present serological surveillance and modeling results, combined with molecular data from ten sea lion isolates, to elucidate the epidemiology of leptospirosis in California sea lions, and investigate whether the disease is endemic in sea lions or results from spill-over from a terrestrial source.

Serum samples collected over an 11-year period from 1,344 California sea lions that stranded alive on the California coast were evaluated using the microscopic agglutination test (MAT) for antibodies to L. interrogans serovar Pomona.¹ Seroprevalence among yearlings was evaluated as a measure of incidence in the population, and antibody persistence times based on temporal changes in the distribution of titer scores were characterized. Multivariate logistic regression was used to determine individual risk factors for seropositivity with high and low titers. The serosurvey revealed cyclical patterns in seroprevalence to L. interrogans serovar Pomona, with 4-5 year periodicity and peak seroprevalence above 50%.⁶ Seroprevalence in yearling sea lions was an accurate index of exposure among all age classes, 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.

Strains of Leptospira interrogans (n = 63) from a variety of domestic animals and ten strains obtained from sea lion kidneys during leptospirosis epizootics since 1971 were propagated in liquid EMJH medium at 29 °C. All strains were identified as L. interrogans serovar Pomona using RFLP analysis.¹⁰ Genomic DNA samples were used as templates for PCR amplification at previously described VNTR loci. PCR products were separated by agarose gel electrophoresis and visualized, and product sizes were calculated by comparison to size standards using linear regression. The number of tandem repeats at nine loci (VNTR4, VNTR7, VNTR10, VNTR23, VNTR27, VNTR29, VNTR30, VNTR31, and VNTR36) was determined for all strains.^8,9 Eight loci (VNTR4, VNTR7, VNTR10, VNTR23, VNTR29, VNTR30, VNTR31, and VNTR36) were used to differentiate serovar Pomona isolates from L. interrogans serovar Copenhageni, used as an out-group in this study. Three loci (VNTR29, VNTR30, and VNTR36) were invariant among serovar Pomona isolates but differentiated serovar Pomona from serovar Copenhageni, while variation among serovar Pomona isolates at five loci (VNTR4, VNTR7, VNTR10, VNTR23, and VNTR31) was used to differentiate isolates within this serovar.

Eleven distinct groups of L. interrogans serovar Pomona isolates were detected using VNTR analysis. Although most cattle isolates shared a common VNTR profile, isolates from pigs and most wildlife species exhibited greater diversity. Remarkably, all California sea lion isolates shared common VNTR profiles, despite separation by time and location of isolation, and were distinct from all other serovar pomona isolates.¹² Isolate Po048, obtained during the first known leptospirosis outbreak among California sea lions,¹¹ differs slightly from all isolates obtained from subsequent sea lion outbreaks and may represent the founder genotype for strains infecting this mammalian host. Isolates obtained during subsequent California sea lions outbreaks were identical to each other. These data strongly suggest that California sea lions are maintenance hosts for serovar Pomona, as it is unlikely that intermittent exposure of California sea lions to the same L. interrogans serovar Pomona clonal population has occurred over a 37-year period.

These 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 dichotomy⁵ between maintenance hosts (infected with "host adapted" strains) of leptospirosis, which experience chronic but largely asymptomatic infections, and accidental hosts (infected with "non-host adapted" strains), which suffer acute illness or death as a result of disease spillover from reservoir species.

References

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