Abstract
Leptospirosis is a bacterial disease caused by pathogenic spirochetes in the genus Leptospira which is common in wildlife, domestic animals and humans worldwide.1 Since 1970, periodic outbreaks of leptospirosis have caused morbidity and mortality of California sea lions (CSL: Zalophus californianus) along the California coast.2,3 Cases occur year round; however, most cases are observed during the 'outbreak' season between the months of July and December, and the magnitude of the annual outbreaks varies, with major epizootics occurring every 3–5 years.4-8 All leptospiral isolates obtained from CSL have been Leptospira interrogans serovar Pomona, and analysis of CSL isolates obtained over the period 1970–2010 shows that the variable nucleotide tandem-repeat (VNTR) profiles are almost identical.9 These data suggest that L. interrogans serovar Pomona is endemic in the CSL population. We hypothesize that asymptomatic 'chronic carrier' CSL, which continue shedding infectious leptospires for months to years post infection, are the mechanism by which uninterrupted circulation of the pathogen in the CSL population occurs. As a first step towards identifying these chronic carriers, we use a combination of techniques to identify CSL that are asymptomatic for leptospirosis, yet are still shedding leptospires. To date, we have found urinary shedding in 3/53 (PCR) and 5/102 (culture) free-ranging wild CSL, and 1/65 (PCR) stranded CSL, all of which had serum chemistry values indicative of normal renal function. At the Marine Mammal Care Center, we detected seroconversion (microscopic agglutination test: MAT) in an animal that was at the center for a flipper amputation and never showed clinical signs of leptospirosis. Shedding was detected in this CSL (PCR) at least 8 weeks after infection. To address the question of chronic carriage, we have assembled preliminary data on CSL that have recovered from leptospirosis: we detected leptospire shedding in a CSL treated for leptospirosis > 6 weeks after admission to The Marine Mammal Center, and Dierauf et al5 identified leptospires, 22 weeks after admission, in the urine of a stranded CSL that had recovered from, and had been treated for, leptospirosis. These results support our hypothesis that CSL can become chronic carriers, but further work is needed. Therefore, we are currently performing a prospective study to determine the prevalence of carrier animals in the CSL population. CSL will be categorized as asymptomatic based on serum chemistry values representative of normal renal function, and carriage will be assessed by three methods to maximize the sensitivity of detection, immunohistochemistry (kidneys) and PCR (urine and kidneys), which have relatively high sensitivity, and culture (urine and kidneys), which is less sensitive but will also yield isolates for strain typing. In showing that carriers exist in significant numbers, we will have identified a mechanism for persistence of Leptospira in the CSL population and clarified the risk to other species using the coastal environment. In addition, information on chronic asymptomatic carriage will allow zoos and aquaria to assess the risks of adopting CSL for their collections.
Acknowledgements
The authors wish to thank all of the volunteers and staff from both the Marine Mammal Center in Sausalito and the Marine Mammal Care Center in San Pedro who helped in sample collection from both stranded and wild-caught California sea lions. We would also like to thank the Prescott Grant Program, the Hellman Family Foundation, and the De Logi Chair in Biological Sciences for their financial support.
References
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