Abstract
Over 35 years of serologic, epidemiologic, and molecular data on California sea lions (Zalophus californianus) indicate that they are a reservoir for Leptospira interrogans serovar Pomona in the marine ecosystem.1-6 However, infections have also been detected in other marine mammals, such as northern elephant seals (Mirounga angustirostris), northern fur seals (Callorhinus ursinus), Pacific harbor seals (Phoca vitulina), Steller sea lions (Eumetopias jubatus), and sea otters (Enhydra lutris), but their epidemiological role is unknown. Distinguishing between maintenance and non-maintenance hosts of multi-host pathogens such as Leptospira is notoriously challenging.7-12 It requires either removal of the putative maintenance host or elimination of the pathogen in the putative maintenance host, both of which are either logistically impossible or ethically untenable. However, an unprecedented fadeout event, during which Leptospira ceased circulating in the California sea lion population for four years (2013 to mid-2017), presented a unique opportunity to assess the epidemiological role of other marine mammals in the marine Leptospira system.
We focused our initial investigation on northern elephant seals, as they were the first marine mammals documented to be affected by leptospirosis during the 2017 reemergence. Using microscopic agglutination testing (MAT) to detect serum antibody titers, we analyzed over 900 elephant seal samples, from three different age classes (0–12 months, 13–24 months, and 25+ months), sampled between 2008–2019. Seroprevalence estimates of the youngest age classes (0–24 months) showed no new exposures during the sea lion fadeout period (2013 to mid-2017). In the oldest age class (25+ months), seroprevalence estimates were lower during the sea lion fadeout period compared with the time periods before and after. In addition, analyses of the antibody titer magnitude and longitudinal changes of titers in individual elephant seals revealed that relatively high titers can be maintained for long periods in elephant seals, indicating that titers seen in the older animals during fadeout were due to historic infections. Combined, data from all age classes provide strong evidence that there were no new Leptospira infections in elephant seals during the sea lion fadeout period. These findings suggest that elephant seals are not a Leptospira reservoir and did not maintain the pathogen during the four-year period that Leptospira infections were absent in the sea lion population. However, the first post-fadeout infections detected in the marine ecosystem occurred in elephant seals in May 2017, indicating that they may have acted as a bridge host, linking an unknown reservoir to California sea lions.
Acknowledgements
The authors would like to thank our collaborators from The Marine Mammal Center, Sausalito, California. The authors would also like to thank the John H. Prescott Marine Mammal Rescue Assistance Grant Program for supporting this work.
*Presenting author
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