Abstract
Brucella pinnipedialis was first isolated from true seals in 1994 and from eared seals in 2008. Although few pathological findings have been associated with infection in true seals, reproductive pathology including abortions, and the isolation of the zoonotic strain type 27 have been documented in eared seals. In this study, a Brucella enzyme-linked immunosorbent assay (ELISA) and the Rose Bengal test (RBT) were initially compared for 206 serum samples and a discrepancy between the tests was found. Following removal of lipids from the serum samples, ELISA results were unaltered while the agreement between the tests was improved, indicating that serum lipids affected the initial RBT outcome. For the remaining screening, we used ELISA to investigate the presence of Brucella antibodies in sera of 231 eared and 1,412 true seals from Alaskan waters sampled between 1975 and 2011. In eared seals, Brucella antibodies were found in two Steller sea lions (Eumetopias jubatus) (2%) and none of the 107 Northern fur seals (Callorhinus ursinus). The low seroprevalence in eared seals indicate a low level of exposure or lack of susceptibility to infection. Alternatively, mortality due to the Brucella infection may remove seropositive animals from the population. Brucella antibodies were detected in all true seal species investigated; harbor seals (Phoca vitulina) (25%), spotted seals (Phoca largha) (19%), ribbon seals (Histriophoca fasciata) (16%), and ringed seals (Pusa hispida hispida) (14%). There was a low seroprevalence among pups, a higher seroprevalence among juveniles, and a subsequent decreasing probability of seropositivity with age in harbor seals. Similar patterns were present for the other true seal species; however, solid conclusions could not be made due to sample size. This pattern is in accordance with previous reports on B. pinnipedialis infections in true seals and may suggest environmental exposure to B. pinnipedialis at the juvenile stage, with a following clearance of infection. Furthermore, analyses by region showed minor differences in the probability of being seropositive for harbor seals from different regions regardless of the local seal population trend, signifying that the Brucella infection may not cause significant mortality in these populations. In conclusion, the Brucella infection pattern is very different for eared and true seals.
Highlights
Alaskan waters accommodate a number of pinniped species, both true seals including Eastern North Pacific harbor seals (Phoca vitulina richardsii), spotted seals (Phoca largha), ribbon seals (Histriophoca fasciata), Arctic ringed seals (Pusa hispida hispida), and bearded seals (Erignathus barbatus), as well as eared seals including Steller sea lions (Eumetopias jubatus) and Northern fur seals (Callorhinus ursinus) [1]
Brucella antibodies were detected in 276/1,122 harbor seals (24.6%), 16/85 spotted seals (18.8%), 9/55 ribbon seals (16.4%), and 21/150 ringed seals (14.0%)
Past evaluations of Brucella infection status in marine mammals have likely been inaccurate if the results were based solely on agglutination tests
Summary
Alaskan waters accommodate a number of pinniped species, both true seals (family Phocidae) including Eastern North Pacific harbor seals (Phoca vitulina richardsii), spotted seals (Phoca largha), ribbon seals (Histriophoca fasciata), Arctic ringed seals (Pusa hispida hispida), and bearded seals (Erignathus barbatus), as well as eared seals (family Otariidae) including Steller sea lions (Eumetopias jubatus) and Northern fur seals (Callorhinus ursinus) [1]. Bearded seals of the Bering Sea distinct population segment, and Steller sea lions in the western distinct population segment have been listed as threatened (“likely to become an endangered species within the foreseeable future”) under the Endangered Species Act (ESA). Listings for seals were based on their predicted negative responses to climate change, while for sea lions and fur seals it is due to population declines for unknown reasons. The International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species considers several of these species as “Data deficient” (“inadequate information to make a direct, or indirect, assessment of its risk of extinction based on its distribution and/or population status”) [3]. Health and disease status of these animal populations are of prime importance for the purpose of management and conservation
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