Abstract

On 4 July 2000, an adult female Steller sea lion, Eumatopiasjubatus, stranded and died on Dillon beach, Sonoma County, California, and was presented for necropsy to the Marine Mammal Center, Sausalito, California, three hours post mortem. On gross examination the animal was emaciated (BCS 2/7) and had nasopharyngeal mites, gastric nematodes, severe hepatic trematodiasis, segmentally severe enteritis, unilateral hydroureter and small, fibrous nodules on gastric and uterine serosa, aortic adventia and renal capsule. Within the right latissimus dorsi muscle was a single, well-demarcated, centrally necrotic, pale yellow, 10.0 x 10.0 x 4.0cm mass, and within the left cranial lung lobe were two firm, white, spherical 1.0 cm and 1.2 cm in diameter subpleural nodules.

Histologically, the unencapsulated, partially demarcated, centrally necrotic, highly cellular muscle mass consisted of loosely arranged, pleomorphic, spindle-shaped or stellate cells occasionally arranged in short or long streaming bundles. Uninucleated neoplastic cells were mixed with multinucleated giant cells, and anisocytosis and anisokaryosis was marked but mitosis few. Pulmonary masses were similar and accompanied by edema and subacute suppurative bronchopneumonia.

The pleomorphism of neoplastic cells and abundance of multinucleate cells in the muscle and pulmonary masses are highly suggestive of rhabdomyosarcoma. These neoplasms often are markedly anaplastic and characteristic cross striations absent. Immunohistochemically, neoplastic cells at both sites were reactive for vimentin, but negative for skeletal myosin, smooth muscle actin, desmin, Factor VIII and pancytokeratin. Normal adjacent muscle stained inconsistently with antibodies specific for myosin and desmin of canine muscle, which indicates incomplete cross-reactivity with otarid skeletal muscle. Differential diagnoses for this neoplasm include leiomyosarcoma, giant cell sarcoma of soft tissues and fibro sarcoma. Further characterization is being pursued using electron microscopy.

Death in flee-ranging pinnipeds commonly is caused by pneumonia, heavy parasite burdens and debilitation.^2,7 In this older animal, these conditions in conjunction with enteritis were likely contributory to mortality and the neoplasm incidental. Over the last two decades, there has been a dramatic decline in the western U.S. Steller sea lion population due to ill-understood causes, but hypothesized to be consistent with a long-term environmental change or novel catastrophe.^8,10 Post mortem data are rare because of the difficulty in recovering carcasses fresh enough to yield beneficial information.

Previous reports of neoplasia in this species are limited to metastatic pulmonary adenocarcinoma⁹, palpebral neoplasia, probable fibroma (Spraker, pers. comm.), and uterine fibroleiomyoma.⁷ However, with the documented increasing incidence of neoplasia in marine mammals^1,2,5,6, it is important to consider and identify predisposing and causative factors, such as infectious agents and biomagnification of exogenous pollutants in top predators predisposing or causing systemic alterations and diseases, such as immunocompromise and neoplasia.

As top predators ourselves, diseases manifested in marine fauna may reflect those to anticipate in the human population and present concerns for sustainable use alternatives of marine populations.

References

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