PRESUMED PRIMARY MEDIASTINAL NEOPLASMS OF SOUTHERN SEA OTTERS (ENHYDRA LUTRIS NEREIS) FROM CALIFORNIA, USA

Lymphoma and thymoma are the most common primary mediastinal neoplasms in human and veterinary patients. Although lymphoma has been widely reported in mustelids and marine mammals, thymoma and other primary mediastinal neoplasms have rarely been reported, especially in free-ranging animals. To date, thymoma has been described in only a few captive mustelids and marine mammals, including domestic ferrets (Mustela furo), an African spotted-necked otter (Lutra maculicollis), and a polar bear (Ursus maritimus); there have been no published reports from free-ranging mustelids or marine mammals. This study describes thymomas (n=2) and a putative primary mediastinal sarcoma (n=1) in free-ranging southern sea otters (Enhydra lutris nereis) that stranded along the central California, USA, coast from 2006 through 2022. Mediastinal neoplasms were the primary cause of death for two of the three cases. In one case, a large, poorly differentiated soft tissue sarcoma obliterated the cranial mediastinum, compressed the heart and lungs, and was associated with severe pleural effusion. The thymomas ranged from small and incidental to large with moderate tissue compression. Other lesions that could have contributed to antemortem morbidity or the demise of these animals included emaciation and shark bite. Although thymomas can be associated with development of myasthenia gravis in humans and domestic animals, available serum from one sea otter was negative for anti-acetylcholine receptor antibodies. However, given our limited sample size and the fact that both sea otters with thymomas were found dead, with unknown prior clinical signs, this association should be investigated for future cases. To our knowledge, these are the first cases of thymoma and primary mediastinal sarcoma reported from southern sea otters, free-ranging mustelids, or marine mammals.