Polyphasic Rhabdomyositis in California Sea Lions (Zalophus Californianus): Pathology and Potential Causes

Abstract

A myositis syndrome has been recognized for more than a decade in California sea lions (CSLs; Zalophus californianus) but a detailed description of the lesions and potential causes of this condition is lacking. The tissues of 136 stranded CSLs with rhabdomyositis were examined. Rhabdomyositis was considered incidental in 67% (91/136) of the CSLs, and a factor contributing to the animal stranding (significant rhabdomyositis) in 33% (45/136). Of the 91 cases with incidental rhabdomyositis, lesions consisted of a few small foci of lymphohistiocytic inflammation. Of the 45 cases with significant rhabdomyositis, 28 (62%) also presented with major comorbidities such as leptospirosis (2 animals) and domoic acid toxicosis (6 animals), whereas 17 (38%) had severe polyphasic rhabdomyositis as the only major disease process associated with mortality. In these animals, most striated muscles had multiple white streaks and diffuse atrophy. Microscopically, there was myofiber necrosis surrounded by lymphocytes and histiocytes admixed with areas of myofiber regeneration, and/or moderate to severe rhabdomyocyte atrophy usually adjacent to intact Sarcocystis neurona cysts. At the interface of affected and normal muscle, occasional T lymphocytes infiltrated the sarcoplasm of intact myocytes, and occasional myofibers expressed MHCII proteins in the sarcoplasm. S. neurona antibody titers and cyst burden were higher in animals with significant polymyositis antibody titers of (26125 ± 2164, 4.5 ± 1.2 cysts per section) and active myonecrosis than animals with incidental rhabdomyositis antibody titers of (7612 ± 1042, 1.7 ± 0.82 cysts per section). The presented findings suggest that S. neurona infection and immune-mediated mechanisms could be associated with significant polyphasic rhabdomyositis in CSLs.

Clinical signs, treatment, and outcome for California sea lions (Zalophus californianus) with Sarcocystis-associated polyphasic rhabdomyositis

“…1 In the cases described in the present report, dysphagia appeared to be secondary to pronounced inflammation, necrosis, and atrophy of the laryngeal, esophageal, and masticatory muscles. 19 Clinically important respiratory compromise was regularly observed in the animals described in the present report. Concurrent verminous and bacterial bronchopneumonia has been documented previously in CSLs with rhabdomyositis.…”

“…Concurrent verminous and bacterial bronchopneumonia has been documented previously in CSLs with rhabdomyositis. 19 Compromise of the diaphragm and intercostal muscles likely reduced the ability to cough, contributing to the accumulation of exudate or development of aspiration pneumonia in some cases. Dysphagia and megaesophagus also likely increased the risk of aspiration.…”

“…Histologic examinations were performed on tissues from 23 of the deceased animals, and lesions were consistent with those previously described for polyphasic rhabdomyositis in CSLs. 19 Mild meningoencephalitis consistent with sarcocystosis was observed in 5 animals, and meningitis was observed in 2. Immunohistochemical staining to identify protozoa in the brain was not performed.…”

“…Sarcocystosis is also a documented cause of hepatic disease and myopathies in multiple species, 5,[12][13][14][15][16] including marine mammals, [13][14][15][16][17][18][19][20] but these are reported less frequently than the neurologic manifestations associated with S neurona. In addition, although Sarcocystis-associated myositis has been recognized in California sea lions (CSLs; Zalophus californianus) for at least a decade, 17,19,20 there is little published information on clinical signs and treatment strategies. 17,20 Polyphasic rhabdomyositis 19 associated with Sarcocystis spp was observed in stranded CSLs treated between 2015 and 2017 at a rehabilitation center in California.…”

“…In addition, although Sarcocystis-associated myositis has been recognized in California sea lions (CSLs; Zalophus californianus) for at least a decade, 17,19,20 there is little published information on clinical signs and treatment strategies. 17,20 Polyphasic rhabdomyositis 19 associated with Sarcocystis spp was observed in stranded CSLs treated between 2015 and 2017 at a rehabilitation center in California. The purpose of the study reported here was to describe clinical signs, treatment, and outcome for CSLs with Sarcocystis-associated polyphasic rhabdomyositis.…”