Abstract

Mast cell tumors are readily reported in veterinary medicine.^1,2 Grading criteria are described for canine cutaneous mast cell tumors (cMCT), however, clear histopathologic criteria are lacking in other species.^3,4 In marine mammals, only two cases of mast cell tumors are reported in the literature to date - a Pacific walrus (Odobenus rosmarus divergens) with pulmonary mast cell neoplasia diagnosed histologically and considered an incidental finding, and a California sea lion (Zalophus californianus) with cMCT excised from the external thoracic body wall.^5,6

Here, we report on cMCTs excised from two asymptomatic, geriatric California sea lions (Zalophus californianus). An 18-year-old, neutered male presenting with a small, smooth pedunculated anal mass found on routine physical examination and a 20-year-old, neutered male noted to have an eyelid mass during a lensectomy procedure. Both masses were excised and submitted for evaluation. Anal mass impressions were cytologically consistent with an intermediately differentiated mast cell tumor. Histopathologic examination of the surgically excised masses identified variably granulated mast cell tumors with 2 and 1 mitoses in 2.37 mm² (ten 400x fields), respectively, and no evidence of vascular invasion. Neoplastic round cells contained fine, metachromatic granules as highlighted by Giemsa staining and were immunopositive for cKIT (CD117). Monitoring for recurrence and sampling of new cutaneous masses is ongoing in both cases.

To investigate species-specific negative prognostic indicators, characterize the biologic behavior of cMCT, and develop potential grading criteria in pinnipeds, further biopsies are warranted. This will allow for informed, effective treatment plans and monitoring protocols moving forward.

Acknowledgments

The authors sincerely thank all of the trainers, veterinarians, and technicians of the National Marine Mammal Foundation, U.S. Army Veterinary Corps, and U.S. Navy Marine Mammal Program for their exceptional animal care. A special thank you to Dr. Carlos Aragon for the surgical consultation, and Melinda Martens, Katie Hodges, Veronica Cendejas, and Celeste Parry for their assistance in data gathering and archival queries.

Literature Cited

1.  Kiupel M. 2017. Mast Cell Tumors. In: Meuten DJ, editor. Tumors in domestic animals, Fifth Edition. Hoboken (NY): John Wiley & Sons. p 176–202.

2.  Tamlin VS, Bottema CDK, Peaston AE. 2020. Comparative aspects of mast cell neoplasia in animals and the role of KIT in prognosis and treatment. Vet Med Sci. 6:3–18.

3.  Patnaik AK, Ehler WJ, MacEwen EG. 1984. Canine cutaneous mast cell tumor: morphologic grading and survival time in 83 dogs. Vet Pathol 21: 469–474.

4.  Kiupel M, et al. 2011. Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior. Vet Pathol 48: 147–155.

5.  Staggs L, Henderson R, Labelle P. 2016. Mast Cell Tumor Detection and Treatment in a California Sea Lion (Zalophus californianus). IAAAM 47th Annual Conference Proceedings, Virginia Beach, VA.

6.  Seguel M, Stimmelmayr R, Howerth E, Gottdenker N. 2016. Pulmonary Mast Cell Tumor and Possible Paraganglioma in a Free-ranging Pacific Walrus (Odobenus rosmarus divergens), Barrow, Alaska, USA. J Wildl Dis. 52:407–410.