Ectopic splenic tissue in 46 dogs, 2000-2024.

Ectopic splenic tissue (accessory spleen or splenosis) occurs as dark-red-to-brown or purple nodules outside the spleen. Accessory spleens are congenital lesions histologically identical to a normal spleen. Splenosis results from implantation of splenic tissue following splenic rupture and lacks features of normal spleen. However, these distinctions have been largely applied to human cases, and the terms are often used interchangeably in domestic animals. Here we describe ectopic splenic tissue in 46 canine surgical biopsy specimens examined at the Athens Veterinary Diagnostic Laboratory, 2000-2024. The omentum (39 cases) and mesentery (5) were the most commonly affected sites. Original diagnoses were accessory spleen (28 cases), splenosis (14), accessory spleen or splenosis (2), and ectopic splenic tissue and normal splenic tissue (1 each). Updated diagnoses, modified after histologic assessment for a fibrous capsule, smooth muscle trabeculae, and white and red pulp, were accessory spleen (37 cases) and splenosis (9). Concurrent splenic lesions were reported in 12 cases in which accessory spleens were diagnosed and only 2 splenosis cases, confirming that the histologic diagnosis of accessory spleen and splenosis is not always correlated with the clinical history and gross findings (no splenic lesions vs. splenic lesions with rupture). For that reason, ectopic splenic tissue may be a more inclusive and better term for these lesions. Hemangiosarcoma was diagnosed in the spleen in 4 of the 12 cases with splenic masses, which underscores the importance of the differentiation between ectopic splenic tissue and hemangiosarcoma.