A 10-year-old spayed female Boxer-mix was presented with a history of several weeks of soft stools, straining to defecate, inappetance, and lethargy and several days of hematochezia, melena, and dyschezia. Physical examination findings included mild tachycardia and tense cranial abdomen. CBC results indicated moderate mature neutrophilia. Ultrasonographic examination of the abdomen revealed a large mass with complex echogenicity in the cranial abdomen, likely associated with the intestines. Cytologic examination of a fine-needle aspirate revealed a population of round, stellate, and spindle-shaped cells arranged individually and in aggregates with occasional cells embedded in an eosinophilic extracellular matrix. The cytologic interpretation was malignant mesenchymal neoplasm with osteosarcoma being the primary differential. Surgical exploration of the abdomen revealed a 10-cm-diameter mass located at the intestinal mesenteric root. The mass occluded blood flow to portions of the gastrointestinal tract. The dog was euthanized due to the nonresectable nature of the tumor. Histopathologic examination revealed an expansile poorly demarcated mesenchymal neoplasm composed predominantly of spindloid and pyriform cells, occasionally embedded in a matrix compatible with osteoid. The diagnosis was extraskeletal osteosarcoma of the intestinal mesenteric root, only rarely reported in dogs.

Rabbits have served as a valuable animal model for the pathogenesis of various human diseases, including those related to agents that gain entry through the gastrointestinal tract such as human T cell leukemia virus type 1. However, limited information is available regarding the spatial distribution and phenotypic characterization of major rabbit leukocyte populations in mucosa-associated lymphoid tissues. Herein, we describe the spatial distribution and phenotypic characterization of leukocytes from gut-associated lymphoid tissues (GALT) from 12-week-old New Zealand White rabbits. Our data indicate that rabbits have similar distribution of leukocyte subsets as humans, both in the GALT inductive and effector sites and in mesenteric lymph nodes, spleen, and peripheral blood. GALT inductive sites, including appendix, cecal tonsil, Peyer's patches, and ileocecal plaque, had variable B cell/T cell ratios (ranging from 4.0 to 0.8) with a predominance of CD4 T cells within the T cell population in all four tissues. Intraepithelial and lamina propria compartments contained mostly T cells, with CD4 T cells predominating in the lamina propria compartment and CD8 T cells predominating in the intraepithelial compartment. Mesenteric lymph node, peripheral blood, and splenic samples contained approximately equal percentages of B cells and T cells, with a high proportion of CD4 T cells compared with CD8 T cells. Collectively, our data indicate that New Zealand White rabbits are comparable with humans throughout their GALT and support future studies that use the rabbit model to study human gut-associated disease or infectious agents that gain entry by the oral route.

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