Meegan Larsen

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2024;28(4):24.

A ten-year-old spayed female border collie dog presented to the referring veterinarian for rectal prolapse. After limited response to antibiotic and topical osmotic treatment, punch biopsies were taken from a palpably thickened and multifocally ulcerated area of mucosa near the mucocutaneous junction of the anus, and were submitted to the AHL for histopathology. Microscopically, the mucosa and submucosa were effaced by a round cell neoplasm that abutted and sometimes invaded into the mucosal epithelium (Fig. 1). Lymphoma was suspected based on the tumour morphology and the location of the neoplasm. Positive staining of the neoplastic round cells for PAX5, a B lymphocyte marker, confirmed the diagnosis of lymphoma and identified the neoplasm as a B-cell lymphoma.

Prolonged tenesmus is a common cause of rectal prolapse in dogs and cats and is most frequently seen in animals with high burdens of intestinal parasites. It can also occur in association with intestinal neoplasia, as in this case. Other underlying causes include intestinal foreign bodies, dystocia, urolithiasis, constipation, congenital defects and prostatic disease. In a small retrospective study of 11 dogs with rectal lymphoma, tenesmus was a common presenting complaint. All cases in this study were identified as B-cell in origin, and the prognosis was favourable with prolonged survival in dogs receiving chemotherapy.

Definitive histologic diagnosis of neoplasms can sometimes be difficult with hematoxylin and eosin staining alone, even with provision of relevant clinical histories and good gross descriptions. Immunohistochemical (IHC) staining uses antibodies to stain proteins expressed by neoplastic cells, and can be an invaluable tool for cell identification and tumour diagnosis. In cases of lymphoma, IHC is required for identification of B or T cell origin; it is not possible to differentiate between B and T lymphocytes with hematoxylin and eosin staining. Immunohistochemical staining for at least two proteins, a B cell protein and a T cell protein, is necessary and, in some cases, additional B cell markers are needed. In this case, the neoplastic cells expressed PAX5, the B cell protein, but not CD3, the T cell protein, providing important information for treatment (Fig. 2).

Figure 1. The neoplasm is composed of sheets of round cells with numerous mitotic figures (arrows). H&E stain

Figure 2. Most neoplastic round cells have positive nuclear PAX5 staining (arrow, left image).

Only a few scattered infiltrating non-neoplastic T lymphocytes have positive cytoplasmic CD3

staining (arrow, right image).

Reference

1. Murphy S, et al. Rectal lymphoma in 11 dogs: A retrospective study. J Small Anim Pract 2012;53:586-591.