Intestinal neoplasia is rare in horses. Lymphoma is the most common form, other intestinal neoplasms include adenocarcinoma, leiomyoma, leiomyosarcoma or gastrointestinal stromal tumors (GIST). Clinical signs are non-specific and include weight loss, colic or diarrhea. The aim of this study was to evaluate tissue samples from 25 horses with intestinal masses. We collected tissue material from 25 horses with masses in the small or large intestine between 2011 and 2017. Included were only samples directly connected to the intestine which were submitted for routine diagnostics. Clinical history was included. Tissue samples were formalin-fixed and embedded using standard methods. Histology and immunohistology (Vimentin, SMA, Desmin, S100, c-Kit, CD20, CD3, FVIII, CD31, cytokeratin) were performed. In 20 cases the clinical history was known. The horses' age ranged from 2-22 years (median 13 years. Patients showed poor body condition, weight loss or cachexia (26%). Six patients were presented with colic. Two had diarrhea or fever. The most common tumors were mesenchymal neoplasias. Four leiomyomas and three leiomyosarcomas were identified. One tumor was characterized as GIST because of c-Kit-positive immunohistochemistry. Lymphoma was found in seven of 25 cases, five of which involved the small intestine, one the large intestine and one both the small and large intestine. Six lymphomas were small-cell type and one intermediate. Five cases were characterized as T-cell lymphomas, two as B-cell lymphomas with immunohistochemistry. In one B-cell lymphoma Congored-positive amorphous eosinophilic extracellular material consistent with amyloid was found. Adenocarcinomas were found in 5/25 cases. Two cases involved the small intestine and three cases the large intestine. Three of five cases showed osseous metaplasia. In three horses benign vessel proliferation (two Angiomatosis, one Lymphangioma) was diagnosed. In two cases chronic inflammation or hematoma in organization was diagnosed. Intestinal neoplasia in horses is rare, there is only one American study and few case reports to our knowledge. A breed predisposition could not be found. No sex disposition was identified. Clinical signs were non-specific, this has also been reported by other authors. A differentiation between benign and malignant changes by history is not possible. Nevertheless, the horses were euthanized before histological examination. Spindle cell tumors were the most common intestinal neoplasm found in this study, in other studies the number of spindle cell tumors was smaller with lymphomas being the most common tumors. In this study more T-cell than B-cell lymphomas were observed. As previously described, small cell type is the most common type of lymphoma. One horse had B-cell lymphoma with amyloid-production which has not yet been described in equine intestinal neoplasia. Rectal biopsies can be helpful in the diagnosis of lymphoma, in most of the cases correlation with immunohistochemistry and clinical signs is necessary for diagnosis. 5/7 adenocarcinomas showed osseous metaplasia which seems to be more frequent in horses than in other species. Vessel invasion was rare (1/5). Intestinal adenocarcinomas metastasize less frequently than in other species. In two animals a reactive process was found. A distinction between tumor and tumor-like lesion was not possible neither clinically nor macroscopically. Histological investigation is necessary to distinguish between reactive lesions and neoplasia and also between benign and malignant tumors. Even in malignant tumors like leiomyosarcomas or adenocarcinoma complete excision can be curative and is associated with longer survival time post surgery.
