The most common tumours of the colon in the dog and cat are polyps, adenocarcinomas and lymphosarcomas, although other types do occur (). Factors which are thought to predispose to colonic and rectal neoplasia include low fibre diets, slow colonic transit times, high levels of bile and fat in the colon, and the presence of longstanding severe colitis ().
Polyps are usually benign and occur most frequently in the distal colon or rectum. They tend to be space-occupying causing local obstruction to the passage of faeces. Occasionally they are associated with subsequent development of adenocarcinomas.
Lymphosarcoma usually occurs as a diffuse tumour of the colon although focal lesions do occur. They rarely cause obstruction, but thickening of the colonic wall results in interference with motility and absorption.
Adenocarcinomas often appear as focal lesions which tend to be proliferative and lead to obstruction of the colon. Ulceration is common as is secondary infection. The consequence of infection and inflammation is fibrous tissue formation and ultimately stricture formation. The commonest site for adenocarcinoma is said to be the distal colon and rectum, with metastasis to the sublumbar lymph nodes ().
Clinical signs accompanying development of intestinal tumours are often vague, insidious and slowly progressing, although in a few cases sudden clinical onset is described (). Patients may have a history of diarrhoea or constipation depending on the type of tumour present, the stage of development, and location — whether diffuse, focal or obstructive. Adenocarcinomas are usually space-occupying so present with tenesmus, dyschezia, haematochezia and possibly diarrhoea. Lymphosarcomas are generally diffuse and involve the majority of the colon and cause chronic diarrhoea which rarely contains blood. Polyps are generally space-occupying, obstructive and result in tenesmus, passage of ribbon-like faeces, occasionally with haematochezia, especially when they ulcerate. Occasionally they cause apparent constipation and dyschezia.
With advanced colonic neoplasia, weight-loss and vomiting may occur together with the symptoms indicated above. Such changes may be due to’ the advanced colonic disease or to metastasis affecting the function of other tissues. Stricture formation may also occur where inflammation is severe and attempts at healing induce fibrosis.
Diagnosis is achieved by radiographic examination of the colon using barium or double contrast studies which may reveal the classic ‘apple core’ filling defect seen with adenocarcinomas (), or a diffuse abnormality of the colonic mucosa as seen with lymphosarcoma (). Where tumours have metastasized the sublumbar lymph nodes may be enlarged (). Unfortunately radiographs will not detect all colonic neoplasms and even when they do detect abnormalities, endoscopic examination with biopsy sampling is essential to confirm the diagnosis and offer a prognosis.
Endoscopy reveals ulceration, general thickening of the mucosa, or space-occupving lesions. Multiple biopsy samples including apparently normal and abnormal tissues should be taken. This ensures a representative assessment of the colon and any tumour present and allows a decision to be made-regarding the most effective method of treatment.
Where adenocarcinoma is diagnosed in the early stages it may be successfully removed by resection of the affected part of the colon. However, if the rectal tissues are involved, this becomes a difficult proposition and attempts at resection may lead to faecal incontinence. Where the adenocarcinoma is well-established and metastasis has occurred to the sublumbar lymph nodes and other tissues, the prognosis is very guarded and chemotherapy in addition to surgery should be instituted.
Polyps are usually operable and even when they occur in the rectum they may be removed using proctoscopy and diathermy. It is important to submit the polyp for histopathological analysis as some may prove to be early carcinomas.
Lymphosarcoma is rarely responsive to therapy especially as it tends to be diffuse and involve the majority of the colon. Cats with lymphosarcoma should be checked for evidence of feline leukaemia virus (FeLV) a positive result indicates a very grave prognosis.
Selections from the book: “Digestive Disease in the Dog and Cat” (1991)