While not common in terms of the frequency with which they are seen in small animal practice, primary intestinal neoplasia
is an important differential diagnosis for a dog or cat with vomiting, especially chronic vomiting, chronic diarrhea, anorexia
and weight loss, particularly animals that are middle-aged and older. Some intestinal neoplasms have been associated with
paraneoplastic syndromes such as hypoglycemia and polyuria/polydipsia, which could be additional clinical complaints.
Etiology and pathophysiology
The most common intestinal tumor in cats is lymphosarcoma; in dogs, intestinal lymphosarcoma intestinal adenocarcinoma and
intestinal smooth muscle tumors are seen with around the same frequency although lymphosarcoma is reported a bit more often
than the other tumor types. Other tumor types that have been described in the bowel include leiomyomas, leiomyosarcomas;
gastrointestinal stromal tumors, plasma cell tumors, mast cell tumors, carcinoids (tumors of neuroendocrine origin) and extraskeletal
osteosarcomas. Benign polyps can be found in the duodenum of cats or the rectum of dogs. Gastrointestinal stromal tumors
(GIST) have been recently described as tumors that arise from the interstitial cells of Cajal, cells that regulate intestinal
motility and peristalsis. GIST have histological features that are very similar to intestinal smooth muscle tumors, but are
distinguished by positive staining for c-kit (CD117). The paraneoplastic syndromes thus described in the older literature
in association with leiomyomas/leiomyosarcomas have the potential to be a result of GIST. In the report that characterizes
GIST, there was a predilection for this tumor in dogs to be seen in the cecum and large intestine whereas the smooth muscle
tumors were more likely to be seen in the stomach and small intestine. In this report, only the dogs with GIST had hypoglycemia.
Intestinal adenocarcinomas in dogs are seen more often in the large intestine than the small intestine.
Animals with intestinal tumors are most commonly middle-aged to older. Common metastatic sites of non-lymphoid intestinal
tumors include regional lymph nodes, liver, spleen, omentum/peritoneal cavity, and lung.
Intestinal tumors can disrupt normal gastrointestinal motility likely as a consequence of disruption of the intestinal smooth
muscle wall and/or accompanying inflammatory changes, or cause obstructive disease. Ulceration of the intestinal mucosa is
a feature of intestinal tumors in some patients, but will not be present in all. Patients with intestinal bleeding secondary
to mucosal ulceration, chronic blood loss can lead to iron deficiency anemia. Iron deficiency anemia can be either regenerative
Patients with primary intestinal neoplasms most commonly present with a history of anorexia, weight loss, diarrhea and vomiting;
melena may be seen in some and if the tumor is located in the large bowel, there may be hematochezia, tenesmus and ribbon-like
stools, or frank bleeding from the anus. Animals, particularly dogs, may exhibit features of both large and small bowel diarrhea
when tumors are located in the cecum or ileocecocolic region. As noted above, some patients with intestinal tumors may exhibit
polyuria/polydipsia and signs of hypoglycemia (tremors, seizures).
The physical examination in patients is variable. Poor body condition will be appreciated in animals with weight loss, and
an abdominal mass may be palpable in some patients. Cats with intestinal lymphoma commonly have enlarged mesenteric lymph
nodes appreciated during abdominal palpation. Abdominal distension and a fluid wave are possible findings in animals with
hypoalbuminemia or peritoneal carcinomatosis. Animals that have developed anemia secondary to neoplasm-associated intestinal
ulceration may have pale mucous membranes. During rectal examination, rectal masses may be palpable; in the author's experience,
some of these masses are very soft and friable and can be easily mistaken for a fold of mucosa, so a hurried rectal examination
is not advised.
Results of routine laboratory testing (CBC, biochemical profile, urinalysis, fecal flotation) helps exclude non-gastrointestinal
origins of clinical signs such as hepatic, renal or adrenal gland disease, although results are not always helpful and can,
in some cases, be confusing (e.g. pre-renal azotemia and an increase in BUN disproportionate to the serum creatinine, which
can reflect gastrointestinal bleeding). Hypoglycemia is a feature of occasional patients with intestinal tumors.