Neoplasia, Abdominal
Basic Information 
Clinical Presentation
History, Chief Complaint
Typical signs associated with abdominal neoplasia include:
Physical Exam Findings
• The physical examination findings confirm the presenting signs, which are weight loss with or without signs of abdominal pain. Persistent or intermittent pyrexia may occur with advanced neoplasia.
• Physical examination of the horse is often unrewarding but is essential, together with a thorough history, to eliminate other more common causes of weight loss such as:
Etiology and Pathophysiology
• The most common neoplasm of the abdominal cavity is mesenteric lipoma, which is common in older horses and especially ponies (>15 years). However, it is often clinically insignificant. When significant, the lipoma may produce signs only related to its physical properties, namely as a space-occupying mass causing a simple intestinal obstruction (caused by compression of the intestine), or more commonly as a pedunculated mass causing an acute strangulating intestinal obstruction.
• Other primary tumors originating in the peritoneal cavity are rare. Mesothelioma is a rare tumor of the mesothelial surfaces. It is most commonly reported in the thoracic cavity, although there have been a few reported cases in which the tumor appeared to develop within the peritoneal cavity.
• There have been isolated reports of omental leiomyoma and mesenteric myofibroblastoma in horses.
• Many other tumors may metastasize to the abdominal cavity. These include lymphoma (lymphosarcoma), squamous cell carcinoma, leiomyoma or leiomyosarcoma (gastrointestinal stromal tumors), adenocarcinoma, melanoma, testicular seminoma, teratoma, transitional cell carcinoma, hepatoblastoma, adrenocortical carcinoma, metastatic granulosa cell tumor, myxosarcoma, and hemangiosarcoma.
• Pheochromocytoma has been associated with acute colic in a small number of horses.
Diagnosis 
Initial Database
• The diagnostic evaluation should consist of a complete physical examination, including rectal examination, routine bloodwork (complete blood count, serum chemistry panel), urinalysis, and peritoneal fluid analysis.
• Many horses with abdominal neoplasia have anemia (as a result of chronic disease or blood loss), leukocytosis (as a result of chronic inflammation), and hyperfibrinogenemia. In a small number of cases of lymphoma, abnormal lymphocytes may be present in the peripheral blood.
• Immune-mediated anemia or thrombocytopenia may occur secondary to neoplasia, especially lymphoma.
• Some affected horses have hypoalbuminemia and hypoproteinemia caused by malabsorption, bowel inflammation, and protein exudation, but other horses have hyperglobulinemia (as a result of chronic inflammation).
• Increased concentrations of the intestinal fraction of the alkaline phosphatase enzyme (IAP) may also indicate the presence of intestinal disease.
• Hypercalcemia has been reported in association with both lymphoma and gastric carcinoma.
• Rectal examination is essential in the investigation of any horse with suspected abdominal neoplasia. Although normal findings may be present in many animals, an increased volume of peritoneal fluid, distension of the intestine, or an abnormal tissue mass or masses increase the index of suspicion of abdominal neoplasia and allow further directed investigations to be selected.
• Peritoneal fluid varies from normal to an exudate. Neoplastic cells from a primary abdominal neoplasm (eg, mesothelioma or lymphoma) are occasionally observed in a sample of peritoneal fluid. In one study comparing the diagnostic features of cases of abdominal neoplasia and abdominal abscesses, peritoneal fluid was classified as an exudate in 12 of 15 horses with intraabdominal abscesses and 14 of 25 horses with intraabdominal neoplasms. Cytologic examination of peritoneal fluid yielded an accurate diagnosis in 11 of 25 horses with neoplasia and in three of 15 horses with abscesses. A mean number of 1.45 cytologic analyses per horse was needed to diagnose neoplasms in the 11 horses in which the analysis was successful in definitively diagnosing the condition.
• In some cases, peritoneal fluid is serosanguineous or frankly hemorrhagic.
• In cases of malignant melanoma of the peritoneal cavity, the peritoneal fluid may be discolored black.
• Neoplastic cells may also be identified in samples of pleural fluid if a pleural effusion is present.
• Pheochromocytoma should be considered in older horses with signs of abdominal pain, sweating, muscle fasciculations, ataxia, azotemia, and intraperitoneal hemorrhage. Identification by per rectum palpation of retroperitoneal swelling in the dorsal aspect of the abdomen also should alert the diagnostician to the possibility of a ruptured pheochromocytoma.
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