Mast cell tumors are an uncommon occurrence in horses . They usually occur as benign, solitary masses on the skin of the head, neck, trunk, and legs. Mineralization of the tumor is common.  In pigs and cattle , mast cell tumors are rare. They tend to be solitary and benign in pigs and multiple and malignant in cattle.  Mast cell tumors are found in the skin of cattle most commonly, but these may be metastases from tumors of the viscera .  Other sites in cattle include the spleen, muscle, gastrointestinal tract, omentum , and uterus. 
Histamine from enterochromaffin-like cells may well be the primary modulator, but the magnitude of the stimulus appears to result from a complex additive or multiplicative interaction of signals of each type. For example, the low amounts of histamine released constantly from mast cells in the gastric mucosa only weakly stimulate acid secretion, and similarly for low levels of gastrin or acetylcholine. However, when low levels of each are present, acid secretion is strongly forced. Additionally, pharmacologic antagonists of each of these molecules can block acid secretion.
RESULTS : The cohort ( n = 48), mean age years (SD = ), % females, 90% Caucasian, was comprised of a non-inflammatory ( n = 26) and an inflammatory ( n = 22) phenotype. There was a significant negative correlation between substance P expression and mast cell count ( P = , r = -). Substance P was found to be expressed more often in female patient biopsies and more intensely in the upper gastrointestinal mucosa as compared to the lower mucosa. There were significantly increased gastrointestinal mucosal immunoreactivity to IL-6 ( P = ) in the inflammatory phenotype compared to non-inflammatory. Additionally, we found significantly increased mast cells ( P = ) in the mucosa of the non-inflammatory phenotype compared to the inflammatory group. This difference was particularly noted in the lower colon biopsies.