Esophagus and Stomach

The esophagus and nonglandular anterior region of the stomach are lined by stratified squamous epithelium (Fig. 6-1). A keratinized thick outer stratum corneum overlies the deeper stratum granularis (flattened cells with shrunken nuclei and large basophilic staining cytoplasmic keratohyaline granules), stratum spinosum (flattened cuboid cells), and stratum basale (columnar cells).¹ Superficial epithelial cells are large cells with abundant pale blue homogeneous cytoplasm, angular cytoplasmic borders, and sometimes small, dense, round to oval nuclei. Bacteria can adhere to the surfaces of these epithelial cells, but inflammatory cells are lacking. Deeper samples may contain some basal epithelial cells, which are round with darker blue, slightly granular cytoplasm, larger ovoid, nuclei, and a higher nucleus to cytoplasm (N:C) ratio.

Fig. 6-1 Squamous epithelial cells scraped from stomach of horse.
Bacterial flora are seen on surfaces of some cells. (Wright-Giemsa stain)

Grossly, the margo plicatus forms a distinct boundary between the white-pink squamous region and the reddish glandular region of the equine stomach. The equine glandular stomach is lined by tall, periodic acid–Schiff–positive columnar epithelial cells that overlie deeper chief cells, parietal cells, mucous neck cells, and rare enteric endocrine cells.¹ Cytologic preparations from the glandular region of the stomach contain clusters and sheets of a uniform population of columnar epithelial cells. On low magnification the cells may have a honeycomb appearance (Fig. 6-2). Individualized cells exhibit a columnar shape with basal, round to oval nuclei, stippled chromatin pattern, and pale blue granular cytoplasm. Surface microvilli may give the apical margin a feathery appearance (Fig. 6-3).

Fig. 6-2 Sheet of columnar epithelial cells from pyloric portion of stomach.
The uniform size and shape of the cells give this cluster a honeycomb appearance. (Wright’s stain)

Fig. 6-3 Columnar epithelial cell imprinted from glandular region of stomach.
Cell has oval, basal nucleus and pale-staining microvillus apical border. (Wright-Giemsa stain)

Gastric fluid pH in the horse is variable. The horse appears to be a continuous, variable gastric hydrochloric acid secretor with intermittent periods of spontaneous alkalization.⁹ Duodenogastric reflux, which is common in the horse, can contribute to this alkalization. Gastric fluid pH values are typically less than 2.0 during feed deprivation but can be greater than 6.0 after free access to timothy grass hay.⁹ Cytologic examination of gastric fluid contains exfoliated and degenerating squamous and columnar epithelial cells, mixed population of bacteria, and possibly plant material (Fig. 6-4).

Fig. 6-4 Smear of gastric fluid from horse containing sloughed squamous epithelial cells, mixed population of bacteria, debris. (Wright-Giemsa stain)

Small and Large Intestine

Both small and large intestinal mucosal cells resemble stomach glandular epithelium (Fig. 6-5). Cells are columnar with pale blue, slightly granular cytoplasm, basal oval nuclei, and striated border composed of apical microvilli. Goblet cells, which have a vacuolated pale staining appearance and eosinophilic staining mucus, are more common in the colon and rectum (Fig. 6-6). Squamous epithelial cells are associated with the terminal rectum and anus. Endocrine cells, Paneth cells (pyramidal-shaped cells with prominent apical, spherical, acidophilic granules), and Brunner’s gland cells (Alcian blue–positive submucosal serous-type intestinal glands) are present in the small intestine and granular cells are present in the colon, but these cells are infrequent and have not been cytologically described (Figs. 6-7 and 6-8).¹ A few lymphocytes are often seen in intestinal specimens from horses because of Peyer’s patches and submucosal lymphoid tissue. Hemosiderin-laden macrophages have also been reported in the lamina propria of clinically normal horses.¹⁰ Although neutrophils were not present in the surface epithelium of the equine rectum, scattered neutrophils and eosinophils have been reported in the rectal lamina propria of healthy horses.¹¹

Fig. 6-5 Row of duodenal columnar epithelial cells with oval, basal nuclei and pale-staining microvillus apical “brush border” similar to gastric columnar epithelial cells. (Wright-Giemsa stain)

Fig. 6-6 Cluster of pale-staining goblet cells from colon of horse.
Note small central to eccentric nuclei and abundant clear to vacuolated cytoplasm. These cells are most prominent in the large intestine. (Wright-Giemsa stain)

Fig. 6-7 Colonic scraping from healthy horse containing numerous clusters of epithelial cells in sheets or forming acinar-like structures. Mast cells are occasionally observed. (Wright-Giemsa stain)

Fig. 6-8 Higher magnification of colonic epithelium.
A small granular cell with fine azurophilic granules is seen toward center of cell cluster. (Wright-Giemsa stain)

Intestinal fluid contains low numbers of squamous epithelial cells from the esophagus and stomach, a few columnar epithelial cells from the intestinal mucosa, a mixed population of bacteria, protozoa, fungal elements, food material, and rare to no inflammatory cells (Figs. 6-9 to 6-11). Minimal information is available on the microbial population inhabiting the gastrointestinal tract of the healthy horse. Proteolytic bacteria comprise a high proportion of culturable bacteria in the equine gastrointestinal tract.¹² Mean pH values reported for the equine duodenum, jejunum, ileum, and hindgut are 6.32, 7.10, 7.47, and 6.7, respectively.¹²

Fig. 6-9 Fluid from large intestine containing a large, dark-staining protozoa in center of field surrounded by plant material, a mixed population of bacteria, and cellular debris. (Wright-Giemsa stain)

Fig. 6-10 Normal equine protozoa.
Large intestinal fluid illustrating the pleomorphic appearance of normal equine protozoa. Note large size of protozoa compared to red cells. Some plant material and cellular debris are adjacent to protozoa. (Wright-Giemsa stain)

Fig. 6-11 Abdominal fluid from horse with chronic cecal impaction.
Two ciliated protozoa have marked difference in size. No evidence of intestinal rupture was found. (Wright-Giemsa stain)

Neoplasia

Neoplasia of the gastrointestinal tract of horses is uncommon and usually occurs in older horses, except for lymphoma, which often occurs in younger horses (Table 6-1). General clinical signs associated with equine gastrointestinal neoplasia include weight loss, anorexia, lethargy, intermittent colic, intermittent fever, and variable fecal consistency. Clinical laboratory findings can include malabsorption (decreased glucose and d-xylose absorption), hypoalbuminemia, hypergammaglobulinemia, anemia (hemorrhage and chronic disease), and hypercalcemia (lymphoma and squamous-cell carcinoma). Peritoneal fluid frequently has an increased nucleated cell count (neutrophilic inflammation), increased protein, and, sometimes, exfoliated neoplastic cells. Microorganisms from the gut lumen may invade gastrointestinal neoplasms, resulting in abscessation and secondary septic peritonitis.

TABLE 6-1 Tumors of the Equine Gastrointestinal Tract

Lymphoma is the most common malignant neoplasm of the equine gastrointestinal tract.^8,13 It occurs most frequently in the small intestines and may be a primary alimentary lymphoma or a multicentric lymphoma that involves the intestines in addition to peripheral lymph nodes and/or the thoracic cavity. Alimentary lymphoma is associated with local to diffuse thickening of the gut wall and marked enlargement of the mesenteric lymph nodes.¹⁴ Neoplastic infiltrates can extend from the lamina propria and submucosa to the serosal surface. The lymphocyte population can vary from sheets of lymphoblasts or granular lymphocytes to a mixed population of large and small lymphocytes (Figs. 6-12 to 6-15).^14–18 Cytoplasmic granules in granular lymphoma cells are readily seen on cytology but may be poorly visible on histopathology.¹⁵ Plasmacytoid cells and plasma cells can be abundant, and giant cells can occasionally be seen in the gut wall and lymph node of horses with lymphoma.^13,14

Fig. 6-12 Equine intestinal lymphoma.
Note mixed population of large, intermediate, and small lymphocytes with coarse chromatin pattern, indistinct nucleoli, and scant cytoplasm. (Wright-Giemsa stain)

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