JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

October 2024

D-P24 (NP)

 

SLIDE A: Signalment (JPC #1817473): Young horse

 

HISTORY: At necropsy, there were numerous elevated subserosal masses in the terminal small intestine that ranged in color from bright red to yellow and brown.

 

HISTOPATHOLOGIC DESCRIPTION: Small intestine: Multifocally, the serosa is expanded up to 1 mm by numerous small-caliber blood vessels, minimal hemorrhage, increased fibrous connective tissue, moderate numbers of macrophages, lymphocytes, plasma cells, fewer eosinophils, ectatic lymphatics, and increased clear space (edema). Macrophages often contain cellular debris and there is multifocal moderate erythrophagocytosis and hemosiderosis. Multifocally, the submucosa is expanded up to 3 mm thick by increased clear space (edema) and increased mature collagen fibrils (fibrosis). Diffusely the submucosa and lamina propria are infiltrated by low to moderate numbers of previously described inflammatory cells, including erythrocyte-laden macrophages. Few intestinal crypts contain necrotic debris and neutrophils (crypt abscesses) and there is mild blunting of intestinal villi. There is rare herniation of crypts into the underlying submucosa.

 

MORPHOLOGIC DIAGNOSIS: 

1. Small intestine, serosa: Serositis, lymphoplasmacytic and histiocytic, subacute, multifocal, moderate, with fibrosis, erythrophagocytosis, hemosiderosis and edema, horse, breed unspecified, equine.
2. Small intestine: Enteritis, lymphoplasmacytic, chronic, diffuse, mild, with crypt abscesses and submucosal erythrophagocytosis.

 

CONDITION: Hemomelasma ilei

 

SLIDE B: Signalment (JPC #4115180-00): 8-month-old quarter horse/appaloosa cross filly

 

HISTORY: Filly was seized by animal control as part of an abuse/neglect case and was euthanized due to poor prognosis. 

HISTOPATHOLOGIC DESCRIPTION: Cecum: There is moderate eosinophilic, histiocytic, and lymphoplasmacytic inflammation expanding the lamina propria. Multifocally expanding the mucosa, compressing crypts, and extending into the underlying submucosa there are multiple longitudinal and cross sections of larval nematodes. Larvae are ~200 µm in diameter and have thick eosinophilic cuticle, platymyarian musculature, vacuolated lateral cords, a muscular esophagus with a triradiate lumen, and a large intestine composed of few multinucleated cells that occasionally contain brown iron pigment and are lined by a brush border muscular. There is thin, circumferential fibrosis surrounding larval nematodes. The submucosa is expanded by previously described inflammatory cells, increased clear space, and dilated lymphatics that contain smooth, eosinophilic fluid and previously described inflammatory cells (edema). Multifocally, the tunica media of arteries in the submucosa are moderately expanded (proliferation) with vacuolation of the tunica media and tunica adventitia (edema; “onion skinning” appearance) and karyorrhectic cellular debris (necrotic vasculitis). Rarely, obscuring the lumen and adhered to the adjacent endothelium is eosinophilic, fibrillar aggregated fibrin with embedded neutrophils and karyorrhectic cellular debris (fibrin thrombi). The serosa is expanded by increased clear space and ectatic lymphatics (edema). The mesothelium is mildly hypertrophied, cuboidal, and crowded (reactive mesothelium).

 

Lung: There is multifocal, extensive intra-alveolar edema that obscures the alveolar septa.

 

MORPHOLOGIC DIAGNOSIS: 

1. Cecum: Typhlitis, eosinophilic, granulomatous and lymphoplasmacytic, diffuse, severe with numerous small larval strongyles, lymphoid depletion and necrotizing and proliferative arteritis, quarter horse/appaloosa cross, equine. 
2. Lung: Pulmonary edema, multifocal to coalescing, moderate.

 

ETIOLOGIC DIAGNOSIS: Intestinal cyathostomiasis

 

CAUSE: Small strongyles

 

GENERAL DISCUSSION: 

Hemomelasma ilei

• Common incidental finding in horses
• Cause is unknown, but thought to be associated with large strongyles (Strongylus edentatus or other Strongylus spp.) (see C-P02)
• Slightly elevated subserosal hemorrhagic plaques usually found along the antimesenteric border of the ileum or rarely on the large bowel, but can occur anywhere along the intestine

 

Small strongyles 

• Small strongyles, or cyathostomins (cyathostomes), are a group containing >50 species of parasites found in the large intestine of horses worldwide
• Adults are essentially nonpathogenic; larvae may cause disease when emerging; if a large number of larvae emerge from hypobisosis simultaneously, devastating necroulcerative and hemorrhagic typhlocolitis with significant morbidity and mortality may result 
• Due to widespread anthelmintic resistance in cyathostomes; as well as suppression of S. vulgaris by anthelmintics, cyathostomes are the most important cause of verminous colic in horses

 

PATHOGENESIS:

Hemomelasma ilei

• Unknown cause
• May be associated with injury to serosal vasculature by migrating larvae of Strongylus edentatus or other Strongylus sp.; however, parasites have not been reported in the lesions, and the incidence has not apparently declined with the reduction in large strongyle infections over the last 25 years

 

Small strongyles 

• Cyathostomes have a direct lifecycle
  1. Infective third-stage larvae ingested and travel to cecum and large colon -> Migrate into deep mucosa or submucosa and encyst -> May molt (into L4) and/or may enter hypobiosis -> Emerge -> Mature into adults in large intestinal lumen 
• There is little resistance to repeat infection
• Hypobiosis
  1. May last as long as 2 years
  2. Larval emergence is based on climate (inhibited during cooler months); mass emergence occurs in late winter, spring, and early summer (northern temperate climates)

 

 

TYPICAL CLINICAL FINDINGS:

Hemomelasma ilei

 

Small strongyles

• Usually affects horses <1 year of age, although horses of any age can be affected
• May see diarrhea, edema, anorexia, and weight loss; has been associated with ceco-colic intussusception

 

TYPICAL GROSS FINDINGS:

Hemomelasma ilei

• Slightly elevated, antimesenteric, subserosal pink to black plaques, 1-4 cm in diameter; occur anywhere in the intestine but most commonly ileum
• Hemorrhages, ulcers, and a catarrhal exudate may be present on the intestinal mucosal surface
• With time, lesions resolve to yellow, brown, or tan fibrotic plaques

 

Small strongyles

• 2-5 mm slightly raised red or black mucosal nodules formed by encysted larvae; on cross section, translucent gray or red larval nematode may be apparent
• Edema and congestion of mucosa and submucosa
• Mass emergence of larvae may result in necroulcerative hemorrhagic typhlocolitis

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Hemomelasma ilei

• Serosal thickening with histiocytic plaques and variably mature fibrous tissue
• Edema, hemorrhage, mixed inflammatory infiltrate; macrophages containing erythrocytes, hemosiderin, and cellular debris 
• Variably mature fibrous tissue

 

Small strongyles

 

ADDITIONAL DIAGNOSTICS:

Small strongyles

 

DIFFERENTIAL DIAGNOSIS:

For serosal plaques:

 

For typhlocolitis in horses

• Salmonella enterica sv Typhimurium, Clostridioides difficile, non-steroidal anti-inflammatory drug intoxication, Paeniclostridium sordellii
• Less common causes include histoplasmosis and anoplocephalid tapeworms

 

COMPARATIVE PATHOLOGY:

Hemomelasma ilei

 

Small strongyles

 

REFERENCES: 

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