Pathology in Practice

Naomi E. Crabtree Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS

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Alicia K. Olivier Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS

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Michael K. Brashier Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS

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Abstract

In collaboration with the American College of Veterinary Pathologists

Abstract

In collaboration with the American College of Veterinary Pathologists

History

A 5-week-old 19.1-kg American Miniature Horse filly was presented because of a 24-hour history of lethargy and diarrhea.

Clinical and Gross Findings

Upon presentation the foal was recumbent with marked fecal soiling present. The horse had a clinically normal body condition and was obtunded, with minimal menace response, bilateral corneal ulcerations, prolonged skin tent, poor jugular refill, and cold extremities. The horse’s heart rate was 64 beats/min, respiratory rate was 24 breaths/min, and rectal temperature was 37.0 °C. The horse’s mucous membranes were dry and purple, with a capillary refill time of 4 seconds. Findings on cardiothoracic auscultation were clinically normal with no murmurs, arrhythmias, or abnormal lung sounds heard. No gastrointestinal motility was auscultated.

Hematology revealed marked leukocytosis (49.0 X 109 cells/L reference range [RR], 5.0 to 11.9 X 109 cells/L), with marked mature neutrophilia (35.3 X 109 cells/L; RR, 2.5 to 6.0 X 109 cells/L), lymphocytosis (10.8 X 109 cells/L; RR, 1.2 to 5.0 X 109 cells/L), monocytosis (2.9 X 109 cells/L; RR, 0 to 0.8 X 109 cells/L); hemoconcentration (45.0%; RR, 26.0% to 42.0%); and moderate thrombocytosis (785.0 X 109 thrombocytes/L; RR, 100 to 400 X 109 thrombocytes/L). Biochemical analyses revealed marked electrolyte derangements, including hyponatremia (106.6 mmol/L; RR, 132.0 to 146.0 mmol/L), hypochloremia (71.2 mmol/L; RR, 98.0 to 106.0 mmol/L), hyperkalemia (6.3 mmol/L; RR, 2.4 to 4.7 mmol/L), hyperphosphatemia (18.2 mmol/L; RR, 2.4 to 4.0 mmol/L), and hypermagnesemia (1.3 mmol/L; RR, 0.6 to 1.0 mmol/L); marked metabolic acidosis (anion gap, 32.0 mmol/L; RR, 6.0 to 16.0 mmol/L; total CO2 concentration, 9.8 mmol/L; RR, 24.0 to 32.0 mmol/L) with hyperlactatemia (11.0 mmol/L; RR, < 2.0 mmol/L); substantial hypoproteinemia (52.0 g/L; RR, 61.0 to 84.0 g/L) and hypoalbuminemia (22.0 g/L; RR, 28.0 to 39.0 g/L); marked azotemia (creatinine concentration, 534.8 mg/dL; RR, 106.1 to 168.0; and BUN concentration, 35.7 mmol/L; RR, 3.6 to 8.6 mmol/L); moderate hyperglycemia (9.5 mmol/L; RR, 3.3 to 6.8 mmol/L); and high activities of creatine kinase (4749 U/L; RR, 57 to 283 U/L) and alkaline phosphatase (213 U/L; RR, 61 to 153 U/L).

Due to the need for emergency stabilization, further diagnostics were not performed. Despite oxygen supplementation, fluid therapy, and IV antimicrobial treatment, the foal died 8 hours after presentation. On postmortem examination, within the distal ileum, cecum, ventral colon, and descending colon (small colon), the intestinal mucosa was diffusely thickened. The mucosal surface was corrugated with multifocal to coalescing areas of congestion and hemorrhage (Figure 1).

Figure 1
Figure 1

Postmortem image of markedly thickened and corrugated mucosa with multifocal to locally extensive areas congestion and hemorrhage of the cecum (CE), ventral colon (VC), and descending colon (small colon [CO]) of a 5-week-old 19.1-kg American Miniature Horse filly that died shortly after onset of treatment for a 24-hour history of lethargy and diarrhea.

Citation: Journal of the American Veterinary Medical Association 259, S2; 10.2460/javma.20.03.0125

Formulate differential diagnoses, then continue reading.

Histopathologic and Microbiological Findings

Histologically within the ileum, cecum and throughout the colon glands were elongated and lined by hyperplastic pseudostratified columnar epithelial cells with marked reduction of goblet cells and locally extensive areas of hemorrhage within the lamina propria (Figure 2). Silver-stained sections of various affected regions demonstrated abundant argyrophilic intracytoplasmic apically located bacilli within epithelial cells. Within the ileum there was marked villous atrophy, gland loss and hyperplastic glands that extended into the underlying submucosal lymphoid tissue (crypt herniation) (Figure 3). Crypts were separated by increased lymphocytes and plasma cells within the lamina propria. Multifocally crypts were mildly expanded by sloughed epithelial cells with scattered neutrophils (crypt abscesses) or were lined by attenuated epithelium. Scattered crypts with epithelial hyperplasia had increased mitotic figures. Small superficial areas of mucosal necrosis were scattered throughout all sections (not shown). Additionally, evidence of sepsis was identified on histologic evaluation including a diffuse suppurative interstitial pneumonia and lymphocytolysis in the spleen.

Figure 2
Figure 2

Photomicrographs of colonic tissue sections from the horse described in Figure 1. A—There is marked mucosal thickening with crypt elongation, goblet cell loss, and locally extensive areas of hemorrhage. H&E stain; bar = 100 µm. B—Colonic crypts are lined by pseudostratified columnar epithelial cells (epithelial hyperplasia) with loss of goblet cells. H&E stain; bar = 20 µm). C and D—Abundant argyrophilic small bacilli are present within the apical cytoplasm of hyperplastic epithelial cells of the colonic glands. Steiner silver stain; bar = 100 µm (C) or 20 µm (D).

Citation: Journal of the American Veterinary Medical Association 259, S2; 10.2460/javma.20.03.0125

Figure 3
Figure 3

Photomicrographs of sections of small intestine from the horse described in Figure 1. A—The small intestine has marked villous atrophy, gland loss, and hyperplastic glands that extend into the underlying submucosal lymphoid tissue (arrow; crypt herniation). H&E stain; bar = 100 µm. B—High magnification of the hyperplastic gland extending into the lymphoid tissue of the submucosa. H&E stain; bar = 50 µm. C—Multifocally, crypts are mildly expanded by sloughed epithelial cells with scattered neutrophils (arrow; crypt abscesses) or are lined by attenuated epithelium (asterisk). H&E stain; bar = 50 µm. D—Scattered crypts have increased mitotic figures (arrows). H&E stain; bar = 20 µm.

Citation: Journal of the American Veterinary Medical Association 259, S2; 10.2460/javma.20.03.0125

Results were negative for fecal bacterial culture and PCR assay panel that included equine coronavirus, Clostridioides difficile toxin A and B, Neoricketsia risticii. The PCR assay result was positive for Lawsonia intracellularis.

Morphologic Diagnosis and Case Summary

Enterocolitis, proliferative, chronic, diffuse, severe with gland elongation and hyperplasia, crypt herniation, crypt abscesses, goblet cell loss, and numerous argyrophilic intraenterocytic bacilli (etiology consistent with L intracellularis) in an American Miniature Horse.

Comments

Lawsonia intracellularis is an obligate intracellular, curved gram-negative organism that preferentially colonizes the small intestine of many species.1,2 The organism invades the intestinal crypt cells, causing proliferation of the epithelium and leading to a grossly corrugated appearance of the intestinal mucosa, giving rise to the disease name equine proliferative enteropathy (EPE).3,4 This proliferative change results in dysfunction at the brush border and a malabsorptive protein-losing enteropathy.5 Fecal shedding via infected epithelial cell sloughing allows for fecal-oral transmission, and transmission via infected animal fomites is considered likely.69

Lawsonia-associated proliferative enteropathy is most common in pigs but has been reported with increasing frequency in horses since its first description in 1982.4,10 EPE is typically a condition of weanling-age horses, although it has occasionally been seen in adults.11,12 This age predilection is speculated to be associated with the stress of weaning, transportation, training, vaccination, and parasite load.13 Additionally, waning maternal antibodies at this age iare suspected to play a role.14,15 EPE is most often described as affecting individual animals, but several outbreaks have been reported.1618 Early reports were predominantly seen in North America; however, cases have been reported from Australia, Europe, South Africa, and Japan.19,20 While no clinical cases have been reported in South America, seropositive animals and fecal shedding have been reported in Brazil.21 Most cases occur in the fall and winter in the northern hemisphere, likely due to when weanlings are present in the population.3

EPE is typically considered a chronic condition with nonspecific clinical signs such as depression, lethargy, anorexia, weight loss, fever, diarrhea, and colic.20,22 Peripheral edema is common, secondary to the development of hypoproteinemia.3 A suspected diagnosis is generally made antemortem based on the clinical picture, identification of hypoproteinemia and hypoalbuminemia, segmental thickening of the small intestine on abdominal ultrasonography, and exclusion of other enteric diseases.20,22 Additional antemortem testing may include fecal or rectal swab sample for PCR assay, serology, or histological or PCR evaluation of surgical biopsies.2325 Due to the risk of false positive results with both fecal PCR assay and serum serology, performing both is recommended.26 Culture is unrewarding due to the intracellular location of the organism.27 Definitive diagnosis can be made postmortem with identification of the characteristic mucosal proliferation, staining of the organism with Warthin-Starry silver stain, immunohistochemical staining, and PCR assay of intestinal tissues, alone or in combination.4,28

Two case series have previously described an atypical, necrotizing presentation of L intracellularis, similar to that seen in pigs and described here.29,30 All 9 of the foals in these series had an acute onset of clinical signs and deteriorated rapidly, despite prompt referral and treatment. This presentation differed substantially from the typical EPE; therefore, Page et al29 suggested referring to these cases as a necrotizing form of EPE (N-EPE). Acute deterioration and death have also been reported in more typical cases of EPE but are typically seen following a protracted disease course and secondary to profound dehydration.10,18 Previous N-EPE cases were seen in foals 6 to 8 months of age, in contrast to the foal in the present case who was only 5 weeks old.29,30 Based the foal’s age, the differential diagnoses considered were the more common causes of diarrhea in this age group, including rotavirus, coronavirus, Salmonella, C difficile, or C perfringens.31 The rapid onset of shock and presence of hemorrhagic diarrhea meant Clostridia or Salmonella infection were highest on the list. The drastic deviation from the typical presentation and expected age predilection meant L intracellularis was not considered as a top differential diagnosis.

Upon review of the EPE literature, most cases exhibited a leukocytosis, as was seen in the present case.5,12,16,17,19,24,27,3237 In the reports describing N-EPE cases, leukocytosis was less common but still seen in 50%.29,30 As most causes of acute enterocolitis are associated with neutropenia, identification of a leukocytosis as profound as that in the present case, in the face of signs consistent with sepsis, may be an indication to consider N-EPE.31

An additional deviation from EPE was seen in this case, with respect to the abundance of organisms identified within enterocytes of the ascending colon. While L intracellularis is known to commonly inhabit the large colon of pigs and other species, this is not typically seen in foals.12 Our comprehensive review of the literature resulted in identification of only 5 manuscripts4,11,19,20,37 describing a colonic distribution in horses. The report by Shimizu et al11 describes histologic changes in the colon; however, it was not indicated whether the organism was identified in enterocytes. Interestingly, the foal in that report was coinfected with Rhodococcus equi, which was speculated to have promoted the more extensive distribution. No known comorbidity was identified to explain the marked colonic colonization in the present case, although age may be a factor. The remaining 4 reports4,19,20,37 describing colonic lesions do so in passing, rather than in reference to specific cases, and no reference for these comments was provided.

Diagnostic clues to add L intracellularis to the differential diagnosis list are of critical importance, as its treatment differs from that for other causes of enterocolitis in young foals. Antimicrobials of choice for L intracellularis include macrolides with or without rifampin, chloramphenicol, and tetracyclines—drugs that would not usually be chosen as a first-line response to a young foal with diarrhea.3,36 Knowing of the possibility of N-EPE may help practitioners tailor their antimicrobial choices accordingly. Consideration of antimicrobial sequelae is also important, given the risk of further disruption of gastrointestinal flora (particularly of the dam if present), macrolide-induced hyperthermia, or tetracycline-induced nephrotoxicity.3,36 Page et al29 suggest the acute death in N-EPE cases may occur due to secondary bacterial colonization and translocation, endotoxemia, and/or disseminated intravascular coagulopathy. As such, broadening the antimicrobial spectrum may be warranted to address these secondary pathogens.29

Supportive care is crucial in cases of EPE, primarily involving the provision of nutritional, fluid and colloidal support. In foals as young as that described here, nutritional support may include stimulation of nursing, milk replacement via nasogastric tube, or parenteral nutrition. Isotonic fluid therapy with electrolyte supplementation may be required if profuse diarrhea is present.4 Synthetic colloid or plasma support may be indicated with severe hypoproteinemia; however, the benefits of these interventions on overall outcome is unclear.5 Additional supportive care can include gastroprotectants and nonsteroidal anti-inflammatory drugs.29 In the case of N-EPE, other suggestions include antithrombotic and antiendotoxic treatment such as low-dose flunixin meglumine, polymyxin B, or hyperimmune plasma.29 While spontaneous recovery has not been documented, long-term (3 to 4 week) treatment has typically been successful in foals with classic EPE.26,38 The outlook is dismal in the case of N-EPE, with a 100% mortality rate in all previous cases.29,30

Lawsonia intracellularis is commonly considered a herd-level problem. Seroprevalences ranging from 17% to 68% have been reported.3,15 Four of the 5 N-EPE cases described by Arroyo et al30 were from farms with current or historical L intracellularis outbreaks, suggesting historical data may help inform regarding differential diagnoses. This prevalence suggests prevention is a key component of disease management. Prevention strategies commonly used in swine production, such as feed-through antimicrobials, are not recommended in horses.4 However, reducing the stress associated with weaning, parasitism, and training, as well as herd monitoring and pest control, is a prudent strategy.4,26 Lawsonia intracellularis vaccination has recently been evaluated, with oral and intrarectal vaccination with the commercially available avirulent live porcine vaccine having resulted in positive titres.39 Intrarectal vaccination has also been shown to provide protection against subsequent challenge.40 Although purportedly safe and effective, it should be noted that these protocols constitute extralabel use.41,42

The present case report describes an unusual presentation of L intracellularis in horses. This is the youngest reported case of EPE in the literature to date. This organism generally causes a treatable protein-losing enteropathy associated with proliferative small intestinal changes, rather than the acutely fatal, widespread necrohemorrhagic enterocolitis seen here. Based on the present case, the authors suggest L intracellularis should be considered in cases involving younger foals than previously reported, even in the face of hemorrhagic diarrhea more commonly associated with other etiologies.

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Contributor Notes

Corresponding author: Dr. Crabtree (naomi.e.crabtree@gmail.com)
  • Figure 1

    Postmortem image of markedly thickened and corrugated mucosa with multifocal to locally extensive areas congestion and hemorrhage of the cecum (CE), ventral colon (VC), and descending colon (small colon [CO]) of a 5-week-old 19.1-kg American Miniature Horse filly that died shortly after onset of treatment for a 24-hour history of lethargy and diarrhea.

  • Figure 2

    Photomicrographs of colonic tissue sections from the horse described in Figure 1. A—There is marked mucosal thickening with crypt elongation, goblet cell loss, and locally extensive areas of hemorrhage. H&E stain; bar = 100 µm. B—Colonic crypts are lined by pseudostratified columnar epithelial cells (epithelial hyperplasia) with loss of goblet cells. H&E stain; bar = 20 µm). C and D—Abundant argyrophilic small bacilli are present within the apical cytoplasm of hyperplastic epithelial cells of the colonic glands. Steiner silver stain; bar = 100 µm (C) or 20 µm (D).

  • Figure 3

    Photomicrographs of sections of small intestine from the horse described in Figure 1. A—The small intestine has marked villous atrophy, gland loss, and hyperplastic glands that extend into the underlying submucosal lymphoid tissue (arrow; crypt herniation). H&E stain; bar = 100 µm. B—High magnification of the hyperplastic gland extending into the lymphoid tissue of the submucosa. H&E stain; bar = 50 µm. C—Multifocally, crypts are mildly expanded by sloughed epithelial cells with scattered neutrophils (arrow; crypt abscesses) or are lined by attenuated epithelium (asterisk). H&E stain; bar = 50 µm. D—Scattered crypts have increased mitotic figures (arrows). H&E stain; bar = 20 µm.

  • 1.

    Cooper DM, Swanson DL, Gebhart CJ. Diagnosis of proliferative enteritis in frozen and formalin-fixed, paraffin-embedded tissues from a hamster, horse, deer and ostrich using a Lawsonia intracellularis-specific multiplex PCR assay. Vet Microbiol. 1997;54:4762. doi:10.1016/S0378-1135(96)01264-3.

    • PubMed
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