History

A 5-month-old 128.4-kg (282.5-lb) Tennessee Walking Horse colt was evaluated at the Mississippi State University College of Veterinary Medicine Animal Health Center because of a 3-week history of inappetence, weight loss, and increasing degree of lethargy. The colt had been weaned 4 weeks before the evaluation and appeared to be healthy at that time. No other animal on the farm was sick at the time of the evaluation.

Clinical and Gross Findings

The colt had evidence of stunted growth, pyrexia (38.9°C [102°F]), and emaciation (body condition score, between 1 and 2) and passed soft feces, which progressed to diarrhea. Abdominal ultrasonography revealed that the small intestinal wall was thickened (up to 9.9 mm). Bacterial culture of fecal samples yielded growth of a Salmonella sp. The colt was treated aggressively for 72 hours with supportive therapy (fluids, plasma, and hetastarch administered IV) and doxycycline (10 mg/kg [4.5 mg/lb], PO, q 12 h). However, the colt became too weak and debilitated to stand. The prognosis was poor to grave, and the owner opted for euthanasia.

Necropsy was performed shortly after euthanasia. The general physical condition of the body was poor; minimal subcutaneous adipose tissue was evident. The rectal mucosa was markedly edematous and bulged from the anus. A thin layer of intra-abdominal adipose tissue was present. Scant malodorous ingesta were in the stomach and colon. The gastric mucosa was considered normal. The jejunum and ileum were palpably thickened, and the mucosae had prominent rugose folds, which could not be obliterated by stretching (Figure 1). Widely disseminated red foci (petechiae) and 2- to 3-mm tan nodules were present along some prominent mucosal folds. In the distal portion of the jejunum and entire extent of the ileum, prominent raised antimesenteric foci (attributed to hyperplastic Peyer's patches) were visible. The cecal and colonic mucosae were smooth with glistening surfaces, occasional reddened foci, and marked thickening due to the presence of mucosal and submucosal edema. The spleen and mesenteric lymph nodes were small. The lungs were considered normal and had no evidence of fluid in airways or on cut surface. The thymus was atrophied.

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Photograph of the ileal mucosa (A) and large colonic mucosa (B) of a 5-month-old colt that had progressively worsening lethargy, inappetence, and weight loss of 3 weeks' duration and recent onset of diarrhea. Notice that the small intestinal mucosa has prominent rugose folds, which extend along its entire length and which could not be obliterated by stretching. No ulceration was present. The colonic mucosa was markedly thickened by edema and had intermittent foci of hyperemia and hemorrhage. The colonic content was watery. Bar in panel A = 1 cm.

Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.529

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Sections of tissues, including all segments of the small intestine, colon, liver, thymus, spleen, mesenteric lymph nodes, and kidneys, were examined histologically. The mucosa of the small intestine had irregular rugose folds, which resulted in a mound-like appearance of the mucosal epithelium (Figure 2). Villi were short, and crypts were markedly elongated, sometimes resulting in a 1:9 villus-to-crypt ratio. The crypts were lined by hyperplastic and hypertrophied epithelial cells, which extended far up the neck of crypts (Figure 3). The crypt epithelial cells had stratification of nuclei, prominent ovoid nuclei with fine chromatin, prominent nucleoli, and abundant apical cytoplasm. The crypt lumina were dilated and contained a few necrotic epithelial cells and degenerate leukocytes. Scattered foci of herniation of crypts into Peyer's patches were also observed. Some sections of ileum had Peyer's patches with depleted central follicles and necrosis and abscess formation within follicles. The colonic lamina propria was expanded by edema and infiltrated with numerous lymphocytes and plasma cells and few neutrophils. A few interepithelial lymphocytes were present in the colonic mucosal epithelium. Colonic glands were dilated, and some contained eosinophilic fluid. Submucosal lymphoid follicles had depletion of lymphocytes.

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Photomicrograph of a section of the small intestine of the horse in Figure 1. Mucosal folds consist of protruding ridges of submucosa covered by markedly hyperplastic mucosa. Much of the mucosal thickening is a result of elongation of crypts, which results in a 1:9 villus-to-crypt ratio in some areas. H&E stain; bar = 500 μm.

Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.529

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Photomicrographs of 2 sections of the small intestine of the horse in Figure 1. A—Section stained with H&E stain. Notice the markedly elongated crypts lined by hyperplastic and hypertrophic epithelial cells that extend far up the neck of crypts. Bar = 100 μm. B—Section stained with an immunohistochemical stain to detect Lawsonia intracellularis. The strong brown staining indicates the presence of the organism in the apical cytoplasm of hyperplastic and hypertrophic crypt epithelial cells. Bar = 50 μm.

Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.529

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The liver had some evidence of lobular atrophy, and mixed inflammatory infiltrates were present in portal areas and did not breach the limiting plates. The thymus was markedly atrophied with complete loss of cortical and medullary distinction. The spleen had marked depletion of small lymphocytes (T and B cells) from the splenic white pulp, particularly from splenic nodules. In a mesenteric lymph node, similar depletion of lymphocytes from cortical lymphoid follicles was observed; cortical and medullary sinuses were distended by edema with intermingled populations of lymphocytes and macrophages. The kidneys had small foci of interstitial mineralization at the cortical medullary junction. Examination of Warthin-Starry silver–stained sections of jejunum and ileum revealed numerous black, curved rod-shaped organisms in the apical cytoplasm of crypt epithelial cells. Based on the gross and histopathologic lesions, a presumptive diagnosis of Lawsonia proliferative enteritis was made. Samples of feces underwent PCR analysis (done at the University of California-Davis School of Veterinary Medicine), and samples of ileum underwent immunohistochemical testing (done at the Purdue University Animal Disease Diagnostic Laboratory); both results confirmed the presence of Lawsonia intracellularis.

Morphologic Diagnosis

Severe proliferative enteritis caused by L intracellularis and acute diffuse severe colitis due to Salmonella sp infection.

Comments

Lawsonia intracellularis is an obligate intracellular, gram-negative, argyrophilic, non–membrane-bound bacterium. It is the causative agent of porcine proliferative enteropathy.^1,2 Porcine proliferative enteropathy is an infectious, economically important, transmissible, enteric disease and causes an annual economic loss of $98 million for the US swine industry.³ Porcine proliferative enteropathy was first described in 1931,³ but the causative agent was identified much later in 1995.⁴ Taxonomically, L intracellularis is classified in delta division of the Proteobacteria.¹ Lawsonia intracellularis is not host restricted and causes proliferative enteritis in other species, including rats, hamsters, guinea pigs, dogs, white-tailed deer, horses, ratites (ostriches and emus), and humans.² In general, few cases of L intracellularis infection in horses have been reported; however, recent retrospective epidemiological studies^5–8 have revealed an increased incidence over the past few years. Notably, L intracellularis infection that results in proliferative enteritis in horses has been reported in the United States, Brazil, Canada, Europe, and Australia.^5–15

Irrespective of host, L intracellularis preferentially infects and localizes in the apical cytoplasm of the crypt epithelium of the small intestine.² Intestinal flora help in the initial colonization by the bacteria. In the absence of resident flora, gnotobiotic piglets fail to develop the disease when infected with pure bacteria that has been cultured in vitro; however, they develop the disease when infected with the crude lesions from infected pigs, which contain a mixed population of intestinal flora and L intracellularis.^2,16

Lawsonia intracellularis induces the upregulation of host genes involved in cell cycle regulation, cell differentiation, and cell proliferation, which is probably the mechanism responsible for the hyperproliferation of crypt epithelial cells detected 3 to 4 days after infection.¹⁷ The lack of normal crypt cells and the loss of normal brush border epithelium results in malabsorption and hypoproteinemia. The shedding of infected cells into the lumen acts as the source of infection via the fecal-oral route.

For the colt of this report, further investigation revealed that the hay it received was procured from a farm on which pig manure was used as a fertilizer. The hay may have been a possible source of infection; however, given the fact that this bacterium is an obligate intracellular bacterium and cannot survive > 2 weeks outside the host at room temperature,³ another source of infection cannot be ruled out. Thymic atrophy may have been a consequence of gram-negative sepsis or stress-induced endogenous glucocorticoid release. The absence of substantial lymph node or splenic reaction to chronic enteric disease seemed unusual and suggested that the horse could have been immunosuppressed, which is in agreement with results of previous in vivo investigations.¹ Colitis and the presence of fetid content in the colon are consistent with Salmonella infection; however, Salmonella infection appeared less important in this case.

Four days after the euthanasia of the colt, a 5.5-month-old 153.8-kg (338.4-lb) Tennessee Walking Horse weanling filly from the same farm was admitted to the hospital because of quiet demeanor and anorexia for 36 to 48 hours. On evaluation, the horse was thin (body condition score, 3/9) and slightly unthrifty in appearance. Physical examination and clinicopathologic abnormalities were similar to those of the colt, and serum albumin concentration (0.7 g/dL; reference range, 2.8 to 3.9 g/dL) was notably low during hospitalization. The filly received treatment similar to that of the colt, including IV administration of fluids, plasma, and hetastarch and administration of an antimicrobial (in this instance, chloramphenicol [50 mg/kg {22.7 mg/lb}], PO, q 8 h). The supposition was that the filly also had L intracellularis infection with possible secondary salmonellosis. Fecal samples underwent bacterial culture and PCR assay for L intracellularis, Clostridium difficile toxins A and B, Neorickettsia risticii, and Salmonella spp. In culture, there was no growth of Salmonella spp; however, results of fecal PCR assay were positive for L intracellularis. Moreover, the horse was seropositive for the organism (titer, 1:240). The filly was hospitalized for 8 days. Following discharge from the hospital, administration of the antimicrobial and supportive treatments was continued for 7 days. Recheck examinations of the filly at 7 and 19 days after discharge from the hospital revealed increasing serum albumin concentrations (1.3 and 1.5 g/dL, respectively) and an overall subjective improvement in thrift. The hay suspected to be a possible source of infection was disposed of, and no further signs of disease consistent with L intracellularis infection have been noted on the farm.