History

A 4-month-old female American Saddlebred was referred for increased respiratory effort, lethargy, inappetance, weight loss, and insufficient growth of 4 weeks' duration. Neither coughing nor nasal discharge had been observed. Clinical signs had become worse despite treatment with ceftiofur sodium.

On admission, the foal was moderately thin, lethargic, 5% to 6% dehydrated, tachypneic (respiratory rate, 49 breaths/min; reference range, 10 to 30 breaths/min), tachycardic (156 beats/min; reference range, 30 to 45 beats/min), and febrile (40.2°C [104.3°F]; reference range, 37.5° to 39.0°C [99° to 102°F]) and had pale mucous membranes, and its respiratory effort was increased. Normal breath sounds were detected during auscultation of the lungs; however, adventitious breath sounds were not detected. Both eyes were affected with severe uveitis, and the conjunctival mucosae were characterized by petechial hemorrhages. Moderate effusion was detected in both tibiotarsal joints, but no signs of pain or lameness were evident.

Results of a CBC included anemia (PCV, 11%; reference range, 32% to 45%) and neutrophilia (17.7 × 10³ cells/ ML; reference range, 5.4 to 14.3 × 10³ cells/ML) with a degenerative left shift (band neutrophils, 0.35 × 10³ cells/ML; reference range, 0 to 0.1 × 10³ cells/ML), and mild toxic changes were detected in the neutrophils. Abnormalities detected on serum biochemical analysis included hypoglycemia (glucose, 66 mg/dL; reference range, 83 to 113 mg/dL); azotemia (urea nitrogen, 55 mg/dL; reference range, 8 to 23 mg/dL); hypoalbuminemia (1.4 g/dL; reference range, 3.5 to 4.4 g/dL); high anion gap (22 mEq/L; reference range, 8 to 16 mEq/L); and low bicarbonate concentration (10 mEq/L; reference range, 26 to 35 mEq/L), compatible with metabolic acidosis. A blood sample obtained aseptically was submitted for bacteriologic culture. No abnormalities were detected on radiography of the thorax. Ultrasonography of the abdomen was performed (Figure 1).

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Ultrasonographic images of the ventral (A) and ventrolateral (B) aspects of the abdomen of a 4-month-old female American Saddlebred horse evaluated for increased respiratory effort, lethargy, inappetance, weight loss, and insufficient growth of 4 weeks' duration. Images were obtained approximately 10 (A) and 20 (B) cm caudal to the xiphoid process by use of a 5-MHz transducer at a display depth of 17 cm.

Citation: Journal of the American Veterinary Medical Association 230, 4; 10.2460/javma.230.4.509

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page

Diagnostic Imaging Findings and Interpretation

Multiple, heterogeneously echogenic, well-defined, encapsulated masses are evident throughout the abdomen, particularly along the ventral aspect (Figure 2). Pinpoint hyperechoic foci with distal acoustic shadowing, compatible with the presence of gas, are seen in several masses. The ultrasonographic findings were compatible with multifocal intra-abdominal abscess formation.

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Same ultrasonographic images as in Figure 1. An abdominal abscess is evident (arrowheads), and hyperechoic foci (arrows) indicative of gas can be seen within the abscess.

Citation: Journal of the American Veterinary Medical Association 230, 4; 10.2460/javma.230.4.509

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Comments

The foal was euthanized and necropsy was performed. Mesenteric lymph nodes were large (approx 5 cm in diameter) and contained white caseous material. One abscess (approx 5 cm in diameter) was locally adhered to the serosa of the ventral aspect of the ascending colon; the subjacent mucosa was characterized by black discoloration for a length of approximately 10 cm orad and aborad from the abscess. A full-thickness perforation (1 cm in diameter) was evident, facilitating communication between the colonic lumen and the abscess. The lungs appeared normal on gross and histologic examinations.

Rhodococcus equi was isolated in pure culture from the eyes and blood (obtained antemortem). Rhodococcus equi and an Actinomyces sp were isolated from the mesenteric lymph nodes and the colonic abscess.

Although infection by R equi most commonly causes pneumonia in 4-to 5-month-old foals, concomitant intra-abdominal lesions have been reported in approximately half of affected foals.¹ Abdominal manifestations of R equi include enterocolitis, typhlitis, and multifocal granulomatous or suppurative mesenteric lymphadenitis. The presence of abdominal abscesses in the absence of lung disease is uncommon in foals infected with R equi; abscesses caused by R equi develop uncommonly in the abdomen without manifestation of pulmonary infection.¹

In the foal reported here, uveitis and tibiotarsal effusion were extrapulmonary signs of R equi infection. Uveitis has been attributed to sterile immune-mediated inflammation rather than direct infection by R equi, but as with this foal, R equi may be isolated from the eyes.² The hematologic changes suggested an infectious process, and severe anemia was attributed to immune-mediated changes or chronic inflammation.

Identification of an abscess that communicates with the alimentary tract lumen, as detected in the foal of this report, is unusual. The presence of gas within an abscess, detected during ultrasonography, was implicative for communication between the intestinal lumen and the abscess. Rhodococcus equi is not a gas-forming organism.

Identification of R equi from bacteriologic culture of blood is reported uncommonly. In 1 study,³ R equi was isolated from the blood of 1% of affected foals during an 8-year period. The case reported here indicates the value of ultrasonography when intra-abdominal abscesses are suspected. Intra-abdominal abscesses attributable to R equi infection may be present in the absence of pneumonia.