Diagnosis:  Rhodococcus equi  in a horse

History:   A 4-month-old Tennessee walking horse colt was submitted to the Indiana Animal Disease Diagnostic Lab at Purdue University for necropsy.  The submitter reported an approximately 1-1/2 month history of pneumonia and osteolytic lesions in the right distal cannon bone and fetlock joint.  The foal was treated with antibiotics and had two arthroscopic surgeries on the affected joint, but did not show significant signs of improvement and was subsequently euthanized.

Gross findings:  The right fetlock joint contained moderate amounts of cloudy tan to white fluid, with numerous strands of fibrin.  The synovial membranes were thickened and covered by tan to yellow exudates that extended to and infiltrated into the deep digital flexor and common digital extensor tendons as well as the interosseous ligament.  The distal physis of the cannon bone was lytic, had roughed irregular surfaces, and was coated by tan to pink, thick, creamy material.

  The lungs were diffusely mottled dark red to pink, wet, heavy, and oozed red froth on cut section.  Numerous (10-15) firm, tan to red, raised, round to irregularly shaped nodules ranging in size from 2X2X1.5 - 8X8X6 cm were scattered throughout all lung fields.  The nodules contained abundant thick , tan to yellow, creamy exudate.     The tracheobronchial lymph nodes ranged in size from 3-7 cm in diameter and were tan to red, slightly firm and, when incised, contained copious amounts of creamy tan material.

Histologic findings:  The distal physis of the cannon bone was segmentally separated from the epiphysis.  At the interface with the epiphysis, the physis was necrotic with a fibrillated surface; there were decreased numbers of cells with few small clusters of degenerate chondrocytes, and it was segmentally infiltrated by numerous neutrophils and fewer macrophages.  Marrow spaces of the metaphysis and epiphysis were infiltrated largely by neutrophils with fewer macrophages and a loose meshwork of fibrous connective tissue.  Macrophages had abundant foamy eosinophilic cytoplasm that was stippled with basophilic rod shaped bacteria.  Bony trabeculae had a scalloped surface, were multifocally fragmented, and segmentally lacked a peripheral rim of osteoblasts.  There were increased numbers of osteoclasts along the peripheral trabeculae.  Inflammatory cells extended through the periosteum and into the adjacent soft tissues forming pyogranulomas.

  Pulmonary airways and adjacent alveolar spaces were multifocally effaced by large aggregates of neutrophils, macrophages, and multinucleated giant cells.  The mononuclear cells contained variable numbers of round, eosinophilic, intracytoplasmic bacteria (bacilli).  Surrounding and coursing through the inflammatory aggregates were thick mats of mature fibrous connective tissue.  An area of coagulative necrosis was focally circumscribed by clusters of neutrophils and fibrous tissue, consistent with an abscess.  Within the less severely affected segments of lungs, the alveolar spaces contained numerous macrophages and fewer neutrophils.  Alveolar septa were segmentally lined by type II pneumocytes.

Cranial ventral aspect of left lung lobe : pulmonary abscess	Right fetlock joint: suppurative tenosynovitis and osteomyelitis

Ancillary findings:  A swab from the fetlock joint and sections of lung, spleen, lymph node, and small intestine were submitted for aerobic bacterial culture.  Rhodococcus equi was isolated from the lungs, fetlock joint, spleen, and lymph node.  A goodpasture gram stain was applied to sections of cannon bone and lung and revealed numerous, intracytoplasmic, gram positive bacilli within macrophages.

Discussion:    Characteristic gross and microscopic lesions, coupled with positive bacterial cultures and identification of intracytoplasmic, gram positive bacilli, were consistent with Rhodococcus equi infection in this foal.  R. equi is a common pathogen in foals that is predominantly seen in animals less than six months of age.  The primary target organs are the lungs, intestines, and associated lymph nodes with resultant pyogranulomatous broncho-pneumonia, ulcerative typhlocolitis, and pyogranu-lomatous lymphadenitis, respectively.  Once the infection is established, hematogenous dissemination may occur to the liver, spleen, bone/joint, and skin.  Additionally, R. equi has been documented to cause abscesses in swine, sheep, cats, cattle, llamas.

  R. equi is a gram positive facultative intracellular bacteria that is found in soil.  Most infections are acquired through inhalation or ingestion of soil-borne organisms and virulence is attributed to at least two predominant mechanisms.  Replication occurs within the cytoplasm of macrophages and is associated, at least in part, to the expression of the plasmid-encoded, virulence-associated protein, VapA.  Additionally, virulence is achieved by preventing lysis through inhibiting the conversion of phagosomes to phagolyosomes.

  Antemortem diagnosis is based on clinical signs, evidence of pulmonary abscess on thoracic radiographs, and aerobic bacterial cultures of transtracheal wash fluid, abdominal fluid, or synovial fluid.

-by Dr. Chad Frank, ADDL Graduate Student

References:

1. Caswell JL, Williams KJ: 2007.  Respiratory system.  IN: Jubb, Kennedy and Palmer's Pathology of Domestic Animals.  5^th ed., Vol. 2.  Ed. Maxie MG.  Saunders-Elsevier, Philadelphia, PA.  pp. 630-632.

2. Hondalus MK: 1997.  Pathogenesis and Virulence of Rhodococcus equi.  Veterinary Microbiology 56: 257-268.

3. Smith BP: 2002.  Large Animal Internal Medicine.  3^rd ed.  Mosby, St. Louis MO.