Overview

• An uncommon infection of dogs and cats.
• Organism-soil saprophyte; enters body through contamination of wounds or by respiratory inhalation.
• A compromised immune system enhances the likelihood of infection.

Systems Affected

• Lymphatic
• Musculoskeletal
• Nervous
• Respiratory
• Skin/Exocrine

Signalment

Dogs and cats of any breed

Signs

• Depends on the site of infection.
• Pleural-pyothorax, resulting in dyspnea, emaciation, and fever.
• Cutaneous-chronic, non-healing wounds; often accompanied by fistulous tracts; if extended, may result in lymphadenopathy, draining lymph nodes, and osteomyelitis.
• Disseminated-most common in young dogs; usually begins in the respiratory tract; lethargy, fever, and weight loss; cyclic fever may be characteristic; CNS may be affected; pleural and/or abdominal effusion may occur.
• May cause pneumonia or pyothorax in cats.

Causes & Risk Factors

• Nocardia asteroides (dogs and cats)
• N. brasiliensis (cats only)
• N. nova (common in Australia; now in United States
• Proactinomyces spp. (rare)

Differential Diagnosis

CBC/Biochemistry/Urinalysis

• Neutrophilic leukocytosis.
• Nonregenerative anemia-with long-standing infections (anemia of chronic disease).
• Chemistries-usually normal; hypergammaglobulinemia may be seen with long-standing infections.

Other Laboratory Tests

N/A

Imaging

Radiographs-may reveal pleural or peritoneal effusion, pleuropneumonia, or osteomyelitis.

Diagnostic Procedures

• Cytology-thoracentesis or abdominocentesis for samples; stain these or other exudates with Romanowsky, gram, and modified acid-fast stains for rapid diagnosis; may reveal gram-positive branching filamentous rods and cocci; cannot be distinguished from Actinomyces spp.
• Culture-diagnostic; aerobic culturing on Sabouraud medium.

Pathologic Findings

• N. asteroides-more suppurative pyogranulomatous reaction than with Actinomyces spp.
• N. brasiliensis-granulomatous reaction with extensive fibrosis.
• Although the organism is usually present, it cannot be distinguished histopathologically from Actinomyces spp.

Drug(s)

• Cultured organism-antibiotic sensitivity testing.
• No culture or results pending-good first-choice drugs: sulfonamides (e.g., sulfadiazine at 100 mg/kg IV, PO as a loading dose followed by 50 mg/kg IV, PO q12h) and sulfonamide-trimethoprim combinations (15–30 mg/kg PO q12h).
• Aminoglycosides-gentamicin (3 mg/kg IV, IM, SC q8h); amikacin (6.5 mg/kg IV, IM, SCq8h).
• Tetracyclines-doxycycline (10 mg/kg PO q24h); tetracycline hydrochloride (15–20 mg/kg PO q8h); minocycline (5–12.5 mg/kg PO q12h).
• Erythromycin 10–20 mg/kg PO q8h; or combined with ampicillin (20–40 mg/kg PO q8h) or amoxicillin (6–20 mg/kg PO q8–12h).
• Amoxicillin or ampicillin plus an aminoglycoside-synergistic combination; consider in any serious infection when culturing is not possible or is pending.
• Average treatment period is 6 weeks; however, medical treatment should extend several weeks past apparent remission of the disease.

Contraindications/Possible Interactions

• Tetracyclines (cats)-may cause fever up to 41.5°C (107°F); discontinue and replace if fever increases during therapy.
• Long-term sulfonamide-trimethoprim may cause anorexia and irreversible bone marrow suppression in cats. Prevent by supplementing with folate administration (1 mg PO q24h).

See Also

Actinomycosis

Abbreviation

CNS = central nervous system

Authors Gary D. Norsworthy and Lisa Restine

Consulting Editor Stephen C. Barr