Overview

• An infectious disease caused by Gram-positive, branching, pleomorphic, rod-shaped bacteria of the genus Actinomyces.
• A. viscosus and A. hodeovulneris-most commonly identified isolates (though most isolates are not identified to the species level); survives in microaerophilic or anaerobic conditions.
• Rarely found as the single bacterial agent in a lesion; more commonly, it is a component of a polymicrobial infection.
• There may be synergism between Actinomyces and other organisms.
• Organ systems affected may include:
  • Skin
  • Respiratory
  • Cardiovascular
  • Musculoskeletal
  • Nervous.

Signalment

• Dogs and cats (uncommon).
• Most common in young male dogs of sporting breeds.

Signs

• Infections-usually localized; may be disseminated; cervicofacial area commonly involved.
• Cutaneous swellings or abscesses with draining tracts-yellow granules (“sulfur granules”) may be seen in associated exudates.
• Pain, fever, and weight loss.
• Exudative pleural or peritoneal effusions; occasionally pericardial effusion noted.
• Cough, dyspnea, decreased ventral lung sounds (empyema).
• Retroperitonitis-lumbar pain; rear limb paresis or paralysis.
• Osteomyelitis of vertebrae or long bones-probably secondary to extension of cutaneous infection; lameness or a swollen extremity may develop.
• Motor and sensory deficits-reported with spinal cord compression by granulomas.
• Pyothorax and subcutaneous bite wounds are the most common presenting signs in cats.

Causes & Risk Factors

• Actinomyces spp. normal inhabitants of the oral cavity of dogs and cats.
• Loss of normal protective barriers (mucosa, skin), immunosuppression, or change in the bacterial microenvironment can predispose; thought to occur as an opportunistic infection.
• Specific risk factors-trauma (bite wound), migrating foreign body (grass awn or, in the western United States, a foxtail), and periodontal disease.

Differential Diagnosis

• Nocardiosis-primary differential diagnosis; Actinomyces not reliably distinguished from Nocardia spp. by Gram staining, cytology, or clinical signs.
• Other causes of chronic draining tracts and pleural or peritoneal effusions must be addressed.

CBC/Biochemistry/Urinalysis

• Nonspecific changes.
• Leukocytosis with a left shift and monocytosis-reported.
• Nonregenerative anemia-may develop.
• Hypoglycemia and hyperglobulinemia-reported.

Imaging

• Radiographs of infected bone-periosteal new bone production, reactive osteosclerosis, and osteolysis.
• Thoracic radiographs-alveolar and interstitial lung patterns with possible lung consolidation; pleural effusion; pericardial effusion; subcutaneous masses on lateral thorax.
• Abdominal radiographs-peritoneal effusion; mass effect in abdomen.
• Vertebral column radiographs-periosteal new bone formation, especially T13–L3.

Diagnostic Procedures

• Pus or osteolytic bone fragments submitted in anaerobic specimen containers for culture (see Anaerobic Infections) can provide a definitive diagnosis; inform the lab to check for actinomycosis; advisable to submit aerobic culture, as well.
• Fresh smears-Gram staining, cytology, and acid-fast staining; staining does not preclude the need for culture; Actinomyces does not stain acid-fast; Nocardia is variable.

Pathologic Findings

Histopathologic examination-sulfur granules can be difficult to find so multiple tissue sections should be submitted; special stains may enhance visualization of organisms; granules are a useful diagnostic tool when present; pyogranulomatous or granulomatous cellulitis with colonies of filamentous bacteria is characteristic.

Drug(s)

• Important to distinguish between Actinomyces and Nocardia for appropriate antimicrobial selection.
• Antibiotics-a retrospective study suggests administration for a minimum of 3–4 months after resolution of all signs; may need to be directed against other associated microbes.
• Penicillins-considered the drug of choice; in most cases, oral therapy can be initiated and parenteral is not needed; amoxicillin should be administered at 20–22 mg/kg q8h PO.

Contraindications/Possible Interactions

• Metronidazole-avoid use; actinomycosis unlikely to respond.
• Aminoglycosides-do not use; ineffective against anaerobic infections.
• A. hordeovulneris-cell-wall deficient variant (l-phase); does not usually respond well to penicillin; consider clindamycin, erythromycin, and chloramphenicol.

Patient Monitoring

Monitor patients closely for recurrence in the months after therapy discontinued.

Prevention/Avoidance

Avoidance of contact with grass awns and prevention of bite wounds.

Possible Complications

Concurrent immune-suppressive disease or therapy may complicate management.

Expected Course and Prognosis

Redevelopment of infection at the initial site may be expected in about half of all cases.

Age-Related Factors

Young outdoor dogs.

Zoonotic Potential

There are no reported cases of actinomycosis being transmitted from animals to man; transmission by bite wound may be possible so appropriate attention should be given to bite wounds.