Issues

Actinomycosis

 

Actinomycosis is an infectious disease caused by Gram-positive, branching, pleomorphic, rod-shaped bacteria of the genus Actinomyces.

• 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.

 

DIAGNOSIS

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.

 

TREATMENT

• Exudative fluid (thorax, abdomen, subcutaneous tissue) should be drained and lavaged.
• A chest tube with continuous suction is needed for cats with pyothorax; dogs are best served with surgical exploration of the chest prior to tube placement in order to identify and remove any grass awns.
• Diseased lung lobes may need to be removed.
• Dogs with solitary masses involving the thoracic or abdominal wall may experience cure with radical surgical excision.

 

MEDICATIONS

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.
• hordeovulneris—cell-wall deficient variant (l-phase); does not usually respond well to penicillin; consider clindamycin, erythromycin, and chloramphenicol.

 

FOLLOW-UP

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.

 

MISCELLANEOUS

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.

Visit your veterinarian as early recognition, diagnosis, and treatment are essential.

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