THE MERCK VETERINARY MANUAL
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Overview of Actinomycosis

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Members of the genus Actinomyces are gram-positive, anaerobic, non-acid-fast rods, many of which are filamentous or branching. Branches are <1 μm in diameter, as opposed to fungal filaments, which are >1 μm in diameter. Although they are normal flora of the oral and nasopharyngeal membranes, several species are associated with diseases in animals.

A bovis is the etiologic agent of lumpy jaw in cattle. It has also been isolated from nodular abscesses in the lungs of cattle and infrequently from infections in sheep, pigs, dogs, and other mammals, including chronic fistulous withers and chronic poll evil in horses. Lumpy jaw is a localized, chronic, progressive, granulomatous abscess that most frequently involves the mandible, the maxillae, or other bony tissues in the head. Disease is seen when A bovis is introduced to underlying soft tissue via penetrating wounds of the oral mucosa from wire or coarse hay or sticks. Involvement of adjacent bone frequently results in facial distortion, loose teeth (making chewing difficult), and dyspnea from swelling into the nasal cavity. Any part of the head can be affected; however, the alveoli around the roots of the cheek teeth are frequently involved. The primary lesion appears as a slow-growing, firm mass that is attached to or part of the mandible. Ulceration forms in some cases, with or without fistulous tracts, and drainage of purulent exudate may occur. Presumptive diagnosis is often based on clinical signs. The diagnosis can be confirmed by culture of the organism from the lesion; however, this requires anaerobic conditions and is frequently negative. A Gram stain of purulent material will reveal gram-positive, club-shaped rods and filaments (sulfur granules). Radiology of the head is also useful; the primary radiographic lesion consists of multiple, centrally radiolucent areas of osteomyelitis surrounded by periosteal new bone and fibrous tissue. As a last resort, a biopsy sample can be taken with a trephine and submitted for histopathology.

The goal of treatment is to kill the bacteria and stop the spread of the lesion. However, the hard mass will usually not regress significantly. Sodium iodide is the treatment of choice in ruminant actinomycosis. Sodium iodide (70 mg/kg of a 10%–20% solution, IV) is given once and repeated several times at 7- to 10-day intervals. If signs of iodine toxicity develop (eg, dandruff, diarrhea, anorexia, coughing, and excessive lacrimation), iodine administration should be discontinued or treatments given at longer intervals. Sodium iodide has been shown to be safe for use in pregnant cows and presents little risk of causing abortion. Concurrent administration of antimicrobials, including penicillin, florfenicol, or oxytetracycline, is recommended. Surgery to debride large mandibular lesions has also been described in conjunction with iodine and antimicrobial therapy. Because A bovis is part of the normal oral flora in ruminants, control focuses on avoiding coarse, stemmy feeds or feeds with plant awns that might damage the mucosal epithelium. When multiple cases are seen in a herd, it is not from the contagious nature of the pathogen but the widespread exposure to a risk factor (eg, coarse feed).

A actinoides is occasionally found as a secondary invader in enzootic pneumonia of calves and seminal vasculitis in bulls.

A israelii is primarily associated with chronic granulomatous infections in people but has also been isolated rarely from pyogranulomatous lesions in pigs and cattle. Treatment involves surgical debridement and administration of penicillin.

A naeslundii has been isolated from suppurative infections in several animal species, the most common being aborted porcine fetuses.

A suis causes pyogranulomatous porcine mastitis, characterized by small abscesses containing thick, yellow pus surrounded by a wide zone of dense connective tissue. Yellow “sulfur granules” may be scattered throughout the pus, as in A bovis in cattle. Chronic, deep-seated abscesses may fistulate. Sows may also develop ventral subcutaneous granulomatous lesions, and occasional pyogranulomatous infections develop in lungs, spleen, kidneys, and other organs. Diagnosis is based on clinical signs and on isolation and identification of the etiologic agent. Treatment is rarely successful, primarily due to the inability of an antibacterial agent to penetrate the infected tissue. Infected tissue may be surgically removed to salvage sows for slaughter.

A hordeovulneris is a rare cause of canine actinomycosis, which can present with either localized abscesses or systemic infections such as pyogranulomatous pleuritis, peritonitis, visceral abscesses, or septic arthritis. A common predisposing factor is the presence of tissue-migrating foxtail grass (Hordeum spp) particles, and the primary route of infection appears to be via inhalation of the bacteria. History and clinical signs may contribute to the diagnosis, but demonstration of the causative agent by Gram stain and bacteriologic culture is necessary for confirmation. Treatment includes surgical debridement and/or longterm treatment with penicillin, cephalosporins, or sulfonamides. Pyothorax is frequently seen in canine actinomycosis and requires repeated drainage of the chest in addition to antimicrobial therapy.

A viscosus causes cutaneous actinomycosis in dogs, which appears as localized subcutaneous abscesses. These usually occur secondary to perforating injuries caused by bite wounds or foreign bodies. The most common sites for abscesses are the head, neck, thorax, and abdomen. A viscosus can also cause pneumonia, pyothorax, and rarely pyogranulomatous meningoencephalitis. Diagnosis may be based on history and clinical signs, including the presence of soft, grayish white granules in the pus or exudate. Cytology (of pus or pleural fluid) is useful and will reveal gram-positive, filamentous organisms. Definitive diagnosis is based on isolation and identification of A viscosus. Treatment of pyothorax with penicillin, sulfonamides, or cephalosporins may be successful if begun early in the clinical course. A successful outcome is more likely with cutaneous infections, which should also be treated with the same antimicrobials.

Last full review/revision November 2013 by Geof W. Smith, DVM, PhD

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