Actinomycetes are gram-positive anaerobic bacteria that cause disease primarily in cattle but also in other ruminant species, swine, and horses. Actinomycosis, or lumpy jaw, is a localized, chronic, progressive, granulomatous abscess and osteomyelitis that most frequently involves the mandible and other bony tissues in the head. Presumptive diagnosis is often based on clinical signs. The diagnosis can be confirmed by culture of the organism or stained aspirates; however, the organism can be difficult to isolate. The goal of treatment is to kill the bacteria, control inflammation, and stop the spread of the lesion. Bony changes from this infection are not reversible.
Actinomycosis, or lumpy jaw, is caused by bacteria in the genus Actinomyces, which are gram-positive, anaerobic, non–acid-fast rods, many of which are filamentous or branching. Branches are < 1 mcm in diameter, as opposed to fungal filaments, which are > 1 mcm in diameter.
Although they are normal microbiota of the oral and nasopharyngeal membranes, Actinomyces spp are associated with disease in a number of animal species.
Actinomyces bovis is the etiological agent of lumpy jaw, which is a pyogranulomatous abscess and osteomyelitis syndrome of cattle. A bovis, the causative organism of actinomycosis, is part of the normal oral microbiota of ruminants. Disease occurs when A bovis is introduced to underlying soft tissue via penetrating wounds of the oral mucosa from coarse or thick-stemmed ("stemmy") feeds, sticks, thorns, or wire. 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.
Actinomyces actinides is occasionally found as a secondary invader in enzootic pneumonia of calves and vesicular stomatitis in bulls.
Actinomyces israelii is primarily associated with chronic granulomatous infections in humans but has also been isolated rarely from pyogranulomatous lesions in pigs and cattle.
Actinomyces naeslundii in several animal species has been isolated from suppurative infections, most commonly in aborted porcine fetuses.
A denticolens (formerly A suis) causes mastitis as well as subcutaneous granulomatous lesions in sows. Some bacteria previously classified as A suis isolates have been reclassified as A denticolens, which has also been reported to cause mandibular lymphadenopathy in horses. Thus, A denticolens is a pathogen in a broad range of animals.
Actinomyces hordeovulneris is a rare cause of actinomycosis in dogs, characterized by skin lesions, pleuritis, peritonitis, or internal abscesses. 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.
Actinomyces viscosus also causes cutaneous and other manifestations of actinomycosis in dogs. Cases usually occur secondary to perforating injuries caused by bite wounds or foreign bodies.
Clinical Findings of Actinomycosis in Cattle and Other Animals
When Actinomyces spp are inoculated into tissue, pyogranulomatous or suppurative disease develops. Lumpy jaw, caused by Actinomyces bovis, is a localized, chronic, progressive, granulomatous abscess in ruminants that most frequently involves the mandible, the maxillae, or other bony tissues in the head.
Any bony location can be affected; however, the mandible and alveoli around the roots of the cheek teeth are most frequently involved (see actinomycosis and lumpy jaw with suppurative lesion images). The primary lesion appears as a slow-growing, firm mass that is attached to, or is part of, the mandible. In some cases, ulceration with or without fistulous tracts forms, and drainage of purulent exudate can occur. Yellow flecks called “sulfur granules” (actually clusters of immune cells and bacterial components) may be scattered throughout the pus.
Involvement of adjacent bone frequently results in facial distortion, loose or maloccluded teeth (making chewing difficult), and, less frequently, dyspnea from swelling into the nasal cavity.
Courtesy of Dr. Geoffrey Smith.
Courtesy of Dr. John Prescott.
A denticolens (previously classified as A suis) has been reported to cause mandibular lymphadenopathy in horses with fever, nasal discharge, and malaise.
A denticolens also causes pyogranulomatous porcine mastitis, characterized by small abscesses containing thick yellow pus surrounded by a wide zone of dense connective tissue. "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.
Cases of A hordeovulneris actinomycosis in dogs can involve either localized abscesses or systemic infections, such as pyogranulomatous pleuritis, peritonitis, visceral abscesses, and septic arthritis.
A viscosus causes cutaneous actinomycosis in dogs, which appears as localized subcutaneous abscesses. The most common sites for abscesses are the head, neck, thorax, and abdomen. Pneumonia and pyothorax have also been described with A viscosus, along with rare cases of pyogranulomatous meningoencephalitis.
Diagnosis of Actinomycosis in Cattle and Other Animals
The process of evaluating cattle and other animals for actinomycosis can include the following steps:
Clinical examination for the presence of a hard immovable mass of or on the mandible or other bony structure
Radiography to demonstrate osteomyelitis, tooth involvement, or pathological fracture (see actinomycosis radiograph)
Anaerobic culture of aspirates; however, negative results of culture do not rule out actinomycete involvement
Biopsy of the lesion
Gram stain of aspirated material; Actinomyces are gram-positive, branching, filamentous bacteria (see Gram-stain image)
Courtesy of Dr. Geoffrey Smith.
Courtesy of Dr. John Prescott.
Presumptive diagnosis of actinomycosis 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 results of culture are frequently negative.
Cytological evaluation is useful because a Gram stain of purulent material will reveal gram-positive, club-shaped rods and filaments.
Radiographic examination of the head is also useful; the primary radiographic lesion consists of multiple central radiolucent areas of osteomyelitis surrounded by periosteal new bone and fibrous tissue.
A biopsy sample can be taken with a trephine and submitted for histological examination.
In canine actinomycosis, history and clinical signs may contribute to the diagnosis; however, demonstration of the causative agent by Gram stain and bacteriological culture is necessary for confirmation.
With cutaneous actinomycosis, soft, grayish-white granules are frequently visible in the pus or exudate. Cytological evaluation (of pus or pleural fluid) is quite useful and will reveal gram-positive, filamentous organisms.
Treatment of Actinomycosis in Cattle and Other Animals
Sodium iodide administered IV in the treatment of ruminants
Antimicrobials, such as penicillin, florfenicol, and oxytetracycline
Oral iodine salts
The goal of treatment for actinomycosis is to kill the bacteria and stop the spread of the lesion. Bony deformations, however, will not regress much.
The treatment of many actinomycete infections involves surgical debridement in addition to antimicrobials because granulomatous inflammation protects the bacteria from antimicrobial exposure in many cases.
In ruminants, the treatment of choice is sodium iodide (70 mg/kg of a 10%–20% solution, IV, repeated several times at 7- to 10-day intervals). If clinical signs of iodine toxicity develop (including dandruff, diarrhea, anorexia, coughing, and excessive lacrimation), administration should be discontinued or treatment intervals increased. Sodium iodide, while not approved as a drug in the US, is marketed and commonly used, and its use is expected to be of low regulatory priority. More information on unapproved drugs can be found on the FDA's Unapproved Animal Drugs website.
Sodium iodide does not have direct antimicrobial activity but may promote cellular oxidative pathways that can inhibit certain bacteria. In the US, the product label for sodium iodide states that it should not be used in pregnant cattle. However, studies have not been able to document negative impacts to pregnancy (1). Concurrent administration of antimicrobials (eg, penicillin, florfenicol, or oxytetracycline) is recommended to enhance inhibition and killing of the pathogen.
Treatment of A denticolens infections in pigs is rarely successful, primarily due to the inability of an antimicrobial agent to penetrate the infected tissue. Infected tissue may be surgically removed to salvage sows for slaughter.
In dogs with A hordeovulneris, treatment includes surgical debridement and longterm treatment with penicillin, cephalosporins, or sulfonamides. Pyothorax is frequently present in canine actinomycosis, and requires repeated drainage of the chest in addition to treatment with antimicrobials. Treatment of pyothorax due to A viscosus 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.
Prevention of Actinomycosis in Cattle and Other Animals
Because A bovis is part of the normal oral microbiota in ruminants, control focuses on avoidance of coarse, stemmy feeds or feeds with plant awns that might damage the mucosal epithelium. Infections are most prevalent in cattle in wintertime or drought situations where high-quality feedstuffs run short, leaving low-quality, stemmy feedstuffs as the only available feeds.
When multiple cases occur in a herd, it is not by a contagious mechanism but rather from the widespread herd exposure to a risk factor (ie, coarse feed).
Key Points
Although they are normal microbiota of the oral and nasopharyngeal membranes, Actinomyces spp are associated with disease in a number of animal species, including lumpy jaw in cattle.
Diagnosis can be confirmed by culture, but false-negative results are common because Actinomyces spp can be difficult to culture.
The treatment of many actinomycete infections involves surgical debridement in addition to antimicrobials because granulomatous inflammation protects the bacteria from antimicrobial exposure in many cases
Control focuses on avoidance of coarse feeds or feeds with plant awns that might damage the mucosal epithelium.
For More Information
Smith BP. Actinomycosis (lumpy jaw). In: Smith BP, Pusterla N, eds. Large Animal Internal Medicine. 6th ed, Elsevier; 2020:811-813.
Sykes JE. Actinomycosis. In: Sykes JE, ed. Greene's Infectious Diseases of the Dog and Cat. 5th ed. WB Saunders; 2021:704-713.
Also see pet health content on actinomycosis in horses.
References
Miller HV, Drost M. Failure to cause abortion in cows with intravenous sodium iodide treatment. J Am Vet Med Assoc 1978;172(4):466-467.
