August 2016



Signalment (JPC #4083133-00):  Two-year-old male intact beagle dog.


HISTORY: This hunting dog developed a large mass on the left lateral thorax wall with a draining tract into the subcutaneous tissue.


HISTOPATHOLOGIC DESCRIPTIONDermis and subcutis (per contributor), left lateral thorax:  Expanding the deep dermis and subcutis, and extending to all borders is a large nodule of pyogranulomatous inflammation which is centered on a 5 x 3 mm vaguely triangular, refractile structure composed of rectangular to oval clear cells with a thick walls (foreign organic material consistent with a grass awn). Multifocally adhered to (and adjacent to) the grass awn there are large colonies of basophilic filamentous 1 x 3-7 µm bacteria admixed with finely granular basophilic material (sulfur granules).  The grass awn and colonies of filamentous bacteria are surrounded by areas of drop-out with scattered eosinophilic and cellular debris and loss of normal dermal architecture (lytic necrosis). These necrotic foci are surrounded by numerous degenerate and fewer viable neutrophils, epithelioid macrophages, fewer lymphocytes, plasma cells and occasional Langhans-type multinucleated giant cells, admixed with eosinophillic beaded fibrillar material (fibrin), hemorrhage and hemosiderin-laden macrophages. Nodules are further surrounded by haphazardly arranged, hypertrophied fibroblasts embedded in loose fibrous connective tissue with numerous small caliber vessels lined by hypertrophic endothelium (granulation tissue).  Granulation tissue progresses to thick bands of mature fibrous connective tissue further away from the necrotic center.  In less affected areas of the dermis and subcutis there are perivascular infiltrates of lymphocytes and plasma cells.  


MORPHOLOGIC DIAGNOSIS:  Dermis and subcutis (per contributor), left lateral thorax:  Dermatitis and panniculitis, pyogranulomatous, chronic, focally extensive, severe with granulation tissue, fibrosis, sulfur granules, large colonies of filamentous bacteria, and a presumed grass awn, consistent with Actinomyces sp., beagle, canine.


ETIOLOGIC DIAGNOSIS:  Actinomycotic dermatitis


CAUSEActinomyces sp.



      Opportunistic, gram-positive, non–acid-fast, filamentous (beaded, branching) anaerobic or microaerophilic rods

      Commensal inhabitants of the oral cavity, intestine, and upper respiratory tract

      Pyogranulomatous inflammation with marked encapsulating fibrosis in domestic animals, livestock and wildlife

o    Infection usually secondary to penetrating wounds of oral mucosa and skin

o    Organized masses (30-3,000 µm or more in diameter) of basophilic or amphophilic staining organisms

o    Commonly bordered by clubbed corona of brightly eosinophilic Splendore-Hoeppli material

      Many species of Actinomyces have been cultured from dogs and cats

      Two primary forms of actinomycosis in dogs:

o    Cervicofacial:  Subcutaneous infection extending into adjacent tissue / bone

o    Thoracic, abdominal and/or retroperitoneal

    Difficult to culture, especially if the animal is on antibiotics

    Most actinomycotic infections mixed with 3-5 additional commensal bacterial species

    A. hordeovulneris, are saprophytes, commonly associated with grass awns



    Infection depends on disruption of mucosal or epidermal barriers by abrasion from coarse roughage (cattle), foreign body penetration (migrating grass awns, dogs), bite wounds (cats) or secondary to chronic periodontal disease

    Spreads by direct extension along tissue planes > invade adjacent structures, including bone

    Hematogenous dissemination is rarely reported

    Pneumonia may follow aspiration or esophageal perforation

    Actinomyces spp. induce neutrophil chemotaxis, activate macrophages and stimulate B-lymphocyte hyperplasia

    Proteolytic enzymes from the macrophages and degranulated neutrophils disrupt connective tissue, facilitating spread of the bacterium through normal tissue planes



    Subcutaneous:  Painful lymphadenopathy associated with granulomas or abscesses, draining sinuses, and radiographic evidence of osteomyelitis in underlying bone

    Thoracic:  Cough, dyspnea, decreased lung sounds and pyothorax

    Abdominal:  Palpable granuloma in GI tract and abdominal distension

    Retroperitoneal:  Back pain and rear leg paresis/paralysis



    Localized infection of the cervicofacial region, thorax, abdomen, retroperitoneal space, and limbs:

o   Usually tender, subcutaneous, firm, lumpy, white, glistening masses

o   +/- draining fistulous tracts

o   Yellow to red pus with yellow/tan granules ("sulfur granules" – grossly visible colonies of bacteria)

o   Infections in cats due to fight wounds are exudative

    Intracavitary infection:

o   Red, velvety proliferation of parietal pleura, peritoneum or omentum; visceral pleura and peritoneum are less often affected

o   Variable amount of reddish-brown ("tomato soup") exudate

    Granulomatous masses may involve one or more adjacent organs or structures

    Involvement of bone causes reactive bone formation



    A central core of neutrophils +/- "sulfur granules", encapsulated by fibrosing granulation tissue, epithelioid macrophages, lymphocytes, plasma cells and occasional Langhans-type giant cells

o   Fibrosis tends to be more common and severe in actinomycosis versus nocardiosis

    "Sulfur granules" are variably-sized, irregular, basophilic rosettes of filamentous, beaded bacteria admixed with necrotic debris; colonies of bacteria may be rimmed by eosinophilic, club-shaped material (Splendore-Hoeppli material) and often contain entrapped degenerate neutrophils

    Chronic lesions may be mineralized

    Osteomyelitis of the mandible or other bones may occur



    Envelope is tri-layer and composed of an inner plasma membrane, a thick cell wall with densely staining internal layer, and a thick capsule with numerous pili



    Clinicopathologic abnormalities:  Nonregenerative anemia, leukocytosis, monocytosis, hypoglycemia, hypoalbuminemia and hyperglobulinemia

    Gram stain (Brown & Brenn):  Gram-positive clumps of tangled filamentous bacteria

    Cytology: Impression smears of cut surface of nodules or smears of "sulfur granules"

    Culture: Facultative or obligate anaerobe

    Polyclonal BCG antibody technique as screening test



    Nocardiosis:  Gram-positive, filamentous rods; +/- "sulfur granules", modified acid-fast stain (Fite-Farraco) will stain Nocardia spp., but not Actinomyces spp.

    Actinobacillosis:  Gram-negative rods

    Botryomycosis (Staphylococcus spp.):  Gram-positive cocci

    Other granulomatous diseases including foreign body reactions, mycobacterial infections, and deep mycoses



    Cattle:  A. bovis causes mandibular and rarely maxillary osteomyelitis (lumpy jaw)

    Pigs:  A. bovis and A. suis cause liver abscesses and mastitis; A. hyovaginalis causes well-circumscribed, necrotizing pneumonia

    Horses:  A. viscosus, A. denticolens, and A. bovis can cause ‘poll evil’ and ‘fistulous withers’ (often in conjunction with Brucella abortus), associated with mandibular lymphadenitis, and abcessation

    Sheep:  Generally resistant to infection



1.       Cheville NF. Ultrastructural Pathology: The Comparative Cellular Basis of Disease. 2nd ed. Wiley-Blackwell, 2009: 487-491.

2.       Fielding CL, Magdesian KG, Morgan RA, Ruby RE, Sprayberry KA. Actinomyces species as a cause of abscesses in nine horses. Vet Rec. 2008; 162:18-20.

3.       Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Skin Diseases of the Dog and Cat. 2nd ed. Oxford, England: Blackwell Science; 2005: 272-275.

4.       Hargis AM, Myers S. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Mosby Elsevier; 2016: 1049, 1077.

5.       Lloyd D. Bacterial skin disease. In: Miller WH, ed. Muller & Kirk's Small Animal Dermatology. 7th ed. St Louis: Elsevier; 2013: 184-222.

6.       Maudlin EA, Peters-Kennedy J.  Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 629, 636-639.

7.       Schumacher VL, Hinckley L, Gilbert K, Risatti GR, Londono AS, Smyth JA. Actinomyces hyovaginalis- associated lymphadenitis in a Nubian goat. J Vet Diagn Invest. 2009; 21: 380-384.

8.       Smith BP. Actinomycosis. In: Smith BP, ed. Large Animal Internal Medicine. 5th ed. St. Louis, MO: Mosby Elsevier; 2015: 743.

9.       Song RB, Vitullo CA, da Costa RC, Daniels JB. Long-term survival in a dog with meningoencephalitis and epidural abscessation due to Actinomyces species. J Vet Diagn Invest. 2015; 27(4):552-557.

10.    Sykes JE. Actinomycosis and nocardiosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. St. Louis, MO: Elsevier Saunders; 2012: 484-495.

11.    Valetine BA. Skeletal muscle. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Mosby Elsevier; 2016: 926, 942.






Actinomyces bovis

Cattle, swine, horses, elk

Mandible, maxilla (lumpy jaw), soft tissues or generalized

A. israelii

Humans, cattle, swine

Soft tissues, may be generalized



Fetus (abortion)

A. odontolyticus


Periodontal disease, eye, subcutis, lungs

A. hordeovulneris


Pleuritis, peritonitis, visceral abscesses, arthritis

A. suis


Mammary gland, fetus (abortion)

A. hyovaginalis



A. viscosus

Humans, swine, cats, dogs, hamsters

Bony and soft tissues, periodontal disease

A. denticolens


Oral cavity commensal

A. howellii


Oral cavity commensal

A. humiferus



Adapted from Jones TC, Hunt Rd, King NW: Veterinary Pathology. 6th ed., pp. 485 Williams and Wilkins, Philadelphia, PA, 1997

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