August 2019



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 DESCRIPTION: Dermis 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 irregular oval, refractile structure composed of rectangular to oval clear cells with 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. Myocytes of the panniculus muscle frequently exhibit vacuolization of the sarcoplasm with loss of cross striations (degeneration) and are surrounded by increased clear space and pale eosinophilic proteinaceous fluid (edema).


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 birefringent foreign material (presumed grass awn), beagle, canine.


ETIOLOGIC DIAGNOSIS: Actinomycotic dermatitis


CAUSE: Actinomyces spp.



       Opportunistic, gram-positive, non–acid-fast, filamentous anaerobic or microaerophilic rods

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

       Infection usually secondary to penetrating wounds of oral mucosa and skin



     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

       Involvement of bone causes reactive bone formation

     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



     Lesions most common on the head, neck, and extremities

     Abscesses, cellulitis, ulcerated nodules, draining tracts, dense fibrous nodules

o   Subcutaneous, firm, lumpy, white, glistening masses

o   Thin serosanguinous to thick purulohemorrhagic exudate

o   White/yellow/tan granules (sulfur granules – grossly visible colonies of bacteria)

o   Regional lymphadenopathy

     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

     Presence of swelling (tumefaction), draining tracts, and sulfur granules together is called an actinomycotic mycetoma



     Pyogranulomatous dermatitis and panniculitis

o   Central cores of neutrophils surrounded by epithelioid macrophages and variable numbers of multinucleated giant cells

o   Masses of filamentous and branching, basophilic- to amphophilic- staining organisms often surrounded by club-shaped, brightly eosinophilic Splendore-Hoeppli material (correspond to sulfur granules)

o   Dense fibrosis +/- granulation tissue between pyogranulomas

o   Fibrosis tends to be more prominent in actinomycosis vs. nocardiosis

o   Epidermis may be ulcerated and variably acanthotic

     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



     Nocardiosis: Gram-positive, filamentous, beaded 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 (fungal mycetomas)

o   Dogs – canine leproid granuloma

o   Cats – atypical mycobacteriosis, feline leprosy, tuberculosis

o   Cattle – Bovine cutaneous opportunistic mycobacteriosis, tuberculosis,



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

o   Nodules, abscesses, and draining tracts

     Pigs: Nodules commonly found on ventral abdomen and in udder

     Horses: Infection is rare

o   Poll evil, fistulous withers, mandibular lymphadenitis, and abscesses

     Goats: Lesions previously described in leg and shoulder



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

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

3.      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.

4.      Mauldin 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.

5.      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.

6.      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.

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


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