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Actinomycosis in Dogs

Actinomycosis is an infectious disease caused by gram positive, branching, pleomorphic (can change shape somewhat between a rod and coccus), rod-shaped bacteria of the genus Actinomyces, most commonly the A. viscosus species. Able to survive with little (microaerophilic) or no oxygen (anaerobic), Actinomyces is rarely found as the single bacterial agent in a lesion. It is more often a component of a polymicrobial infection with several bacteria present. In fact, there may even be synergism between Actinomyces and other organisms.

Symptoms and Types

• Pain and fever
• Infections on the face or neck area; usually localized but may be spread out
• Skin swellings or abscesses with draining tracts; sometimes yellow granules
• Inflammation of the cellular tissue behind the peritoneum, the smooth membrane which lines the abdomen (retroperitonitis)
• Inflammation of the bone or vertebrae (osteomyelitis), especially long bones such as those found in the limbs; this is secondary to the skin infection
• When associated with spinal cord compression, motor and sensory deficits (i.e., trouble walking, touching, etc.) 

Causes

Actinomycosis is thought to occur as an opportunistic infection; i.e., Actinomyces spp. is a normal inhabitant of dog's mouth, but cuts, scrapes, or bite wounds in the mucosa or skin can cause an imbalance in the bacterial microenvironment. Other risk factors include periodontal disease and immunosuppressive disorders.

Diagnosis

You will need to give a thorough history of your dog’s health, including the onset and nature of the symptoms, to the veterinarian. He or she will then perform a complete physical examination as well as a biochemistry profile, urinalysis, complete blood count, and electrolyte panel. X-rays of dogs with actinomycosis will typically demonstrate periosteal (outer layer of bone) new bone production, reactive osteosclerosis (hardening of bone), and osteolysis (dissolution of bone).

For a more definitive diagnosis your veterinarian will submit a specimen of pus or osteolytic bone fragments for culturing. Gram staining, cytology, and acid-fast staining may also be employed.