Osteomyelitis can be caused by bacterial or fungal agents or may be idiopathic. Cocci, bacilli, and filamentous bacteria such as members of the Actinomycetes have all been determined to be causes of osteomyelitis. Differential diagnoses for Gram-positive filamentous rods in the family Actinomycetales causing osteomyelitis in the dog, include members of the more frequently encountered genera Nocardia and Actinomyces. Bacteria gain access to the bone via several routes but are most often associated with direct inoculation (such as percutaneous injuries, compound fractures, or secondary to foreign bodies such as surgical or other material including dirt and wood) and fracture instability. Less frequently, the route is hematogenous, as has been found with Propionibacterium acnes. Agents isolated from osteolytic lesions in dogs and cats have included Gram-positive Staphylococcus spp., Streptococcus spp., Gram-negative Escherichia coli, Proteus spp., Klebsiella spp., Pseudomonas spp., anaerobic Clostridium spp., Peptostreptococcus spp., Actinomyces spp., Bacterioides spp., Fusobacterium spp., and rarely Brucella canis, Nocardia spp., and Mycobacterium avium. Fungal causes include Coccidioides immitis, Blastomyces dermatitidis, Histoplasma capsulatum, Cryptococcus neoformans, and Aspergillus spp. Osteosarcoma with associated cellulitis is a reported noninfectious cause of osteomyelitis in dogs. Our report details the diagnosis, treatment, and resolution of osteomyelitis in a dog caused by a unique agent, Nocardiopsis composta
