The clinical, laboratory, radiographic, and histologic features and the response to therapy in three dogs with actinomycosis are reported. One dog (dog 1) had a 12-cm nonresectable mass extending from the ventrolateral chest wall into the left ventricular myocardium. Another dog (dog 2) had a diffuse peritonitis with "sulfur granules" and two large masses. One of these masses was nonresectable involving adjacent abdominal structures. A third dog (dog 3) had a subvertebral mass at T1-3 producing quadraplegia. Two dogs had periosteal reactions involving adjacent sternebrae (dog 1) or ribs and vertebral bodies (dog 3) that are characteristic of Actinomyces spp infections. In dogs 1 and 2 the diagnosis was based on the morphologic and tinctorial properties of free sulfur granules and/or tissue granules. Culture results were variable. Tissue from dog 1 yielded no growth, while polymicrobial infections, which included Actinomyces spp, were identified in dogs 2 and 3. Actinomyces odontolyticus was isolated from dog 3. Although the actinomycotic granulomas were either not excised or only partially excised from dogs 1 and 2, both animals were cured by the oral administration of high doses of penicillin G for 19 and 6 months, respectively. Dog 3 responded dramatically to the same antibiotic therapy given for 5 months. However, within 4 months of discontinuing treatment an abscess and draining fistulous tracts developed in the left axillary region. Two surgical fistulectomies and additional penicillin therapy were required to cure this animal. These cases and the current veterinary and human literature on actinomycosis are used to propose a rational approach to the treatment of actinomycosis in the dog.
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