Actinomycosis is a slowly progressive, pus-forming infection caused by branching bacteria of the genus Actinomyces. Classically, actinomycosis is a disease of cattle, but it also occurs in other animal species, and as a facial and oral infection in man. Actinomyces bacteria normally live in the mouth, the nasal passages near the throat, and the bowels. Actinomyces may cause disease due to poor oral hygiene, dental and periodontal problems, trauma and following oral surgical procedures. Pulmonary actinomycosis may develop as a result of aspiration of actinomyces that originated from dental plaque and diseased gums. Several species are associated with the disease in dogs: Actinomyces bovis, Actinomyces hordeovulneris, Actinomyces canis and Actinomyces viscosus. Hunting or field dogs in southern areas are most commonly affected. Actinomyces bovis causes loose teeth and difficulty breathing due to swelling of the nasal cavity. Actinomyces hordeovulneris causes abscesses in the liver and spleen and generalized infections in the cavities surrounding the lungs and bacterial arthritis.
Two forms of actinomycosis are usually seen in dogs. The more common is a skin abscess or mycetoma. This form, resulting from a skin wound, usually responds well to treatment. Actinomyces viscosus causes chronic pneumonia, inflammation of cavities surrounding the lungs, and skin abscesses associated with fever, pain and swelling of the skin. Canine interdigital actinomycosis is especially likely to result from foxtail, grass awns, or quills foreign body penetration.
Skin form of actinomycosis is diagnosed based on skin cell examination, bacteriologic culture, or tissue examination. Abscesses, draining fistulous tracts, and granulomas that develop can be misinterpreted as neoplasms on radiographs or ultrasonograms. Treatment of actinomycosis depends on the bacterium species that causes the disease. The skin form of actinomycosis may require surgical cleaning, drainage, and administration of antibiotics. High doses of penicillins given for prolonged periods (weeks to months) is the treatment of choice.1 Amoxycillin and clindamycin may or may not be effective.2 Minocyclin hydrochloride (Minosin, Arestin), and rifampin (Rifadin, Rifactane) have also been used. Major limiting factor associated with rifampin is rapid development of resistance, thus the need to combine this antibiotic with other agents. The drug is very toxic to the liver.4 In some dogs there may be recurring infections that will require several surgical procedures.
actinomycosis - in dogs and cats it is a localized infection most often under the skin around the head and neck, bone, chest and abdomen.
mycetoma - chronic infection of the skin and underlying tissues, with swelling, draining sinuses and grains, giving the appearance of a tumor