Members of two genera of Actinomycetaceae, Nocardia and Actinomyces , may cause human disease. One species, A. israelii , causes the majority of human actinomycoses. An obligate anaerobe, A. israelii is a normal resident of gingivodental crevices and tonsillar crypts. A. bovis , which causes lumpy jaw in cattle, rarely causes human disease. Men are affected more often than women.
Most patients have intact immune defenses.
In many cases, actinomycosis follows an event that disrupts the integrity of a mucosal barrier. As do other anaerobic infections, actinomycosis thrives in the presence of necrotic tissues and foreign bodies and frequently participates in polymicrobial infections. A fourth category is pelvic actinomycosis, which appears in women who use IUDs. The disease may also present with widespread hematogenous dissemination to multiple organ systems. Wherever the site of infection, actinomycosis characteristically produces chronic granulomatous inflammation that burrows deeply through contiguous tissues without respect to fascial planes.