Actinomyces bacteria normally live in the mouth and in the nasal passages near the throat. One species, Actinomyces bovis, has been identified infrequently in infections in horses, including chronic fistulous withers and chronic poll evil (see Bone, Joint, and Muscle Disorders in Horses: Fistulous Withers and Poll Evil). Disease occurs when this bacterium is introduced to underlying soft tissue through penetrating wounds of the mouth (such as those that occur from chewing wire or coarse hay). Involvement of the nearby bone frequently results in facial distortion, loose teeth (making chewing difficult), and difficulty breathing due to swelling of the nasal cavity. Treatment is rarely successful in longterm cases in which bone is involved, due to the poor penetration of antibacterial drugs into the affected area. In less advanced cases, your veterinarian may prescribe an antibiotic.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS