Resolution of hypertrophic osteopathy in a 2‐year‐old filly

Summary This article reports the successful management of a 2-year-old Thoroughbred filly that presented with clinical signs of hypertrophic osteopathy, diffuse granulomatous dermatitis, lethargy and inappetence. An infectious primary focus was suspected due to an elevated white blood cell count (WBC), marked neutrophilia and dramatically increased serum amyloid A (SAA) and plasma fibrinogen levels. After failure to respond to a range of antimicrobial treatments, an actinomycetes bacterial infection, such as Mycobacterium avium or Rhodococcus equi, was suspected and the horse was started on rifampicin and clarithromycin treatment, which resulted in a rapid and pronounced improvement in clinical signs. After 10 weeks of treatment the skeletal manifestations had regressed, both clinically and radiographically, in addition to normalisation of the WBC count, SAA and fibrinogen levels.