Atypical Multibacterial Granulomatous Myositis in a Horse: First Report in Italy

Infectious causes of myositis are reported relatively uncommonly in horses. Among them,
bacterial causes include Streptococcus equi subsp. zooepidemicus, Actinobacillus equuli, Fusobacterium
spp. Staphylococcus spp, and Corynebacterium pseudotuberculosis. Infection can be spread to muscles via
haematogenous or extension from skin lesions. Parasitic myositis has also been documented. In this
report, a 12 year-old Italian Quarter Horse mare presented with diuse subcutaneous nodules and
masses ranging from 2 3 to 5 20 cm in size, and adherent to subcutis and muscles that were first
macroscopically and cytologically diagnosed as pyogranulomas. Subsequently, histological, molecular,
bacteriological, and biochemical investigations were performed. All the data obtained allowed to
diagnose a severe and diuse multibacterial granulomatous myositis caused by Corynebacterium
pseudotuberculosis and Corynebacterium amycolatum. Following the therapy and an initial disappearance
of most of the lesions together with a general improvement of the mare, the clinical condition
deteriorated, and new nodules appeared. Matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) and PCR techniques revealed the presence of bacteria as Glutamicibacter creatinolyticus
and Dietzia spp. To the authors’ knowledge, this case report represents the first description of
multibacterial granulomatous myositis due to Corynebacterium pseudotuberculosis, Corynebacterium
amycolatum, Glutamicibacter creatinolyticus, and Dietzia spp. in a horse reared in Italy.