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

Abstract

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 diffuse 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 diffuse 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.

Keywords: Corynebacterium pseudotuberculosis; Dietzia spp.; Glutamicibacter creatinolyticus; bacterial myositis; horses.

Figure 1

(A) Several single or multiple subcutaneous nodules and masses, from 3 to 20 cm in size. (B) Nodules mainly localized at shoulder regions. (C) Nodules adherent to muscles, painless and firm in consistency. (D) On cut section, evidence of necrotic caseous material surrounded by a fibrotic capsule with small purulent-like foci externally to the necrotic tissue.

Figure 2

Histological examination: (A) widespread inflammatory infiltrate invading the endomysium (HE, 10× magnification); (B) mixed inflammatory infiltrate constituted by granulocytes neutrophils and eosinophils, macrophages, lymphocytes, plasma cells, epithelioid and multinucleated giant cells (HE, 40× magnification); (C) collagenolytic degeneration areas (Masson Trichrome, 20×), and foci of necrosis associated with calcification (HE, 5× magnification); (D) Gram stain allowed to detect small aggregates of rod-shaped, straight or slightly curved blue Gram-positive microorganisms showing an irregular swelling at one or both end (“Club Shaped”), and grouped together in a characteristic way often forming a “V” or a “L” (arrows) (Gram, 100× magnification).

Figure 3

Biochemical gallery (RapID CB Plus, Remel, ThermoFisher, Italy) for strain 1.

Figure 4

Agarose gel electrophoresis of the quadruplex PCR products for the identification of the two strains isolated from a horse. Lane 1- 1kb Gene Ruler DNA Ladder (Thermo Scientific); Lane 2-Negative Control; Lane 3-Corynebacterium pseudotuberculosis biovar Ovis DNA (strain 1002); 4-Corynebacterium pseudotuberculosis 258 Equi; 5- DNA from strain 1, isolated from a sick equine; 6-DNA from strain 2, isolated from a sick horse. The arrows indicate the target genes and fragment sizes expected for the Corynebacterium pseudotuberculosis amplification profiles.