Disseminated idiopathic myofasciitis in a ferret

Andrew Brooks, Michelle Yee

Animal Health Laboratory, University of Guelph, Guelph, ON (Brooks), Links Road Animal and Bird Clinic, Toronto, ON (Yee)

AHL Newsletter 2021;25(2):18.

Formalin-fixed tissues were submitted to the AHL from an 8-month-old, spayed female ferret that presented with a history of mobility problems, weight loss, lethargy and weakness.  Severe leukocytosis and neutrophilia were evident in the complete blood count.  Disseminated idiopathic myofasciitis (DIM) was suspected clinically and the ferret was humanely euthanized.  

Histopathology revealed marked areas of suppurative to pyogranulomatous inflammation in skeletal muscle (Fig. 1), including muscle tissue in the tongue and esophagus.  The inflammation consisted mostly of neutrophils with fewer macrophages, lymphocytes, plasma cells and rare eosinophils.  There was myofibre atrophy and loss of myofibres within the lesions.  The inflammation in the muscular wall of the esophagus was particularly prominent (Fig. 2).  Other histological findings included extramedullary hematopoiesis in the spleen and reactive lymphoid hyperplasia in the lymph nodes.  In the absence of discernible infectious agents, the morphological diagnosis of multifocal suppurative to pyogranulomatous myositis supported the clinical diagnosis of DIM. 

DIM in ferrets is an idiopathic disease with a poor prognosis.  An immune-mediated pathogenesis is suspected. DIM usually has a rapid clinical progression and affects young adult ferrets of both sexes, often under 18 months of age.  Clinical signs often include weakness, lethargy, fever, depression, anorexia and there is frequently a moderate to marked neutrophilia.  Clinical chemistry is not a reliable diagnostic test for DIM since muscle enzymes, such as CK, may not be elevated.  At postmortem, the gross lesions may include discoloration the skeletal muscle and muscle atrophy.  The esophagus may be dilated, and the spleen and lymph nodes may be enlarged.  The characteristic histological lesion is multifocal, suppurative to pyogranulomatous inflammation of the skeletal muscle and connective tissues of the limbs, head and trunk.  Cardiac muscle and smooth muscle of the stomach, intestine and urinary bladder may also be affected.  Myositis involving the circumference of the esophagus may be a hallmark of DIM in ferrets.  The diagnosis of DIM is based on the clinical presentation, lack of response to treatment, and the characteristic lesions in the skeletal muscle.  AHL

References

1. Ramsell KD, Garner MM. Disseminated idiopathic myofasciitis in ferrets. Vet Clin North Am Exot Anim Pract 2010;13:561-575. 

2. Garner MM, et al. Myofasciitis in the Domestic Ferret. Vet Path 2007;44:25-38.

Figure 1. Suppurative to pyogranulomatous inflammation in the skeletal muscle. H&E stain.

Figure 2. Inflammation involving the muscular wall of the esophagus (black arrows). H&E stain.