Disseminated idiopathic myofasciitis in a ferret
Jan Shapiro, Penny Waite
A 6-mo-old pet ferret was examined at a veterinary clinic after a 1-mo history of pyrexia, weakness, and lethargy. A respiratory tract infection was suspected, and a course of antibiotics was undertaken. One month later, the clinical signs had not abated with antibiotic treatment and the ferret was losing weight, so it was surrendered to the clinic. On re-examination, the veterinarian found the animal to be pyrexic, weak, lethargic, and dehydrated, with excessively warm and mildly swollen limbs, and a heart murmur. The ferret was euthanized. Postmortem findings were nonspecific, and included thin body condition, mottled and congested lungs, and a mildly enlarged mesenteric lymph node. The veterinarian collected a variety of tissues including multiple muscle samples, which were fixed and submitted to AHL-Kemptville for histopathology.
Significant lesions were restricted to muscle tissue and were dramatic. Intense multifocal-to-coalescing infiltrates of neutrophils admixed with small numbers of mononuclear cells were in myocardium, skeletal muscle and the associated fascia (Fig. 1), and smooth muscle of the esophagus, trachea, and colon, often extending into the submucosa. Suppurative-to-pyogranulomatous myositis was multifocally accompanied by necrosis of myofibers, and occasional skeletal myofiber regeneration, and there were more chronic areas of stromal collapse. Special stains for bacteria, mycobacteria, and mycotic agents detected no etiologic agents.
The clinical history and histologic lesions in this case are consistent with disseminated idiopathic myofasciitis (DIM) of ferrets. This is an uncommon, progressive disease with acute or subacute onset, causing inflammation of smooth, striated, and cardiac muscle and the associated fascia. DIM usually affects ferrets younger than 18 mo, with a reported range of 5-24 mo and an average age of 10 mo. The most common clinical signs are a high fever, lethargy, weakness, inappetance, pain, and dehydration that is often refractory to treatment. Signs reported less commonly are labored fast breathing, tachycardia, heart murmur, subcutaneous swelling, and enlarged lymph nodes. Most ferrets develop moderate-to-marked mature neutrophilia, but clinical chemistry is usually not diagnostic, including lack of elevation of creatine kinase.
The cause of DIM is not known. First recognized in 2003, the early cases had an association with a specific, now discontinued, canine distemper vaccine, leading to the supposition that DIM is an immune-mediated disease. However, the epidemiology of cases since then has not confirmed a causative role of vaccination. A genetic predisposition has been speculated but not proven. An infectious cause has not been ruled out, but to date, no consistent pathogen has been detected in confirmed cases. AHL
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Figure 1. Skeletal muscle showing severe pyogranulomatous myositis (H&E stain)