Infectious Myositis in Dogs and Cats

Infectious myositis may be due to a wide variety of pathogens. Numerous microbial agents have been implicated in inflammatory myopathies in dogs and cats. Most infectious myopathies are polymyopathies and represent a facet of inflammatory systemic disease.

Viral myositis is rare; however, feline immunodeficiency virushas been shown to induce subclinical myopathy in an experimental setting (1).

Infection with Clostridium spp is the most common cause of bacterial myositis; affected patients may present with focal disease affecting one or more muscles in one limb.

Causative agents of protozoal myositis are Toxoplasma gondii (toxoplasmosis), Neospora caninum (neosporosis), Hepatozoon americanum (hepatozoonosis), and Babesia canis and Babesia gibsoni (babesiosis). Ehrlichiosis and Rocky Mountain spotted fever can also result in myositis.

Clinical signs of infectious myositis in dogs and cats typically include myalgia and fever. Patients with clostridial infections may have palpable crepitus resulting from gas accumulation and may have a history of a bite wound or other trauma. Myositis caused by T gondii and N caninum infections typically occurs in young dogs and results in rigid, hyperextended pelvic limbs.

Diagnosis is confirmed by demonstrating the presence of the likely causative infectious agent in the patient with myositis. In some cases, the microorganism can be detected and identified through muscle biopsy; in others, serial serological testing or culture is needed. PCR assay or paired serum titer measurements may be valuable. Serum CK activity is likely elevated. Radiographic imaging may show periosteal bone proliferation in dogs with hepatozoonosis.

Treatment and prognosis for dogs and cats with infectious myositis vary depending on the infectious agent and extent of disease.

For other inflammatory myopathies in dogs and cats, see the Overview for this chapter.