Avian Influenza
What's Inside

College Links









Make a Gift box

  Home > Neuromuscular Diagnostic Laboratory > Inflammatory Myopathies

Inflammatory Myopathies


Inflammatory Myopathies



Rhabdomyolysis and S.equi

What is it?
Severe, acute rhabdomyolysis can be a rare complication of an upper respiratory tract infection caused by Streptococcus equi.

How does S.equi cause rhabdomyolysis?
The exact mechanism associating rhabdomyolysis and S. equi is not known but there are two current methods of thought. The first is that the myositis is caused by a syndrome similar to streptococcal toxic shock syndrome in humans which is when release of inflammatory cytokines combines with the effect of hypotension and hypoperfusion resulting in shock. The second proposed mechanism is that the upper respiratory tract infection with S.equi disseminates into the blood which then enters the muscles. As the bacteria multiply and produce exotoxins and proteases, skeletal muscle is damaged.

How common is this disease?
Rhabdomyolysis as a complication of a Streptococcus equi infection is rare.

What are the clinical signs?
Firm, swollen and painful epaxial and gluteal muscles, stiff gait, reluctance to move, and hind limb ataxia are most commonly seen. Tachycardia, tachypnea, fever, and nasal discharge may be present and are due to the concurrent streptococcal infection.

Are there any groups of horses that are more susceptible?
No breed, age, or sex predilection is noted at this time.

How is this diagnosed?

Routine diagnostic results
Complete blood count
Increased white blood cells with a marked increase in neutrophils
Mildly elevated fibrinogen

Blood chemistry
Creatinine kinase values of over 50,000 U/L
Mildly increased BUN and phosphorus
Mildly decreased albumin, sodium, and chloride

Additional diagnostics
Muscle biopsy of the semimembranosus or semitendinosus muscle is ideal. Findings consistent characteristic of rhabdomyolysis include swelling of the muscle cells, vacuolization, homogenization of sarcoplasm, and necrosis with macrophage infiltration.

Diagnosis of S. equi should be confirmed by either culture, PCR, or immunofluorescence.

What are other diseases that could look similar?
Polysaccharide storage myopathy
Nutritional myodegeneration
Toxic myopathy
Neurological conditions (ex) spinal cord abscess)

What is the treatment?
Flushing guttural pouches if infected, penicillin and rifampin and potentially corticosteroids may be indicated.

Does the S.equi vaccine increase or decrease the horse’s chance of getting rhabdomyolysis?
At this time the S.equi vaccine status of the horse does not appear to increase or decrease the horse’s chance of developing rhabdomyolysis.

What is the prognosis?
The prognosis for a horse with a S. equi infection complicated by rhabdomyolysis is guarded if horses become recumbant.  


Immune Mediated Myositis

What is IMM?
Immune mediated myositis (IMM) is a disorder in which the immune system of horses attacks the skeletal muscles causing rapid atrophy of the muscles along the topline.

What causes IMM?
This immune mediated disease is probably caused by a loss of self tolerance by the immune system to the antigens present on the horse’s own muscle cells. This means that the immune system fails to recognize muscle cells as a part of the body and begins to see them as foreign. Certain infectious agents or possibly vaccines are thought to “trigger” this reaction. The most common disease believed to trigger IMM in the horse is Streptococcus infections. However, in only 40% of IMM cases is an infectious process ever identified.

How common is IMM?
Immune mediated myositis is rare in horses but it is the most common cause of rapid atrophy of the topline in Quarter and Paint horses.

What are the clinical signs?
Rapid loss of muscle mass, usually symmetric, is seen in the back (epaxial) and rump (gluteal) muscles. The atrophy along the horse’s topline typically becomes apparent within one to 3 days. Depression, loss of appetite and stiffness are also seen. If advanced, horses can have difficulty standing.

Are there any groups of horses that are more susceptible to IMM?
Quarter Horses and Paints are the primary breeds affected by IMM, however, Thoroughbreds, ponies, and Icelandic Horses have also been diagnosed with IMM. Horses under the age of 8 years old or over the age of 17 years old are more likely to develop the disease.

How is it diagnosed?

Horses with a recent history of respiratory disease or that have had recent exposure to horses with respiratory disease are at higher risk for developing IMM.

Routine diagnostic results
Blood chemistry results will typically show a chronic increase in serum creatine kinase (CK) and aspartate transaminase (AST) of up to 10,000 U/L.

Additional diagnostics
A definitive diagnosis requires muscle biopsies of the gluteal or epaxial muscles, semimembranosus muscle is often normal. Extensive lymphocyte infiltration and mononuclear vasculitis with regenerative and atrophied myofibers are characteristic features of IMM. We recommend submitting several muscle samples; 1) biopsies of atrophied rump/back muscles (can be in formalin using a large gauge trucut) and fresh semimembransus to rule out other diseases.

What are other diseases that look similar to IMM?
Equine Motor Neuron Disease
Malabsorption/cachetic conditions
Cushings disease in older horses
Homozygous for type 1 PSSM
Neurogenic muscle atrophy (trauma)

How is it treated?
IMM is remarkably responsive to corticosteroids. Typical therapy consists of dexamethasone (0.05 mg/kg) for 3 days, followed by prednisolone (1 mg/kg for 7 to 10 days) tapered by 100 mg/week over 1 month. We recommend a CBC be performed and antibiotic therapy is recommended if a leukocytosis, hyperfibrinogenemia or lymphadenopathy is present.

Are there any management strategies that can help a horse with IMM?
Horses recovering from muscle atrophy should be fed a concentrate in addition to hay that is balanced for vitamins and minerals and has high quality protein. (alfalfa hay may help here). If IMM has occurred within a month of vaccination, we recommend spreading out necessary vaccines with at least 6 weeks in between to see if one of these triggers a reaction. S equi vaccines are not generally recommended for horses that have had IMM.

What is the prognosis for a horse with IMM?
IMM usually resolves without long term consequences. Improvement in appetite should occur within 48 hours of steroid therapy, muscle atrophy should stop and then within 2-3 months muscle mass should return. In some cases there may be areas in a muscle that have a permanent divot. Without corticosteroid treatment, atrophy will eventually resolve and return of muscle mass may take a longer period of time.
Some horses may show signs of recrudescence of the syndrome particularly within in a year or two of initial presentation. Call your veterinarian to begin steroid therapy as soon as you notice muscle atrophy.



Notice of Privacy Practices