Causes of Immune-Mediated Myopathies in Horses 

Review possible causes, therapies, and outcomes for horses affected by this inflammatory myopathy.
Atrophied horse with immune-mediated myopathy, IMM
Horses with a genetic susceptibility that are homozygous for the MYH1 mutation might not fully recover from IMM in terms of persistent muscle atrophy. | Adobe Stock

Histologically, immune-mediated myositis (IMM) is an inflammatory myopathy in horses characterized by myonecrosis and the presence of lymphocytes, especially T lymphocytes. Clinically, IMM presents with acute tying-up (rhabdomyolysis), muscle pain, stiffness, reluctance to move, and rapid muscle atrophy. This atrophy is particularly noticeable in horses’ epaxial and gluteal muscles, often occurring within 24 hours of disease onset.  

Genetic Cause of IMM in Horses

In Quarter Horses (QH), researchers have identified an autosomal mutation in the myosin heavy chain 1 (MYH1), a gene that encodes the fast-twitch type 2X myosin heavy chain. This genetic mutation makes Quarter Horses susceptible to IMM. Immune-mediated myositis episodes can occur in horses with this mutation following infection or recent vaccination (within a few weeks). Infections associated with IMM in genetically susceptible horses include influenza virus, equine herpesvirus-1 and -4, Streptococcus equi, and S. equi zooepidemicus. However, over half of all IMM cases have an unknown trigger.  

According to Monica Aleman, PhD, MVZ Cert, Dipl. ACVIM, from the University of California, Davis, “The MYH1 mutation confers susceptibility to IMM. Upon inciting and other unknown factors, the MYH1 mutation results in lymphocytic infiltration and destruction of type 2X myofibers.  

Still, cases of IMM can occur even in horses without the MYH1 mutation.  

Clinical Laboratory Data 

As one would expect in cases of rhabdomyolysis, affected horses often have mild to severe elevations in muscle enzyme activities. Study results show median creatine kinase (CK) and aspartate aminotransferase (AST) values of 101,071 U/L and 12,270 U/L, respectively. Researchers have observed an inflammatory leukogram with neutrophilia in about 60% of cases, as well as hyperfibrinogenemia.  

Treating Horses With IMM 

If you know the primary cause of IMM (e.g., underlying infection), address and treat it immediately. Begin supportive therapy right away for the best outcome. This includes administering intravenous glucocorticoids for a few days before tapering the dose and switching to an oral corticosteroid for several weeks (two to six weeks, typically).  

Mortality rates are usually low with prompt therapy; however, horses with a genetic susceptibility that are homozygous for the MYH1 mutation might not fully recover in terms of persistent muscle atrophy. Acute kidney injury is also a possibility if treating dehydrated horses that have myoglobinuria. 

IMM and Anaplasmosis 

Muscle disease occurs in approximately 18% of horses with equine granulocytic anaplasmosis. Ixodes spp ticks are responsible for transmitting the intracellular Gram-negative rickettsial organism Anaplasma phagocytophilum to horses. A. phagocytophilum infects and replicates in leukocytes. Common laboratory abnormalities include anemia, leukopenia, lymphopenia, and thrombocytopenia. Horses with muscle involvement might have normal white blood cell counts. Hyperfibrinogenemia is often seen in cases of IMM and, as with other cases of infection-associated IMM, CK and AST levels are elevated. 

“Muscle manifestations in horses with anaplasmosis should prompt the clinician to test for MYH1 mutation in Quarter Horse and related breeds,” advised Aleman.  

Tetracycline is the medication of choice for treating anaplasmosis. Glucocorticoid therapy is recommended for managing the IMM component. Affected horses should recover, though they might require several weeks to regain lost muscle mass. 

Key Takeaways 

Rapid muscle loss occurs with IMM, particularly within the epaxial and gluteal muscles. Horses with a mutation in the MYH1 gene might be susceptible to IMM following infection or vaccination. Prompt treatment, including glucocorticoids, will improve outcome and restoration of muscle mass.  

Reference 

Aleman M. Inflammatory and immune-mediated myopathies, What do we know? Vet Clin North Am Equine Pract. In Press.  

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