Clinical Implications and Hospital Outcome of Immune‐Mediated Myositis in Horses

Abstract

BackgroundImmune‐mediated myositis (IMM) is a cause of rhabdomyolysis, stiffness, and muscle atrophy predominantly affecting Quarter horses. Limited information is available with regard to outcome, prognostic indicators, and associations with concurrent diseases.Hypothesis/ObjectivesTo report outcomes and associations between outcome and clinical and laboratory parameters, and presence of concurrent illness.AnimalsSixty‐eight horses; 52 Quarter horses and related breeds and 16 other breeds.MethodsRetrospective cohort study (1991–2014). Medical records of horses with histological diagnosis of IMM were reviewed. Data recovery included signalment, laboratory variables, therapy, and outcome. Logistic regression was used to quantify the association between potential prognostic factors and survival to discharge.ResultsQuarter horses were younger (mean < 4 years, range 3 months–21 years) than other breeds (mean < 10 years, range 1–23 years). Pathogens causing concurrent or recent infection included S. equi equi, S. equi zooepidemicus, C. pseudotuberculosis, Anaplasma phagocytophilum, herpes virus‐1, and influenza. The most common clinical signs consisted of rapidly progressive diffuse symmetrical muscle atrophy (80%), stiff gait (74%), and fever (44%). All horses that received medical therapy immediately upon admission survived to discharge (survival proportion = 87%). Leucocytosis was a common finding (60%). Horses with concurrent fever and other illness had a poor prognosis for hospital discharge.Conclusions and Clinical ImportanceHorses with IMM can have a favorable outcome. Horses with concurrent fever and another illness had decreased probability of survival to discharge.

An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses

“…Quarter Horses with gross muscle atrophy were highly likely to possess the MYH1 mutation with 93% of nonER QH with atrophy being My/My or My/N and 29% of atrophied ER horses also were My/N. Quite unexpectedly, however, nearly 60% of nonER horses with the MYH1 mutation did not have gross muscle atrophy, the most common clinical sign reported for IMM . It is possible that atrophy developed later after muscle biopsy in some horses or that horses died before developing atrophy.…”

“…Both innate and adaptive immunity could trigger IMM. The innate immune response could drive reactivity to self and affect the type of adaptive immune response that occurs when the mutant form of type 2X myosin heavy chain is released from myofibers after muscle damage (ER, trauma, and vaccination) . Shared epitopes among bacteria, such as the M protein of group A Streptococcus spp.…”

“…This observation suggests that the MYH1 mutation may be the cause of previously reported S. equi ‐associated rhabdomyolysis in horses. Agents such as S. equi zooepidemicus , C. pseudotuberculosis , equine herpes virus, and equine influenza virus previously have been associated with triggering IMM and therefore potentially could be triggers for rhabdomyolysis in QH with the MYH1 mutation through the innate response either to primary muscle damage or by adaptive immunity . Atrophy, fever, and infection in the presence of rhabdomyolysis appear to be strong indicators to perform MYH1 genetic testing in QH.…”

A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses

“…Inflammatory myopathies are infectious or immune-mediated disorders that are characterized by the presence of lymphocytes in the skeletal muscle. Immune - mediated myositides (IMMs) are an important cause of morbidity and, in some cases, mortality in several species including humans [ 1 ], dogs [ 2 ], and horses [ 3 , 4 ]. Common clinical features include malaise, muscle atrophy , and weakness with a histopathologic hallmark of inflammatory infiltrates, particularly lymphocytes, surrounding blood vessels , and within myocytes [ 5 , 6 ].…”

“…Common clinical features include malaise, muscle atrophy , and weakness with a histopathologic hallmark of inflammatory infiltrates, particularly lymphocytes, surrounding blood vessels , and within myocytes [ 5 , 6 ]. There are several different IMM subtypes including inclusion body myositis in humans [ 7 ], polymyositis and dermatomyositis in dogs and humans [ 5 ], canine masticatory myositis [ 8 ] , and equine IMM [ 3 , 4 ]. Equine IMM is characterized by CD4+, CD8+ , and CD20+ lymphocytic infiltrates surrounding blood vessels and infiltrating myofibers without evidence of rimmed vacuoles [ 3 , 6 ].…”