From dog to man: the broad spectrum of inflammatory myopathies

Myosin Heavy Chain Myopathy and Immune-Mediated Muscle Disorders

Several inflammatory myopathies have an infectious or immune-mediated basis in the horse. Myosin heavy chain myopathy is caused by a codominant missense variant in MYH1 and has 3 clinical presentations: immune-mediated myositis, calciphylaxis, and nonexertional rhabdomyolysis in Quarter Horse-related breeds. An infarctive form of purpura hemorrhagica affects numerous breeds, presenting with focal firm, painful muscle swelling, and subsequent infarction of multiple tissues. While Streptococcus equi equi is often the inciting cause, anaplasmosis, sarcocystis, piroplasmosis, viruses, and vaccines can also be inciting agents. This article describes the diagnosis, pathophysiology, and treatment of these inflammatory myopathies.

Successful diagnosis and treatment of canine polymyositis: utilizing MRI and immunohistochemistry for accurate detection

BACKGROUND

Inflammatory myopathy is generally categorized into generalized inflammatory myopathies (gIM), which affect muscles throughout the body, and focal inflammatory myopathies (fIM), which are localized to specific muscles or muscle groups. This report details a case of immune-mediated polymyositis in a dog, successfully diagnosed using MRI and IHC and managed with immunosuppressive therapy.

CASE PRESENTATION

A 5-year-old castrated male Poodle was admitted to our hospital presenting with lethargy and exercise intolerance. Biochemical analysis revealed significantly elevated serum levels of aspartate aminotransferase (AST) and creatine kinase (CK). Physical examination showed muscle atrophy in the hind legs, but further orthopedic and neurological examinations identified no additional abnormalities. MRI demonstrated hyperintense and heterogeneous signal changes across the muscles, including contrast enhancement, suggesting inflammatory myopathy. This diagnosis was confirmed through histopathological examination, which revealed inflammatory lesions with fibrous tissue proliferation within the muscle tissue. To investigate the presence and type of inflammatory cells and vascular changes, aiding in the differential diagnosis of inflammatory myopathies, immunohistochemistry (IHC) was performed, revealing positive findings for CD8+, CD4+, and VEGF in the evaluated tissue, leading to a diagnosis of polymyositis.

CONCLUSIONS

The dog was diagnosed with immune-mediated polymyositis and treatment was initiated with prednisolone at 1 mg/kg twice daily and azathioprine at 2 mg/kg once daily. Following the administration of these immunosuppressive agents, CK levels returned to normal, and the dog's exercise intolerance and lethargy resolved. The thickness of the hind legs also increased progressively. The dog has maintained an improved condition under continued immunosuppressive therapy for four months. This case highlights the critical role of MRI and immunohistochemistry in diagnosing immune-mediated polymyositis, demonstrating their alternative capability in cases where conventional electromyography (EMG) is not feasible in this context.

Pathologic Changes in and Immunophenotyping of Polymyositis in the Dutch Kooiker Dog

Earlier, we described a breed-specific inflammatory myopathy in Dutch Kooiker dogs (Het Nederlandse Kooikerhondje), one of the nine Dutch breeds. The disease commonly manifests itself with clinical signs of difficulty walking, muscle weakness, exercise intolerance, and/or dysphagia. In nearly all dogs' creatine kinase (CK) activity was elevated. Histopathology reveals the infiltration of inflammatory cells within the skeletal muscles. The objective of this study was to further investigate and characterize the histopathological changes in muscle tissue and immunophenotype the inflammatory infiltrates. FFPE fixed-muscle biopsies from 39 purebred Kooiker dogs were included and evaluated histopathologically according to a tailored classification scheme for skeletal muscle inflammation. As in other breed-related inflammatory myopathies, multifocal, mixed, and predominantly mononuclear cell infiltration was present, with an initial invasion of viable muscle fibres and the surrounding stroma leading to inflammation, necrosis, and tissue damage. Immunophenotyping primarily revealed lymphohistiocytic infiltrates, with CD3+ T-cells being the predominant inflammatory cell type, accompanied by CD8+ cytotoxic T-cells. The concurrent expression of MHC-II class molecules on myofibres suggests their involvement in initiating and maintaining inflammation. Additionally, CD20+ B-cells were identified, though in lower numbers compared to T-cells, and IBA-1-positive macrophages were frequently seen. These findings suggest a breed-specific subtype of polymyositis in Kooiker dogs, akin to other breeds. This study sheds light on the immune response activation, combining adaptive and innate mechanisms, contributing to our understanding of polymyositis in this breed.

Inflammatory and Immune-Mediated Myopathies, What Do We Know?

Inflammatory myopathies or myositis encompass diseases characterized by the presence of inflammatory cellular infiltrates, mainly polymorphonuclear cells and/or lymphocytes, in muscle. This is in contrast to most forms of muscle disease characterized by myodegeneration that results in macrophage infiltration. Inflammatory myopathies could have infectious or noninfectious causes. Noninfectious causes consist of primary (genetic, autoimmune) or acquired immune-mediated disease. Focal, multifocal or diffuse, acute or recurrent forms of disease can occur. This article will mainly review immune-mediated myopathies in horses. Myositis directly caused by infection such as Clostridium spp and others will not be discussed here.

Clinical Diagnosis of Rhabdomyolysis without Myoglobinuria or Electromyographic Abnormalities in a Dog

A 2-year-old female neutered Old German Shepherd was presented for acute non-ambulatory tetraparesis. Upon presentation to the emergency department, hematology and biochemical blood tests revealed no abnormalities aside from mildly elevated C-reactive protein levels (22.5 mg/L, reference range 0.0-10.0) and immeasurable creatine kinase (CK) activity. Neurological evaluation the next day revealed ambulatory tetraparesis, general proprioceptive deficits, mild ataxia and dubious diffuse myalgia. Withdrawal reflexes were weak on both thoracic and pelvic limbs. The CK was determined to be significantly elevated at that point (32.856 U/L, ref. range 10.0-200.0). Urinalysis revealed no abnormalities. An electromyographic (EMG) study of thoracic limb, paraspinal and pelvic limb muscles revealed no abnormalities. A magnetic resonance imaging (MRI) study of the cervicothoracic spinal cord was performed and revealed no abnormalities. A presumptive clinical diagnosis of rhabdomyolysis without myoglobinuria or EMG abnormalities was formed. Muscular biopsies were declined due to the rapid clinical improvement of the dog. A follow-up showed the progressive decline of CK activity to normal values and clinical remission of signs. A diagnosis of rhabdomyolysis was concluded based on clinical signs, consistent CK activity elevations and the response to supportive treatment for rhabdomyolysis, despite the absence of myoglobinuria and EMG abnormalities. Rhabdomyolysis should not be excluded based on the lack of EMG abnormalities or myoglobinuria in dogs.

An Inflammatory Myopathy in the Dutch Kooiker Dog

The Dutch Kooiker dog (het Nederlandse Kooikerhondje) is one of nine Dutch dog breeds. As of 1960, a number of heritable diseases have been noted in this breed. One is an inflammatory myopathy that emerged in 1972, with numbers of affected dogs gradually increasing during the last few decades. The objective of this paper is to describe clinical signs, laboratory results, electromyography and histopathology of the muscle biopsies of the affected dogs. Method: Both retrospectively as well as prospectively affected Kooiker dogs were identified and categorized using a Tiered level of Confidence. Results: In total, 160 Kooiker dogs-40 Tier I, 33 Tier II and 87 Tier III-were included. Clinical signs were (1) locomotory problems, such as inability to walk long distances, difficulty getting up, stiff gait, walking on eggshells; (2) dysphagia signs such as drooling, difficulty eating and/or drinking; or (3) combinations of locomotory and dysphagia signs. CK activities were elevated in all except for one dog. Histopathology revealed a predominant lymphohistiocytic myositis with a usually low and variable number of eosinophils, neutrophils and plasma cells. It is concluded that, within this breed, a most likely heritable inflammatory myopathy occurs. Further studies are needed to classify this inflammatory myopathy, discuss its treatment, and unravel the genetic cause of this disease to eradicate it from this population.

Case report: Atypical and chronic masticatory muscle myositis in a 5-month old Cavalier King Charles Spaniel. Clinical and diagnostic findings, treatment and successful outcome

Masticatory muscle myositis (MMM) is the second most common inflammatory myopathy diagnosed in dogs, but it is rarely described in puppies. The disease is characterized by the production of autoantibodies against 2M myofibers contained in masticatory muscle, although the cause of this production is still unclear. The aim of the present case report was to describe the clinical presentation, diagnostic findings, treatment, and follow-up of an atypical case of chronic masticatory muscle myositis in a very young dog. A 5-month old Cavalier king Charles Spaniel (CKCS) was presented to the AniCura Istituto Veterinario Novara with a two weeks, progressive history of lethargy and difficulty in food prehension. Neurological examination revealed bilateral masticatory muscle atrophy, mandibular ptosis with the jaw kept open, inability to close the mouth without manual assistance, jaw pain, and bilateral reduction of palpebral reflex and menace reaction; vision was maintained. A myopathy was suspected. Computed tomography (CT), magnetic resonance imaging (MRI), enzyme-linked immunosorbent assay test for 2M antibodies, and histopathological examination of masticatory muscle biopsy confirmed the diagnosis of MMM. Glucocorticoids treatment was started and clinical signs promptly improved. To the authors' knowledge, this is the first case describing mandibular ptosis in a dog affected by chronic MMM, successfully managed with medical treatment and the first report describing the CT and MRI findings in a young CKCS affected by MMM.

Case Report: Persistent Moderate-to-Severe Creatine Kinase Enzyme Activity Elevation in a Subclinical Dog

A 4-year-old, male-castrated, mixed breed dog was presented for a routine wellness examination at which time a moderate increase in serum creatine kinase (CK) enzyme activity (hyperCKemia) (15,137 IU/L; reference interval 10–200 IU/L), and moderate increases in alanine transaminase and aspartate aminotransferase enzyme activities were first identified. There was no history of clinical abnormalities (e.g., lethargy, lameness, anorexia, dysphagia, weakness, gait abnormalities, or exercise intolerance) and the physical examination was unremarkable. The dog was screened for several relevant potential infectious diseases known to cause inflammatory myopathies and was treated empirically with clindamycin. The serum total CK enzyme activity remained increased, which prompted recommendations for an echocardiogram, electromyogram (EMG), and muscle biopsy acquisition. The echocardiogram and electrocardiographic monitoring were unremarkable. The EMG and muscle biopsies were declined by the owner. The dog was evaluated several times in the subsequent 5 years and remained subclinical with unremarkable physical examinations despite a persistent moderate-to-severe hyperCKemia. Differential diagnoses considered most likely in this dog were an occult/latent hereditary muscular dystrophic disorder or idiopathic hyperCKemia, a phenomenon not yet reported in the veterinary literature. This report describes for the first time, clinical and diagnostic features of a subclinical dog with persistent moderate-to-severe hyperCKemia.

Differential acute-phase protein responses in dogs seropositive or seronegative for Neospora caninum

Neospora caninum is one of the most prevalent Apicomplexa parasites that causes abortion in cattle, as it infects dogs as its definitive host, causing subclinical disease or active neosporosis, marked by meningoencephalitis, and myopathies with muscle and neuromuscular signs of disease. This study aimed to evaluate the acute phase protein response in dogs seropositive and seronegative for N. caninum. Serum samples of 72 dogs were tested by an immunofluorescence antibody test using N. caninum NC-1 strain, and the study population was divided into four groups: symptomatic - muscular and/or neuromuscular signs - and seropositive (n = 16); symptomatic and seronegative (n = 9); asymptomatic and seropositive (n = 34); and asymptomatic and seronegative (n = 13). C-reactive protein (CRP) was measured via immunoturbidimetric assay and serum haptoglobin (Hp) via hemoglobin-binding capacity assay. In the symptomatic groups, seropositive dogs had higher levels of Hp, but not CRP, while seronegative dogs had higher CRP levels. There was no difference in CRP concentration in asymptomatic dogs. Dogs with neuromuscular signs had higher concentrations for Hp in the group seropositive. Hp concentration did not differ between dogs seropositive and seronegative dogs for each group. Serum Hp and CRP could not sufficiently alone flag subclinical infections. Measurement of CRP and Hp concentrations could be clinically valuable to the diagnosis of neurological diseases, and their relative change may indicate the stage of the infection, although their sole use is not able to support the diagnosis of canine neosporosis. Further studies are encouraged to evaluate the specific dynamics of acute phase proteins in canine neosporosis. Serum samples of 72 dogs were tested by an immunofluorescence antibody test using N. caninum NC-1 strain, and the study population was divided into four groups: (1) dogs with muscular and/or neuromuscular signs and seropositive for N. caninum; (2) dogs with muscular and/or neuromuscular signs and seronegative for N. caninum; (3) dogs seropositive for N. caninum with no neuromuscular signs; and (4) healthy dogs and seronegative for N. caninum. The study evaluated if N. caninum infection could have pathophysiological changes activating the acute phase response and an increase in the concentration of acute phase proteins in serum.

Clinical presentation, MRI, histopathology and outcome in a cat with immune-mediated masticatory myositis

CASE SUMMARY

A 4-year-old female spayed domestic shorthair cat was presented with facial swelling, ocular discharge and intermittent bilateral exophthalmos. Haematology revealed mild eosinophilia. Serum biochemistry showed a markedly elevated creatine kinase activity. MRI of the head revealed diffuse and severe changes of the masticatory muscles, including irregular areas compatible with fluid or necrosis within the abnormal muscle tissue. Cytological analysis of the left temporal muscle revealed eosinophilic and macrophagic inflammation. Bacterial and fungal cultures were negative. Serological titres against Toxoplasma gondii were compatible with previous exposure. A canine ELISA against masticatory muscle type IIM fibre proteins was positive at 1:4000 (reference interval <1:100). Histopathological examination of the left temporalis muscle revealed moderately severe and multifocal myositis. A diagnosis of immune-mediated masticatory myositis was made and immunosuppressive therapy was started. The cat initially responded to tapering doses of prednisolone, but subsequent relapses required therapy modulation. At the time of writing, 27 months after the initial diagnosis, the cat was in remission, but was diagnosed with diabetes mellitus, probably secondary to chronic glucocorticoid use.

RELEVANCE AND NOVEL INFORMATION

To our knowledge, this is the first case report to describe the MRI appearance of masticatory myositis in a cat and the second to describe the clinical presentation, histopathology, response to treatment and outcome in a cat with this condition.

Leishmania spp.-Infected Dogs Have Circulating Anti-Skeletal Muscle Autoantibodies Recognizing SERCA1

Leishmania spp. infection is associated with an inflammatory myopathy (IM) in dogs. The pathomechanism underlying this disorder is still elusive, however, the pattern of cellular infiltration and MHC I and II upregulation indicate an immune-mediated myositis. This study aimed to investigate the presence of autoantibodies targeting the skeletal muscle in sera of leishmania-infected dogs and individuate the major autoantigen. We tested sera from 35 leishmania-infected dogs and sera from 10 negative controls for the presence of circulating autoantibodies with indirect immunofluorescence. Immunoblot and mass spectrometry were used to identify the main target autoantigen. Immunocolocalization and immunoblot on immunoprecipitated muscle proteins were performed to confirm the individuated major autoantigen. We identified circulating autoantibodies that recognize skeletal muscle antigen(s) in sera of leishmania-infected dogs. The major antigen was identified as the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 1 (SERCA1). We also found that canine SERCA1 presents several identical traits to the calcium-translocating P-type ATPase of Leishmania infantum. In the present study, we defined circulating anti-SERCA1 autoantibodies as part of the pathogenesis of the leishmania-associated IM in dogs. Based on our data, we hypothesize that antigen mimicry is the mechanism underlying the production of these autoantibodies in leishmania-infected dogs.

The Dog Model in the Spotlight: Legacy of a Trustful Cooperation

Dogs have long been used as a biomedical model system and in particular as a preclinical proof of concept for innovative therapies before translation to humans. A recent example of the utility of this animal model is the promising myotubularin gene delivery in boys affected by X-linked centronuclear myopathy after successful systemic, long-term efficient gene therapy in Labrador retrievers. Mostly, this is due to unique features that make dogs an optimal system. The continuous emergence of spontaneous inherited disorders enables the identification of reliable complementary molecular models for human neuromuscular disorders (NMDs). Dogs’ characteristics including size, lifespan and unprecedented medical care level allow a comprehensive longitudinal description of diseases. Moreover, the highly similar pathogenic mechanisms with human patients yield to translational robustness. Finally, interindividual phenotypic heterogeneity between dogs helps identifying modifiers and anticipates precision medicine issues.

This review article summarizes the present list of molecularly characterized dog models for NMDs and provides an exhaustive list of the clinical and paraclinical assays that have been developed. This toolbox offers scientists a sensitive and reliable system to thoroughly evaluate neuromuscular function, as well as efficiency and safety of innovative therapies targeting these NMDs. This review also contextualizes the model by highlighting its unique genetic value, shaped by the long-term coevolution of humans and domesticated dogs. Because the dog is one of the most protected research animal models, there is considerable opposition to include it in preclinical projects, posing a threat to the use of this model. We thus discuss ethical issues, emphasizing that unlike many other models, the dog also benefits from its contribution to comparative biomedical research with a drastic reduction in the prevalence of morbid alleles in the breeding stock and an improvement in medical care.

Polyphasic Rhabdomyositis in California Sea Lions (Zalophus Californianus): Pathology and Potential Causes

A myositis syndrome has been recognized for more than a decade in California sea lions (CSLs; Zalophus californianus) but a detailed description of the lesions and potential causes of this condition is lacking. The tissues of 136 stranded CSLs with rhabdomyositis were examined. Rhabdomyositis was considered incidental in 67% (91/136) of the CSLs, and a factor contributing to the animal stranding (significant rhabdomyositis) in 33% (45/136). Of the 91 cases with incidental rhabdomyositis, lesions consisted of a few small foci of lymphohistiocytic inflammation. Of the 45 cases with significant rhabdomyositis, 28 (62%) also presented with major comorbidities such as leptospirosis (2 animals) and domoic acid toxicosis (6 animals), whereas 17 (38%) had severe polyphasic rhabdomyositis as the only major disease process associated with mortality. In these animals, most striated muscles had multiple white streaks and diffuse atrophy. Microscopically, there was myofiber necrosis surrounded by lymphocytes and histiocytes admixed with areas of myofiber regeneration, and/or moderate to severe rhabdomyocyte atrophy usually adjacent to intact Sarcocystis neurona cysts. At the interface of affected and normal muscle, occasional T lymphocytes infiltrated the sarcoplasm of intact myocytes, and occasional myofibers expressed MHCII proteins in the sarcoplasm. S. neurona antibody titers and cyst burden were higher in animals with significant polymyositis antibody titers of (26125 ± 2164, 4.5 ± 1.2 cysts per section) and active myonecrosis than animals with incidental rhabdomyositis antibody titers of (7612 ± 1042, 1.7 ± 0.82 cysts per section). The presented findings suggest that S. neurona infection and immune-mediated mechanisms could be associated with significant polyphasic rhabdomyositis in CSLs.

A Mutation in the Mitochondrial Aspartate/Glutamate Carrier Leads to a More Oxidizing Intramitochondrial Environment and an Inflammatory Myopathy in Dutch Shepherd Dogs

BACKGROUND

Inflammatory myopathies are characterized by infiltration of inflammatory cells into muscle. Typically, immune-mediated disorders such as polymyositis, dermatomyositis and inclusion body myositis are diagnosed.

OBJECTIVE

A small family of dogs with early onset muscle weakness and inflammatory muscle biopsies were investigated for an underlying genetic cause.

METHODS

Following the histopathological diagnosis of inflammatory myopathy, mutational analysis including whole genome sequencing, functional transport studies of the mutated and wild-type proteins, and metabolomic analysis were performed.

RESULTS

Whole genome resequencing identified a pathological variant in the SLC25A12 gene, resulting in a leucine to proline substitution at amino acid 349 in the mitochondrial aspartate-glutamate transporter known as the neuron and muscle specific aspartate glutamate carrier 1 (AGC1). Functionally reconstituting recombinant wild-type and mutant AGC1 into liposomes demonstrated a dramatic decrease in AGC1 transport activity and inability to transfer reducing equivalents from the cytosol into mitochondria. Targeted, broad-spectrum metabolomic analysis from affected and control muscles demonstrated a proinflammatory milieu and strong support for oxidative stress.

CONCLUSIONS

This study provides the first description of a metabolic mechanism in which ablated mitochondrial glutamate transport markedly reduced the import of reducing equivalents into mitochondria and produced a highly oxidizing and proinflammatory muscle environment and an inflammatory myopathy.

Treatment Outcome of 22 Dogs With Masticatory Muscle Myositis (1999-2015)

Medical records of dogs diagnosed with masticatory muscle myositis (MMM) at Ryan Veterinary Hospital of the University of Pennsylvania during a period of 17 years (from 1999 to 2015) were reviewed. Twenty-two dogs were included in this retrospective case series study. Immunosuppressive doses of prednisone were prescribed to all dogs. Twenty dogs had full recovery of masticatory function. The mean (SD) improvement in the vertical mandibular range of motion (vmROM) was 5.3 (3.1) cm during the first 4 weeks of treatment (weeks 1-4) and 2.8 (2.2) cm during the subsequent 8 weeks (weeks 5-12). The vmROM continued to improve for several more months. Six dogs had a relapse, but the clinical signs were more severe in dogs when no longer receiving prednisone compared to dogs still on prednisone at the time of relapse. When diagnosed and treated appropriately, MMM has a good prognosis with relatively quick return to masticatory function. Early discontinuation of prednisone therapy should be avoided. Approximately 1 year of therapy is recommended prior to discontinuing the medication. Educating the client about how to perform muscle palpation, determine vmROM at home, recognize signs of pain, and notice behavioral changes may help in the early detection of relapses.

Magnetic resonance imaging of masticatory muscles in basset hounds

OBJECTIVES

To describe low- and high-field MRI characteristics of occult masticatory muscle lesions in basset hounds.

MATERIALS AND METHODS

Retrospective review of the MR images of the heads of 44 basset hounds that had been imaged for reasons unrelated to suspected muscle disease.

RESULTS

In most basset hounds, there was reduced bulk of the superficial part of the temporalis and masseter muscles together with bilaterally symmetrical T2W and T1W hyperintense signal with reduced contrast enhancement compared to normal muscle. Interpretation of various pulse sequences suggested that the affected muscle areas contained adipose tissue with no evidence of inflammation, and this was confirmed on biopsy in one dog. This pattern of changes is different from MRI changes in inflammatory myopathies.

CLINICAL SIGNIFICANCE

Poor muscle bulk and MRI signal changes in the superficial parts of the temporalis and masseter muscles in basset hounds appear to be normal findings in this breed and should not be misinterpreted as evidence of an inflammatory myopathy.

Myositis, Ganglioneuritis, and Myocarditis with Distinct Perifascicular Muscle Atrophy in a 2-Year-Old Male Boxer

A 2-year-old male, intact Boxer was referred for chronic diarrhea, hyporexia, labored breathing, weakness and elevated creatine kinase, and alanine aminotransferase activities. Initial examination and diagnostics revealed a peripheral nervous system neurolocalization, atrial premature complexes, and generalized megaesophagus. Progressive worsening of the dog's condition was noted after 36 h; the dog developed aspiration pneumonia, was febrile and oxygen dependent. The owners elected humane euthanasia. Immediately postmortem biopsies of the left cranial tibial and triceps muscles and the left peroneal nerve were obtained. Postmortem histology revealed concurrent myositis, myocarditis, endocarditis, and ganglioneuritis. Mixed mononuclear cell infiltrations and a distinct perifascicular pattern of muscle fiber atrophy was present in both muscles. This is a novel case of diffuse inflammatory myopathy with a distinct perifascicular pattern of atrophy in addition to endocarditis, myocarditis, and epicarditis.

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

Background

The cause of immune-mediated myositis (IMM), characterized by recurrent, rapid-onset muscle atrophy in Quarter Horses (QH), is unknown. The histopathologic hallmark of IMM is lymphocytic infiltration of myofibers. The purpose of this study was to identify putative functional variants associated with equine IMM.

Methods

A genome-wide association (GWA) study was performed on 36 IMM QHs and 54 breed matched unaffected QHs from the same environment using the Equine SNP50 and SNP70 genotyping arrays.

Results

A mixed model analysis identified nine SNPs within a ~ 2.87 Mb region on chr11 that were significantly (P_unadjusted < 1.4 × 10^− 6) associated with the IMM phenotype. Associated haplotypes within this region encompassed 38 annotated genes, including four myosin genes (MYH1, MYH2, MYH3, and MYH13). Whole genome sequencing of four IMM and four unaffected QHs identified a single segregating nonsynonymous E321G mutation in MYH1 encoding myosin heavy chain 2X. Genotyping of additional 35 IMM and 22 unaffected QHs confirmed an association (P = 2.9 × 10^− 5), and the putative mutation was absent in 175 horses from 21 non-QH breeds. Lymphocytic infiltrates occurred in type 2X myofibers and the proportion of 2X fibers was decreased in the presence of inflammation. Protein modeling and contact/stability analysis identified 14 residues affected by the mutation which significantly decreased stability.

Conclusions

We conclude that a mutation in MYH1 is highly associated with susceptibility to the IMM phenotype in QH-related breeds. This is the first report of a mutation in MYH1 and the first link between a skeletal muscle myosin mutation and autoimmune disease.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0155-0) contains supplementary material, which is available to authorized users.

Immune-Mediated Muscle Diseases of the Horse

In horses, immune-mediated muscle disorders can arise from an overzealous immune response to concurrent infections or potentially from an inherent immune response to host muscle antigens. Streptococcus equi ss. equi infection or vaccination can result in infarctive purpura hemorrhagica (IPH) in which vascular deposition of IgA-streptococcal M protein complexes produces ischemia and complete focal infarction of skeletal muscle and internal organs. In Quarter Horse–related breeds with immune-mediated myositis, an apparent abnormal immune response to muscle antigens results in upregulation of major histocompatibility complex class (MHC) I and II on muscle cell membranes, lymphocytic infiltration of lumbar and gluteal myofibers, and subsequent gross muscle atrophy. Rarely, an inflammatory event results in myositis with subsequent systemic calcinosis characterized by a pathognomonic hyperphosphatemia and high fatality rate. This review presents an overview of these immune-mediated myopathies and highlights clinical and pathological features as well as the suspected pathophysiology.

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

BACKGROUND

Immune-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/OBJECTIVES

To report outcomes and associations between outcome and clinical and laboratory parameters, and presence of concurrent illness.

ANIMALS

Sixty-eight horses; 52 Quarter horses and related breeds and 16 other breeds.

METHODS

Retrospective 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.

RESULTS

Quarter 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 IMPORTANCE

Horses with IMM can have a favorable outcome. Horses with concurrent fever and another illness had decreased probability of survival to discharge.

Major Histocompatibility Complex I and II Expression and Lymphocytic Subtypes in Muscle of Horses with Immune-Mediated Myositis

BACKGROUND

Major histocompatibility complex (MHC) I and II expression is not normally detected on sarcolemma, but is detected with lymphocytic infiltrates in immune-mediated myositis (IMM) of humans and dogs and in dysferlin-deficient muscular dystrophy.

HYPOTHESIS/OBJECTIVES

To determine if sarcolemmal MHC is expressed in active IMM in horses, if MHC expression is associated with lymphocytic subtype, and if dysferlin is expressed in IMM.

ANIMALS

Twenty-one IMM horses of Quarter Horse-related breeds, 3 healthy and 6 disease controls (3 pasture myopathy, 3 amylase-resistant polysaccharide storage myopathy [PSSM]).

METHODS

Immunohistochemical staining for MHC I, II, and CD4+, CD8+, CD20+ lymphocytes was performed on archived muscle of IMM and control horses. Scores were given for MHC I, II, and lymphocytic subtypes. Immunofluorescent staining for dysferlin, dystrophin, and a-sarcoglycan was performed.

RESULTS

Sarcolemmal MHC I and II expression was detected in 17/21 and 15/21 of IMM horses, respectively, and in specific fibers of PSSM horses, but not healthy or pasture myopathy controls. The CD4+, CD8+, and CD20+ cells were present in 20/21 IMM muscles with CD4+ predominance in 10/21 and CD8+ predominance in 6/21 of IMM horses. Dysferlin, dystrophin, and a-sarcoglycan staining were similar in IMM and control muscles.

CONCLUSIONS AND CLINICAL IMPORTANCE

Deficiencies of dysferlin, dystrophin, and a-sarcoglycan are not associated with IMM. Sarcolemmal MHC I and II expression in a proportion of myofibers of IMM horses in conjunction with lymphocytic infiltration supports an immune-mediated etiology for IMM. The MHC expression also occured in specific myofibers in PSSM horses in the absence of lymphocytic infiltrates.

Dynamic exophthalmos and lateral strabismus in a dog caused by masticatory muscle myositis

CASE DESCRIPTION

A 5.5-year-old neutered male mixed-breed dog was presented for evaluation of a 5-month history of deviation of the right globe upon opening the mouth and a 1-month history of conjunctivitis in the same eye. Clinical findings, diagnostic imaging results, treatment and follow-up are described.

CLINICAL FINDINGS

When the mouth was opened, the right globe deviated rostrally and laterally. There was no pain or resistance to opening the mouth, or resistance to retropulsion of the globe. No other abnormalities were observed. Computed tomography was performed, revealing a contrast-enhancing region associated with the right masseter muscle impinging into the right orbit; this was more pronounced with the mouth open. Cytology revealed a small number of mildly to moderately atypical mesenchymal cells. Histopathology was consistent with polyphasic myositis, with a predominance of lymphocytes and plasma cells. No infectious agents were identified. Serum antibody titers for Toxoplasma gondii and Neospora caninum were negative. Serum titers for 2 M antibody were positive at 1:500, consistent with a diagnosis of masticatory muscle myositis.

TREATMENT AND OUTCOME

Therapy with prednisone was initiated. During a follow-up period of 5 months, there was no recurrence of clinical signs, and the dose of prednisone was reduced by 25%.

CLINICAL SIGNIFICANCE

To the authors' knowledge, this is the first reported case of masticatory muscle myositis causing dynamic exophthalmos and strabismus in a dog.

Principles and application of range of motion and stretching in companion animals

Optimal function after injury, surgery, or in patients with chronic conditions requires adequate motion in joints, muscles, tendon, fascia, and skin. Range of motion and stretching exercises are commonly used in companion animal rehabilitation programs to maintain or improve motion of musculoskeletal tissues. Range of motion exercises and stretching prevent adhesions from forming, help scar tissue remodeling, may improve muscle tone, and prevent future injury from occurring. Stretching is used to avoid loss of motion or to regain lost joint motion. Stretching is done manually, using external coaptation, or using therapeutic exercises. Careful documentation of range of motion is necessary.

Use of MRI for the Early Diagnosis of Masticatory Muscle Myositis

The medical records of two dogs that were diagnosed with masticatory muscle myositis (MMM) were reviewed. The reported clinical signs included intense pain when opening the mouth and restricted jaw movement. MRI detected widespread, symmetrical, and inhomogeneously hyperintense areas in the masticatory muscle. Electromyography (EMG) demonstrated severe and spontaneous pathologic activity in the temporal and masseter muscles. With early therapeutic treatment, remission of symptoms occurred within 2 mo, and no relapses were observed for the subsequent 2 yr. The gold standard for the diagnosis of MMM is the 2M antibody test, but the purpose of this study was to evaluate the use of MRI as an accurate and efficient diagnostic tool for the initiation of early therapy for the treatment of muscle myositis.

Inflammatory Myopathy with Severe Tongue Atrophy in Pembroke Welsh Corgi Dogs

A disease characterized by tongue and facial muscle atrophy has been recognized sporadically among Pembroke Welsh Corgi (PWC) dogs in Japan. The present study describes the pathologic findings of this canine syndrome. Histopathologic examinations were performed in 2 dogs, including a case of muscular biopsy. Identification and characterization of autoantibodies were attempted by fluorescent antibody test (FAT) and Western blot (WB) by using sera from 7 PWC dogs with typical clinical features, 6 PWC dogs with other clinical signs, and 2 from other breeds with polymyositis. Clinically, the 7 affected PWC dogs exhibited dysphagia with severe tongue atrophy, facial muscular atrophy, and occasional walking difficulty. Histopathologic examinations of the 2 dogs with clinical symptoms revealed moderate to severe inflammatory lesions characterized by lymphohistiocytic infiltration and muscular atrophy in the tongue and/or femoral muscles. The tongue lesions were very severe and accompanied by diffuse fatty infiltration. There were no major lesions in the nervous tissues examined. By FAT, an autoantibody against the cross striation of skeletal muscle was detected in sera from 5 affected PWC dogs. By using WB analysis, the autoantibodies recognized a 42-kDa molecule in striated muscle but not in the nervous tissues. All of the findings indicated that the unique disease of PWC dogs might be generalized inflammatory myopathy, whereas the detailed etiology concerning the dominant involvement of tongue muscles and the role of the autoantibody in the canine disease remain to be clarified.

Vacuolar myopathy in a dog resembling human sporadic inclusion body myositis

Sporadic inclusion body myositis (sIBM) is the most common myopathy in people over the age of 50 years. While immune-mediated inflammatory myopathies are well documented in dogs, sIBM has not been described. An 11-year-old dog with chronic and progressive neuromuscular dysfunction was evaluated for evidence of sIBM using current pathologic, immunohistochemical and electron microscopic diagnostic criteria. Vacuoles and congophilic intracellular inclusions were identified in cryostat sections of multiple muscle biopsies and immunostained with antibodies against amyloid-β peptide, amyloid-β precursor protein, and proteosome 20S of the ubiquitin–proteosome system. Cellular infiltration and increased expression of MHC Class I antigen were observed. Cytoplasmic filamentous inclusions, membranous structures, and myeloid bodies were identified ultrastructurally. These observations constitute the first evidence that both the inflammatory and degenerative features of human sIBM can occur in a non-human species.