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

Genetics of Muscle Disease

In the field of equine muscle disorders, many conditions have a genetic basis. Therefore, genetic testing is an important part of the diagnostic evaluation. Validated genetic tests are currently available for 5 equine muscle disorders: hyperkalemic periodic paralysis, malignant hyperthermia, glycogen branching enzyme disease, type 1 polysaccharide storage myopathy, and myosin heavy chain myopathy. These diseases should be tested for in the appropriate breeds with clinical signs of disease or as part of breeding management. Genetic testing in veterinary medicine is not regulated, and therefore, any new genetic test offered in horses should be carefully evaluated and confirmed to be a valid test before use.

Nonexertional Rhabdomyolysis

Although horses most commonly develop exertional rhabdomyolysis, there are numerous causes for nonexertional rhabdomyolysis (nonER) that pose a serious health threat to horses. Their etiologies can be broadly categorized as toxic, genetic, inflammatory/infectious, nutritional, and traumatic and a variety of diagnostic tests are available to discern among them. This study discusses causes of nonER as well as diagnostics and treatments that are specific to each etiology. General treatment of acute rhabdomyolysis is covered in detail in the study in this issue on sporadic and recurrent exertional rhabdomyolysis.

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.

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.

ECEIM consensus statement on equine kidney disease

The aim of this consensus statement is to summarize and appraise scientific evidence and combine this with the clinical experience of a panel of experts to optimize recommendations on how to recognize and manage kidney disease in horses.

Myosin heavy-chain myopathy in 2 American quarter horses

A 1.5-year-old American quarter horse gelding (case 1) and an 11-month-old American quarter horse filly (case 2) were presented for acute onset pelvic lameness and lethargy. Case 1 had nasal discharge, while case 2 developed rapid muscle atrophy. Both horses had elevated serum creatine kinase activity. The horses showed similar polyphasic histiocytic and lymphoplasmacytic myositis with necrosis, mineralization, and regeneration. Additionally, case 1 had Streptococcus equi subsp. equi-induced suppurative retropharyngeal lymphadenitis with renal purpura hemorrhagica and myoglobinuric nephropathy. A focal pulmonary abscess caused by Actinobacillus equuli was found in case 2. Genetic testing revealed case 1 as heterozygous and case 2 as homozygous for the E321G MYH1 variant, supporting the diagnosis of myosin heavy-chain myopathy, with concomitant bacterial disease as potential triggers.

Common and atypical presentations of Anaplasma phagocytophilum infection in equids with emphasis on neurologic and muscle disease

BACKGROUND

Comprehensive descriptions of equids with granulocytic anaplasmosis (EGA) with neurologic or muscle disease and other atypical presentations are scarce in the literature.

OBJECTIVE

Describe the clinical signs, laboratory findings, treatment, and outcome of equids with EGA with emphasis on neurologic and muscle disease.

ANIMALS

Thirty-eight horses, 1 donkey.

METHODS

Retrospective study. Equids with EGA were included. The electronic data base was searched from January 2000 to December 2022 using the words anaplasmosis, ehrlichiosis, granulocytic, and rickettsia. Signalment and clinical data were reviewed. Data were evaluated for normality using Shapiro-Wilk test. Parametric and nonparametric statistics were used for normally and non-normally distributed data.

RESULTS

Common (41%) and other (59%) presentations were seen in horses ≥ 4 years of age (median, 14 years) with an overrepresentation of males (77%). Neurologic disease was common (41%), mainly presenting as diffuse symmetrical proprioceptive ataxia. Brain disease was less common manifesting as obtundation and cranial nerve deficits. Muscle disease was less common, with QH breeds with the variant causing myosin heavy chain myopathy (MYHM) having severe disease. Cavitary effusion, cardiomyopathy and disseminated intravascular coagulation (DIC) were uncommon. Clinical laboratory results varied depending on disease stage. Muscle enzyme activities were significantly higher in horses with muscle disease. Outcome was favorable with prompt tetracycline treatment. Death and long-term sequelae were not reported.

CONCLUSIONS AND CLINICAL IMPORTANCE

Common and atypical presentations of EGA have a favorable outcome with prompt tetracycline treatment. Quarter horse breeds with muscle disease should be genotyped for MYHM.

Streptococcus equi Subspecies equi

Strangles, caused by the bacteria Streptococcus equi subsp equi, is a highly contagious disease of equids classically characterized by a high fever and enlarged lymph nodes of the head. Diagnostic sampling depends on the stage of the disease. The goal of treating strangles is to control transmission and to eliminate infection while providing future host immunity. Daily temperature checking and isolation of febrile horses is the key to controlling outbreaks. Eradication of this disease will not be possible until S equi carriers are eliminated from the equine population.

Systemic calcinosis in a Quarter Horse gelding homozygous for a myosin heavy chain 1 mutation

Case Description

A 9‐year‐old Quarter Horse gelding was presented for lethargy, decreased appetite, polyuria and polydipsia (PU/PD), and severe muscle wasting suggestive of immune‐mediated myositis.

Clinical Findings

The horse displayed lethargy, fever, tachyarrhythmia, inappetence, PU/PD, and severe epaxial and gluteal muscle wasting. Clinicopathologic findings were consistent with previously reported cases of systemic calcinosis in horses, including increased muscle enzyme activity, hyperphosphatemia, increased calcium‐phosphorus product, hypoproteinemia, and an inflammatory leukogram. A diagnosis of systemic calcinosis was established by histopathologic evaluation of biopsy specimens from skeletal muscle, lung, and kidney.

Treatment and Outcome

Symptomatic treatment was complemented by IV treatment with sodium thiosulfate to reverse calcium‐phosphate precipitation in soft tissue and PO aluminum hydroxide to decrease intestinal phosphorus absorption and serum phosphorus concentration.

Clinical Relevance

This is the first report in the veterinary literature of an antemortem diagnosis of systemic calcinosis in the horse that was successfully treated and had favorable long‐term outcome.

Prevalence of clinical signs and factors impacting expression of myosin heavy chain myopathy in Quarter Horse-related breeds with the MYH1E321G mutation

BACKGROUND

The prevalence of clinical signs and factors triggering muscle atrophy and rhabdomyolysis associated with an MYH1E321G mutation in Quarter Horses and related breeds (QH) remain poorly understood.

HYPOTHESIS/OBJECTIVES

Determine the prevalence and potential triggers of atrophy and stiffness in horses homozygous reference (N/N), heterozygous (My/N), and homozygous (My/My) for the MYH1E321G mutation.

ANIMALS

Two-hundred seventy-five N/N, 100 My/N, and 10 My/My QH.

METHODS

A retrospective case-control study using a closed-ended questionnaire completed by clients of the Veterinary Genetics Laboratory at the University of California, Davis. History of clinical signs, disease, vaccination and performance were analyzed by genotype using contingency testing.

RESULTS

Atrophy occurred in proportionately more horses with MYH1E321G (My) than N/N QH and more frequently in My/My than My/N QH (P < .001; My/My 8/10 [80%], My/N 17/100 [17%], N/N 29/275 [11%]). More My/My horses had rapid atrophy (P < .001), with recurrence in 50%. Fewer My/My horses recovered versus My/N QH (P < .001). Stiffness was common across genotypes (P = .100; My/My 4/10 [40%], My/N 18/100 [18%], N/N 48/275 [17%]). Three months before the observed atrophy and stiffness, 47% of MYH1E321G QH were vaccinated or had respiratory or gastrointestinal disease. Horses achieving 100% expected performance did not differ across genotypes (50% My/My, 71% My/N, 55% N/N), but, only 4/10 My/My QH were competing. My/N horses achieved national or world championships or both.

CONCLUSION AND CLINICAL IMPORTANCE

Approximately 20% of My/N QH develop rapid atrophy. Atrophy is more common (80%) in homozygous My/My QH and less likely to resolve. Inciting causes such as vaccination and infection are inapparent in over half of cases.

Myofibre Hyper-Contractility in Horses Expressing the Myosin Heavy Chain Myopathy Mutation, MYH1E321G

Myosinopathies are defined as a group of muscle disorders characterized by mutations in genes encoding myosin heavy chains. Their exact molecular and cellular mechanisms remain unclear. In the present study, we have focused our attention on a MYH1-related E321G amino acid substitution within the head region of the type IIx skeletal myosin heavy chain, associated with clinical signs of atrophy, inflammation and/or profound rhabdomyolysis, known as equine myosin heavy chain myopathy. We performed Mant-ATP chase experiments together with force measurements on isolated IIx myofibres from control horses (MYH1^E321G−/−) and Quarter Horses homozygous (MYH1^E321G+/+) or heterozygous (MYH1^E321G+/−) for the E321G mutation. The single residue replacement did not affect the relaxed conformations of myosin molecules. Nevertheless, it significantly increased its active behaviour as proven by the higher maximal force production and Ca²⁺ sensitivity for MYH1^E321G+/+ in comparison with MYH1^E321G+/− and MYH1^E321G−/− horses. Altogether, these findings indicate that, in the presence of the E321G mutation, a molecular and cellular hyper-contractile phenotype occurs which could contribute to the development of the myosin heavy chain myopathy.

Prevalence of the E321G MYH1 variant in Brazilian Quarter Horses

BACKGROUND

In the Quarter Horse (QH), myosin heavy chain myopathy (MYHM), which is characterised by nonexertional rhabdomyolysis or immune-mediated myositis (IMM) with acute muscle atrophy, is strongly associated with the missense E321G MYH1 mutation.

OBJECTIVES

To document the existence of MYHM in the Brazilian QH population, this study includes a case report of two related QH foals with the E321G MYH1 mutation that had clinical signs of MYHM, with histological confirmation of IMM in one of the foals. This prompted an investigation the aim of which was to determine the allele frequency of the E321G MYH1 variant across QHs using a DNA archive in Brazil.

STUDY DESIGN

Cross sectional.

METHODS

To estimate the allele frequency of the E321G MYH1 variant in Brazilian QHs, 299 DNA samples from QHs used in different disciplines (reining, barrel racing, halter, cutting and racing) were analysed. DNA fragments containing the region with the mutation were amplified by PCR and used for direct genomic sequencing.

RESULTS

Of the 299 genotyped QHs, 44 animals (14.7%) were heterozygous (My/N) for the E321G MYH1 variant, and 255 (85.3%) were homozygous for the wild-type allele (N/N), implying an allele frequency of 0.074. Reining horses had a significantly higher prevalence of heterozygosity than horses in other disciplines (P = .008).

MAIN LIMITATIONS

The DNA samples were collected from 2010 to 2014. As only registered QHs were evaluated, the results may not reflect the actual incidence in the general population of Brazilian QHs.

CONCLUSIONS

The reported cases of MYHM and the high prevalence of the MYH1 mutation found in the assessed Brazilian QH population, particularly in reining QHs, suggests that MYHM should be included in genetic screening. Reasonable control measures are important to prevent an increase in the incidence of MYHM in QHs in Brazil.

Differential Expression of IGF1, IGFBP5, MSTN, and MYH1 Across Different Age Classes in American Quarter Horses

The objective of this study was to determine the influence of age on expression of insulin-like growth factor 1 (IGF1), insulin-like growth factor binding protein (IGFBP5), myostatin (MSTN), and myosin (MYH1) genes which are related to growth and muscle development in the American Quarter Horse. Thus, horses (n = 10) from weanling, yearling, 2-, 3-, and 10-year-old age classes were sampled and gene expression was assessed by RT-qPCR. ΔC_T was calculated using the hypoxanthine-guanine phosphoribosyltransferase gene as an internal normalizer. The generalized linear model was used to determine differentially expressed genes, by pairwise comparison between ages. Among technical replicates, the coefficient of variation ranged from 1.0 to 5.2% and was lower than the variation observed between biological replicates (2.1-12.9%). IGF1 demonstrated significantly lower expression in the 3-year-old age class than in weanlings and yearlings, but the 10-year-old age class displayed a significantly higher level than 2- and 3-year-old age classes. Expression of IGFBP5 was highest in weanlings compared with all other age classes. Expression of MSTN was significantly higher in weanlings than in other age classes, whereas 10-year-old horses had an intermediate level of expression, but significantly different from yearlings, 2- and 3-year-old fillies. Finally, expression of MYH1 was lower in 2- and 10-year-old horses than in weanlings and yearlings, whereas 3-year-old fillies demonstrated an intermediate level of expression. Differential expression patterns observed in this preliminary study provide insight into the physiological changes occurring throughout the life span of horses. These patterns could also help explain the variation in performance and endurance between individuals at different developmental stages.

Genetics of Equine Muscle Disease

There are 5 single-gene mutations that are known to cause muscle disease in horses. These mutations alter the amino acid sequence of proteins involved in cell membrane electrical conduction, muscle energy metabolism, muscle contraction, and immunogenicity. The clinical signs depend on the pathway affected. The likelihood that an animal with a mutation will exhibit clinical signs depends on the mode of inheritance, environmental influences, and interactions with other genes. Selection of a genetic test for use in diagnostic or breeding decisions requires a knowledge of clinical signs, mode of inheritance, breeds affected, and proper scientific test validation.

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

BACKGROUND

An E321G mutation in MYH1 was recently identified in Quarter Horses (QH) with immune-mediated myositis (IMM) defined by a phenotype of gross muscle atrophy and myofiber lymphocytic infiltrates.

HYPOTHESIS/OBJECTIVES

We hypothesized that the MYH1 mutation also was associated with a phenotype of nonexertional rhabdomyolysis. The objective of this study was to determine the prevalence of the MYH1 mutation in QH with exertional (ER) and nonexertional (nonER) rhabdomyolysis.

ANIMALS

Quarter Horses: 72 healthy controls, 85 ER-no atrophy, 56 ER-atrophy, 167 nonER horses selected regardless of muscle atrophy.

METHODS

Clinical and histopathologic information and DNA was obtained from a database for (1) ER > 2 years of age, with or without atrophy and (2) nonER creatine kinase (CK) ≥ 5000 U/L, <5 years of age. Horses were genotyped for E321G MYH1 by pyrosequencing.

RESULTS

The MYH1 mutation was present in a similar proportion of ER-no atrophy (1/56; 2%) and in a higher proportion of ER-atrophy (25/85; 29%) versus controls (4/72; 5%). The MYH1 mutation was present in a significantly higher proportion of nonER (113/165; 68%) than controls either in the presence (39/42; 93%) or in absence (72/123; 59%) of gross atrophy. Lymphocytes were present in <18% of muscle samples with the MYH1 mutation.

CONCLUSIONS AND CLINICAL IMPORTANCE

Although not associated with ER, the MYH1 mutation is associated with atrophy after ER. The MYH1 mutation is highly associated with nonER regardless of whether muscle atrophy or lymphocytic infiltrates are present. Genetic testing will enhance the ability to diagnose MYH1 myopathies (MYHM) in QH.

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.