Genetic myopathies are muscle disorders that occur because of hereditary or spontaneous genetic mutations, with clinical signs present at birth or developing later with age. The conditions are primarily noninflammatory and present clinically as various degrees of altered muscle function. Clinical signs vary according to condition. Signalment, age of onset of clinical signs, clinical pathology, and genetic testing, where available, are used for diagnosis. Generally, no effective treatments to reverse clinical disease exist beyond supportive therapy. Breeding affected or carrier animals is discouraged.
Caprine Myotonia
Myotonia congenita in goats is due to an autosomal dominant mutation that has incomplete penetrance in the skeletal muscle chloride channel. Goats with this mutation have been selected for as a breed and are commonly referred to as fainting goats. Clinical signs range from stiffness after rest to marked general rigidity after visual, tactile, or auditory stimulation and usually develop by 6 weeks old. These signs remain throughout the animal’s life but are not progressive. A diagnosis of myotonia is made by identifying the characteristic “dive bomber” discharges in electromyography and/or by genetic testing.
Congenital Muscular Dystonia in Belgian Blue Cattle
Congenital muscular dystonia in Belgian Blue calves is caused by an autosomal recessive mutation in the gene encoding the neuronal glycine transporter. Affected calves exhibit clinical signs of lateral recumbency, low head carriage, and transient muscle spasms after tactile or auditory stimulation.
Phosphorylase Deficiency in Charolais Cattle
Phosphorylase deficiency, a mutation in the myophosphorylase gene in Charolais cattle, produces clinical signs of exercise intolerance and muscle necrosis very similar to nutritional myodegeneration. The disease has been recognized in many countries, including the US (1) and New Zealand (2). Affected animals become exercise intolerant, might collapse when forced to exercise, and develop muscle necrosis characterized by increased serum CK and prolonged recumbency. Supportive care during episodes has allowed many young animals to survive and, without the owner's knowledge, enter the breeding herd.
Porcine Malignant Hyperthermia
Malignant hyperthermia in swine is due to an autosomal recessive genetic mutation in the skeletal muscle ryanodine receptor 1 gene (RYR1) that causes abnormal meat quality in swine. Pietrain, Poland China, and certain strains of Landrace pigs are affected. During transportation or anesthesia, pigs develop increased body temperature, extreme rigidity of the skeletal muscles, and lactic acidosis. At slaughter, affected muscles become pale, soft, and exudative, which diminishes meat quality. A genetic test is available to diagnose affected and carrier animals.
Porcine RN(–) Glycogen Storage Disease
The RN(−) (rendement Napole) phenotype is common in Hampshire pigs. It is due to an autosomal dominant mutation in the protein kinase adenosine monophosphate (AMP)-activated gamma 3 subunit gene (PRKAG3), which encodes the gamma 3 isoform of AMP-activated protein kinase (AMPK). Clinically, pigs appear healthy; however, the 70% increase in glycogen content in skeletal muscle causes poor meat quality at slaughter.
Pseudomyotonia in Cattle
Bovine congenital pseudomyotonia is an impairment of muscle relaxation induced by exercise that prevents animals from performing rapid movements because of an autosomal recessive myopathy. Different mutations in the ATP2A1 gene that encodes the Ca2+-ATPase in skeletal muscle sarcoplasmic reticulum have been identified in the Chianina, Belgian Blue, and Romagnola breeds. Clinical signs include muscle cramping, stiffness, inability to move rapidly, and collapsing episodes after exertion. Condition is distinguished from true myotonia by the lack of electrical activity on an electromyogram.
For More Information
Ekinci İB, Chłodowska A, Olejnik M. Ionophore toxicity in animals: a review of clinical and molecular aspects. Int J Mol Sci. 2023;24(2):1696.
Gadelha IC, Fonseca NB, Oloris SC, Melo MM, Soto-Blanco B. Gossypol toxicity from cottonseed products. ScientificWorldJournal. 2014:231635.
References
Angelos S, Valberg SJ, Smith BP, et al. Myophosphorylase deficiency associated with rhabdomyolysis and exercise intolerance in 6 related Charolais cattle. Muscle Nerve. 1995;18(7):736-740. doi:10.1002/mus.880180710
Johnstone AC, McSporran KD, Kenny JE, Anderson IL, Macpherson GR, Jolly RD. Myophosphorylase deficiency (glycogen storage disease Type V) in a herd of Charolais cattle in New Zealand: confirmation by PCR-RFLP testing. N Z Vet J. 2004;52(6):404-408. doi:10.1080/00480169.2004.36459
