Atypical Myopathy In Grazing Horses

Atypical myopathy (AM) is a seasonal pasture myopathy which often leads to death. At present, the etiology is unknown, but a variety of causes have been hypothesized and investigated. AM is preceded by specific weather conditions, including rainy, stormy weather with little sunshine and a low temperature, but absence of frost. This causes outbreaks to occur mostly in autumn. Only horses on a pasture or horses that have been on a pasture are affected by AM. Bare, sloping pastures that contain humid areas or watercourses and trees with accumulations of dead leaves favor AM. Young, untrained horses are mostly affected by the disease, although older horses might develop AM as well. Clinical signs are recumbency, sweating, trembling, myoglobinuria, muscle stiffness and general weakness. Horses often show increased heart and respiratory rate, and congestive mucosae. Plasma creatine kinase and troponin I levels are increased, as a result of muscle damage, and a prolonged QTcf interval is found. At post mortem histological examination of skeletal muscle tissue severe lesions that resemble Zenker necrosis/degeneration are found. At biochemical level, acquired multiple acyl-CoA dehydrogenase deficiency (MADD) is found in affected horses. History, clinical signs, blood and urine analysis, muscle biopsy and post mortem examination are used to diagnose AM. Since the etiologic agent is unknown, there is no adequate therapy available. Treatment is symptomatic and consists mainly of administration of fluids and medication to support and stabilize metabolism, muscles and electrolyte levels. Therefore it is wise to accustom some preventive measures, including stabling the horse when predisposing weather conditions are forecasted and providing nutritional supplements. During the autumn of 2009, a large outbreak occurred. Although this was a large outbreak, it was less severe because of the relatively high survival rate (25%). For the future, a lot of investigation is needed to clear up the many uncertainties around AM, and since AM is a severe disease with a high mortality rate, a useful therapy is necessary.