Equine Atypical Myopathy (EAM) is a seasonal, pasture-associated disorder of unknown etiology. The disease is characterized by a generalized degeneration of muscle fibers which can lead to sudden death due to collapse of the cardio respiratory system in over 90 per cent of cases.
Affected horses can either die peracutely or show profuse sweating, muscle weakness, recumbency, difficulty breathing and death after 12 to 72 hours. Since the etiology is unknown no effective prophylaxis exists and affected horses can only be treated symptomatically.
Large outbreaks have been reported in Europe and in 2010, 224 new cases were reported. A very similar, if not the same disease has been observed in the United States.
Environmental factors such as regular access to pasture and certain weather conditions seem to influence the incidence of the disease which occurs seasonally, mostly in autumn with fewer cases in springtime and sporadic cases in winter.
Young horses kept full time at pasture without any food supplementation are most frequently affected. Further risk factors as adjacent streams and trees, dead leaves and branches in pastures and wet windy, unpleasant chilly weather conditions.
Horse owners are advised to remove manure from pastures, provide clean drinking water and salt blocks and bring horses in from the pasture during rough weather.
Causes of EAM are thought to be ionophores, mycotoxins and phytotoxins and has been associated with ingestion of maple leaves covered with European tar spot. Stress and metabolic imbalances might predisposes horses to develop the disease.