Anouk Frieling, MSc Equine Sciences, BSc (Hons)
Atypical myopathy (AM) is a seasonal, non-exercise related, disease causing acute rhabdomyolysis (rapid breakdown of damaged muscle tissue) which affects the muscles of the horse
Atypical Myopathy Causes and Pathophysiology
Equine atypical myopathy mainly affects the respiratory muscles and muscles that support the skeleton and maintain balance
Figure 1: During autumn the seeds from the sycamore tree fall on the pasture on which horses graze. These seeds can be toxic and horses ingesting these seeds can develop atypical myopathy.
After ingestion, HGA interferes with metabolic processes
Clinical signs and Diagnosis
Clinical signs often start with the sudden onset of muscle weakness and stiffness
Figure 2: Atypical Myopathy affects the muscles of the horse. Horses that lie down and do not stand up is one of the clinical signs of AM.
Diagnosing AM based on the clinical signs is difficult and can often lead to a wrong diagnosis as the clinical signs are very similar to those of other equine diseases
Treatment
As mentioned, currently there is no specific treatment for AM affected horses
One of the symptoms of AM is secreting dark urine which is a signs of dehydration. Therefore, providing fluids is essential for rehydration, and will enhance renal clearance of excess metabolites and correction of blood parameters
As previously discussed, due to AM the fatty acid metabolism in muscle tissue is disturbed and will shift from aerobic to anaerobic
Studies suggest that the supplementation of vitamin E, selenium, carnitine and riboflavin can support AM affected horses and can increase chances of survival
Prevention
Preventing AM is important as currently there is no specific treatment. Firstly, to prevent AM in grazing horses, ingestion of toxic sycamore seeds or spouts should be prevented by checking the environment of the horse. If found, seeds and sprouts should be removed from the horse’s environment
Conclusion
Atypical myopathy is an acute seasonal disease in grazing horses, affecting the muscles of the horse. The onset of the disease is caused due to ingestion of sycamore seeds, sprouts or leaves which contain the toxic compound HGA. After intoxication the disease progresses rapidly and is therefore highly fatal. Clinical signs include muscle weakness and stiffness but the signs of the disease are very similar to other muscle diseases in horses, making it difficult to correctly diagnose a horse with AM. Currently, there is no specific treatment for the disease as the full pathophysiology is still unknown. Available treatment is mainly supportive to be able to increase the chances of survival for affected horses. Because there is no treatment available, it is important to implement good pasture management and prevent horses from ingesting toxic composites.
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