This seasonal myopathy is attributed to the toxin, hypoglycin A, found in seeds (samara) of the box elder tree (Acer negundo), the sycamore maple (Acer pseudoplatamus), and several other maples, including the Japanese maple, silver maple, sugar maple, and mountain maple trees. Hypoglycin A induces a Multiple Acyl-CoA Dehydrogenase Deficiency (MADD). This syndrome causes a mitochondrial enzyme deficiency impacting lipid metabolism in type I myocytes. The disease is most commonly seen in autumn or spring following a few days of bad weather.
Signs of Atypical Myopathy
Affected horses suddenly show signs of stiffness, with dark coloured urine (myoglobinuria). They become weak and rapidly progress to recumbency. Cardiac changes, tachycardia and dysrhythmias, are common. Interestingly, despite these signs the horses do not appear to be overly painful and will continue to consume feed despite lying down. Unfortunately the mortality rate is very high despite therapy. Horses less than 5.25 years have a 87.1% chance of dying. Younger animals are more sensitive to the toxins. The survival rate increases to 50% if the horse is still alive by day 3.
Consistent with widespread skeletal muscle damage the enzymes CK and AST are markedly elevated. Hypocalcaemia is common. The acylcarnitine profile, hypoglycin A, or MCPA-carnitine concentration will confirm the diagnosis.
Treatment is for any cause of muscle damage (rhabdomyolysis), including intravenous fluids, anti-inflammatory drugs and anti-oxidants. It has been suggested that forcing carbohydrate (over lipid) metabolism may be helpful. This is through intravenous glucose, insulin and carnitine.
Tags: Musculoskeletal; Toxicities