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Protecting Horses from Seasonal Pasture MyopathyBy Kentucky Equine Research Staff · October 2, 2017

After many years of work, researchers pinpointed hypoglycin A toxicity as the underlying cause of atypical myopathy. The next step in understanding this devastating disease involves identifying factors that can help predict an affected horse’s chance of survival so owners can make informed, appropriate treatment and management decisions.

“Atypical myopathy occurs when horses consume seeds and leaves from trees of the Acer species, including maple and box elder,” reminded Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER).

Also called pasture-associated myopathy or seasonal pasture myopathy, this disease appears similar to rhabdomyolysis (tying-up). Affected horses show stiffness; difficulty stiffness or walking; production of dark urine; rapid breathing; and inability to rise once recumbent.

The clinical signs associated with muscle disease result from the hypoglycin A toxin. After ingestion, hypoglycin A is metabolized in the muscle cells, producing chemicals called acylcarnitines, which inhibit the normal function of muscle cells by blocking the production of energy from fat. As acylcarnitine levels rise, they spill over into the blood where they can be measured. In fact, horses with seasonal pasture myopathy have characteristic acyclarnitine profiles in their blood.

Approximately 75% of affected horses either die or are humanely euthanized within two days of onset of clinical signs.

“There is no cure for hypoglycin A toxicity, and there is little information regarding factors associated with a better outcome. Thus, helping owners decide whether or not to treat their horses remains very challenging,” Crandell summarized.

According to a recent study on seasonal pasture myopathy*, “Treatment is costly, labor- and resource-intensive, and although treatment significantly increases an animal's probability of survival, outcome is currently difficult to predict on initial assessment of a case.”

To help owners make the best possible decision in a difficult situation, Boemer and colleagues measured the acylcarnitine profiles of horses with and without seasonal pasture myopathy. They found that certain acylcarnitines could help predict survival. Unfortunately, this test can only be performed in select research laboratories, and facilities do not offer this testing on an “emergency basis.” Nonetheless, the research team suggested, “Our results provide a major incentive for such laboratories to implement, in their own organization, the analysis of corresponding samples on a daily basis to provide prompt and comprehensive diagnostics.”

“Further research in this field will help limit the unnecessary suffering of horses unlikely to survive and to help clinicians focus on horses with favorable chances of survival,” noted Crandell.

Although not a cure for seasonal pasture myopathy, vitamin E plays an important role in muscle function.

Horses should consume approximately 1,000 IU of vitamin E daily, which can often by achieved by feeding quality, green forage. Supplemental vitamin E, such as KER’s Nano•E, a rapidly bioavailable, water-soluble product easily top-dressed on feed, can be provided if you are concerned your horse is not receiving adequate levels.

*Boemer, F., J. Detilleux, C. Cello, et al. 2017. Acylcarnitines profile best predicts survival in horses with atypical myopathy. PLoS One. 12(8):e0182761.