Seasonal Tying-Up: Toxins or Deficiency?

In Minnesota, cases of pasture myopathy (muslce disease caused by pasture) associated with severe non-exercise-induced rhabdomyolysis (also know as tying-up) are often attributed to the toxins found in white snakeroot. However, researchers at the University of Minnesota said factors other than white snakeroot could be causing the syndrome.

"In Europe, there is a condition called 'atypical myopathy' (AM) that is characterized by the same clinical signs as white snakeroot toxicity," explained Carrie Finno, BSc, DVM, one of the researchers involved in the study (she's now a resident at the University of California, Davis). "However, the white snakeroot plant is not found in Europe."

Atypical myopathy is often reported during the fall months following inclement weather. The study horses (14 in all) had clinical signs similar to the European cases, which included sudden muscle stiffness that progressed to the point that the horse was unable to stand, and this often resulted in the death of the horse.

"The other striking similarity between our cases and cases of AM is that they all occurred during the fall months (September-November) when there was often inclement weather," she said.

Specific causes for atypical myopathy are still unknown, but weather-related toxin accumulation in pasture plants is thought to be the cause. Changes in weather can cause plants to store nutrients or toxins in higher concentrations than they would normally, as a defense mechanism against the elements. This usually causes the plant to be less desirable to horses, but in situations where food is scarce, horses might still eat the plant.

Finno said her greatest concern with pasture myopathy is the fact that signs can progress rapidly.

"The progression is very fast, with most horses becoming recumbent in less than 12 hours without treatment," Finno said. "Any horse with signs of pasture myopathy should be treated aggressively with intravenous fluids, antioxidants, and anti-inflammatories as the progression of the condition is rapid and often fatal.

"In pastured horses in the Midwest, it is important to supplement their diet with hay in the fall as grass becomes scarce in order to discourage the horses from grazing on less palatable--and potentially toxic--plants," Finno said. "Pasture myopathy should be considered in any horse that is housed primarily on pasture in the Midwest and presents with the following clinical signs: dark red urine, muscle fasiculations, overall weakness or recumbency, and signs of colic or choke."

Researchers who completed the study published in the Oct. 1 issue of the Journal of the American Veterinary Medical Association were Finno; Stephanie Valberg, DVM, PhD, Dipl. ACVIM; Arno Wunschmann, Dr.Med.Vet., Dipl. ACVP; and Michael Murphy, DVM, PhD.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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