What Causes Equine Grass Sickness?

Scientists are getting closer to determining what toxins could be behind this deadly condition.

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We all know that horses residing at pasture spend the majority of their days grazing. But did you know that, in certain parts of the world, grazing could put a horse at risk for contracting a potentially fatal disease? And what's more, researchers still aren't sure what causes the disease, called equine grass sickness (EGS).

At the 2014 British Equine Veterinary Association Congress, held Sept. 10-13 in Birmingham, U.K., R. Scott Pirie, BVM&S, PhD, CertEP, CertEM(IntMed), Dipl. ECEIM, MRCVS, reviewed the current knowledge about the causes of EGS. Pirie is a senior lecturer in equine internal medicine at the University of Edinburgh Royal (Dick) School of Veterinary Studies, in Scotland.

An often fatal neurologic disease, EGS primarily affects grazing horses. Horses with more severe forms of the disease experience colic, difficulty swallowing, reflux of stomach contents, excessive salivation, high heart rate, impacted intestines, muscle tremors, and patchy sweating. Horses with less severe forms experience sudden and extreme weight loss, drying of the nasal membranes, and difficulty swallowing. Only mild cases that receive intensive care survive.

Since its first reported occurrence in 1909 in eastern Scotland, grass sickness has appeared in most northern European countries and in South America. And, Pirie said, although "a vast array of etiological hypotheses have been proposed and addressed experimentally," the disease's cause remains unknown. Still, researchers have several ideas on what could be causing EGS

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Written by:

Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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