The exact causes of grass sickness are still controversial, although there is strong evidence implicating Clostridium botulinum type C toxin. Grass sickness can occur at any age after weaning and at any time of year, but it is most common in the spring and in horses between 2 and 7 years of age. It is rarely seen in housed stock and is most common in Great Britain. Horses with grass sickness develop paralysis of their gut. Visible signs include patchy sweating, twitching of the muscles of the shoulders and flanks, a dropping of the penis, “droopy” eyelashes, drooling, hard feces, and the regurgitation of food out through the nose. Some horses assume a “tucked-up” stance with the legs held close together. Less visible signs include an increase in the size of the intestines and stomach, displacement of the large intestine, rupture of the gastric system due to fluid, and degeneration of neurons. There is no reliable laboratory test for diagnosis.

The severity of the toxicity varies: some horses survive for weeks or months and, in some instances, the horse can recover. In more severe cases, death occurs within 24 hours to 1 week. Mildly affected horses can survive with nursing care and a wide variety of feeds. For severe cases, veterinarians often recommend euthanasia on humane grounds, because the disorder is very painful.

Last full review/revision July 2011 by William B. Thomas, DVM, MS, DACVIM (Neurology); Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Kyle G. Braund, BVSc, MVSc, PhD, FRCVS, DACVIM (Neurology); Cheryl L. Chrisman, DVM, MS, EDS, DACVIM (Neurology); Caroline N. Hahn, DVM, MSc, PhD, DECEIM, DECVN, MRCVS; Charles M. Hendrix, DVM, PhD; Maureen T. Long, DVM, PhD, DACVIM; Robert J. Mackay, BVSc, PhD; Karen R. Munana, DVM, MS, DACVIM (Neurology); Charles E. Rupprecht, VMD, MS, PhD; Josie L. Traub-Dargatz, DVM, MS, DACVIM; Susan L. White, DVM, MS, DACVIM