Equine Grass Sickness (alternatively termed 'equine dysautonomia') is a rare but predominantly fatal illness in horses. Grass sickness may affect all types of horse, pony and donkey, and has affected some well known horses including thoroughbred stallions.
Research has identified a number of risk factors which may increase the likelihood of a horse developing grass sickness including: soil disturbance, worming with Ivermectin based dewormers, movement to new pasture, stress, grazing on a field which has previously produced a case of grass sickness.
The cause remains unknown, however present research suggests that toxin production from the bacterium Clostridium botulinum type C is involved. Clostridium botulinum is a soil-borne bacterium, which may be better known for producing clinical signs of botulism. Research has suggested that Clostridium botulinum may cause grass sickness when the spores of C. botulinum type C are ingested and produce their toxin locally within the intestine.