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Equine grass sickness (EGS) is a debilitating and usually

fatal degenerative disease of the neurological system

affecting grazing horses, ponies and donkeys.

Emma Houghton BVetMed Cert AVP(EM) MRCVS,

Endell Equine Hospital

The UK has the highest incidence of EGS

worldwide with studies showing there are

specific high-risk areas throughout the country.

EGS was first reported in 1909 in Scotland

and since then numerous theories have been

proposed for the cause. Research in the past

20 years has been directed towards the

association with Clostridium botulinum type

C and its neurotoxins. Studies have shown

that horses with EGS have lower antibody

titres to C. botulinum type C. In addition, it

has been shown that horses with higher

antibody titres have a reduced risk of

developing the disease. It is not believed to

be the ingestion of the preformed C. botulinum

type C neurotoxins which result in the

pathogenesis of EGS but that a combination

of risk factors triggers the production of C.

botulinum type C neurotoxins by bacteria

present within the gastrointestinal tract.

Veterinary surgeon

Emma Houghton

XLEquine practice

Endell Equine Hospital




Risk Factors

Grazing at pasture

Recent movement to a new yard or changed field within the previous 2 weeks

Change of feed type within the previous 2 weeks

Previous occurrence of cases at yard

Increased soil nitrogen content, pasture disturbance or higher herbage within pastures,

in particular Ronunculus species (buttercups)

Age: peak incidence in 2-7 year old horses

Good body condition score

Use of an ivermectin anthelmintic

Weather: cooler, drier weather and irregular ground frosts

Increased risk of disease has been shown with:

Figure 2: Ptosis (drooping) of both left and

right upper eyelids

Figure 1: Dysphagia with food and saliva

emerging from both nostrils