Grass sickness, alternatively termed equine dysautonomia, is a rare but predominantly fatal illness in

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

s. Grass sickness may affect all types of horse,

pony A pony is a type of small horse ('' Equus ferus caballus''). Depending on the context, a pony may be a horse that is under an approximate or exact height at the withers, or a small horse with a specific conformation and temperament. Compared ...

and

donkey The domestic donkey is a hoofed mammal in the family Equidae, the same family as the horse. It derives from the African wild ass, ''Equus africanus'', and may be classified either as a subspecies thereof, ''Equus africanus asinus'', or as ...

, and has affected some well known horses including the thoroughbred stallions Dubai Millennium,

Moorestyle Moorestyle (1977–1984) was a British Thoroughbred racehorse and sire. He was unfashionably bred, sold cheaply as a yearling and began his career in minor races. As a three-year-old however, he improved to become the one of the outstanding ...

and Mister Baileys.

Clinical signs

Grass sickness is a

polyneuropathy Polyneuropathy ( poly- + neuro- + -pathy) is damage or disease affecting peripheral nerves (peripheral neuropathy) in roughly the same areas on both sides of the body, featuring weakness, numbness, and burning pain. It usually begins in the hand ...

affecting the central, peripheral and enteric

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

s. The majority of visible clinical signs are related to paralysis within the digestive tract although nerve damage occurs throughout the body. There are three forms of grass sickness: * acute grass sickness (AGS) - horses display signs of colic and require euthanasia or die within 48 hours * subacute grass sickness (SAGS) - horses display clinical signs similar to AGS but with less severity and may survive up to 7 days * chronic grass sickness (CGS) - horses present with severe and rapid weight loss and a selected portion of these cases may survive. Clinical signs common to all subsets include: depression, anorexia, colic (moderate with AGS/SAGS and mild with CGS), excess salivation, constipation, nasogastric fluid secretion, patchy sweating, muscle tremors and eyelid drooping.

Cause

The cause remains unknown, but the toxin produced from the bacterium ''

Clostridium botulinum ''Clostridium botulinum'' is a Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce the neurotoxin botulinum. The botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans ...

'' type C may be involved. ''Clostridium botulinum'' is a soil-borne bacterium, which may be better known for producing clinical signs of botulism. It may cause grass sickness when the spores of ''C. botulinum'' type C are ingested and produce their toxin locally within the intestine. It is also linked to pasture mycotoxicosis, which comprises a very large and diverse population of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

Risk factors

The main risk factor for grass sickness, as the name may suggest, is grass. The disease is almost always seen in grazing animals, although there are isolated reports of the condition occurring in stabled horses. Grass sickness is most frequently seen in young horses aged between two and seven, and is particularly prevalent during April, May and June, and later in the autumn, after a spurt of grass growth.

Diagnosis

Diagnosis of grass sickness in the live animal requires a thorough clinical examination including a rectal examination. Definitive diagnosis can only be made at surgery (where biopsies of the gut are taken) or at

post-mortem An autopsy (post-mortem examination, obduction, necropsy, or autopsia cadaverum) is a surgical procedure that consists of a thorough examination of a corpse by dissection to determine the cause, mode, and manner of death or to evaluate any d ...

(where samples from the nerves are taken). Differential diagnoses for grass sickness are varied and include any other cause of colic and weight loss, tying-up,

laminitis Laminitis is a disease that affects the feet of ungulates and is found mostly in horses and cattle. Clinical signs include foot tenderness progressing to inability to walk, increased digital pulses, and increased temperature in the hooves. Seve ...

botulism Botulism is a rare and potentially fatal illness caused by a toxin produced by the bacterium ''Clostridium botulinum''. The disease begins with weakness, blurred vision, feeling tired, and trouble speaking. This may then be followed by weakne ...

, choke and dental problems.

Treatment

There is no treatment for grass sickness. A proportion of CGS cases survive following periods of intensive nursing. Overall, the mortality rate of equine grass sickness is over 95%.

History

The first cases of grass sickness were recorded in eastern

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...

in 1909.

References

{{Reflist Horse diseases