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Spinal Research


Saddle-up Answers - Grass Sickness

Several readers asked for more information on grass sickness, following the tragic death of top racehorse, Dubai Millennium, from this disease. 

Grass sickness is a disease of horses, ponies and donkeys that causes damage to parts of the nervous system, producing the main symptom of gut paralysis. The cause is unknown but it is thought to be due to a type of toxin in grass.

It was first recognised in 1907 following an outbreak in army horses in Scotland.  The UK has the highest incidence of grass sickness, but it also occurs in Northern Europe.   A very similar condition  called mal seco (dry sickness), has now been recognised in Argentina, the Falklands and Chile. 

Within the UK, the highest incidence is in eastern Scotland, particularly Perthshire and Angus.  It is less common in England and Wales, although still a significant problem in many areas including Northumberland, Yorkshire, Co. Durham and East Anglia. 

It can affect all breeds and sexes, and at all ages with the greatest number of cases in 2 - 7 year olds.  Very young foals do not appear to be affected, probably because they are eating little grass, and older horses may develop a degree of resistance.  It occurs most commonly between April and July.  Most cases occur in horses spending considerable time at grass, but there have been a few cases in horses with no access to pasture.

It appears to be most common in horses newly arrived on the premises and stress may be implicated.

 In severe cases, the onset is sudden and the horse may die (or require euthansia, within two days.  Severe colic symptoms, difficulty in swallowing, abdominal distention, and constipation occur.   Milder cases may show similar symptoms, including rapid weight loss over a week or more.   

In chronic cases, there may be mild, intermittent colic, reduced appetite and varying degrees of difficulty in swallowing.  Diagnosis can be difficult in the early stages and there is no non-invasive test.  

Treatment is rarely considered in acute cases, but mild and chronic cases may respond to early treatment with a reasonable chance of recovery.  If you suspect grass sickness, call your vet immediately!

Additional information of interest - from Cob Nut:

The last FHAGBI (Friesian Horse Association of Great Britain and Ireland) newsletter included an article on grass sickness, which suggested that it may be due to cyanide poisoning. Cyanide in the soil can be concentrated in certain plants, especially white clover, and if the horses eat these they are then at risk. Thus the factors which this article suggested needed to coincide for horses to be at risk were (a) a soil with a high incidence of cyanide, and (b) the presence of plants which would collect and concentrate that cyanide.

I do not know how authoritative this article was, and indeed, since it referred to the need for further research subjects, it would appear to be at the stage of hypothesis rather than proven link.  It is, however, an interesting angle on the subject.


Information about grass sickness on the internet.
Animal Health Trust

1999 BEVA Congress report

Liverpool University - Leahurst current information and research

Cornell University - article


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