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
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
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
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