Grass Sickness

Equine grass sickness (EGS) is a disease of the nerves that co-ordinate intestinal movement and peristalsis. More specifically, scientists think the disease is caused by toxins released from the bacteria Clostridium botulinum that damage these and other nerves throughout the horse's body.

 The signs of disease shown by horses with EGS reflect this damage to the nervous system:

  • Damage to nerves of the intestine causes normal gut motility to cease. This leads to distension of small intestine and impaction of large intestine with food material.
  • Nerve damage and release of adrenalin-like substances causes an increase in heart rate.
  • Damage to nerves that cause sweating causes intermittent "patchy" sweating, often on the neck and flanks.
  • Damage to the nerves controlling saliva production can cause profuse salivation.

Grass sickness occurs in two different forms. The sudden-onset (acute) form of the disease is characterised by signs of colic. In fact, it is very difficult to differentiate this cause of colic from the many other types of intestinal problem that are classed as "colic".

The second type of EGS is much slower in onset and takes the form of progressive weight loss. Over a period of up to two weeks, horses become thinner with a markedly "tucked-up" abdomen. Some people describe this as a "Greyhound" appearance (see above). Horses affected by this slower onset (chronic) form of disease become very depressed and have difficulty eating food.

There is a common misconception that grass sickness only happens in Scotland, but this is incorrect. There is little doubt that the highest prevalence of the disease is in Scotland, but EGS has been diagnosed in most counties of England and Wales. The disease does show regional clustering. Particular regions have a higher prevalence of the disease, although the underlying reasons for these differences are poorly understood at present. Generally, the southern counties of England have a lower prevalence of EGS than northern counties.

  • For more information about grass sickness see the 1 June '06 issue of Horse & Hound

    Grass sickness is a potential killer that most commonly occurs in the spring and autumn when grass growth is at its peak

    Equine grass sickness (EGS) is a disease of the nerves that co-ordinate intestinal movement and peristalsis. Common signs of the condition include distension of small intestine and impaction of large intestine with food material; an increased heart rate and intermittent "patchy" sweating, often on the neck and flanks; and profuse salivation. The condition is often fatal and currently there is no cure.  

    Research has shown that when a horse on particular premises is diagnosed with the disease, there is an increased risk of other horses nearby also suffering the disease for a period of approximately one month. The radius of increased risk is at least 10km but there is likely to be a gradient of risk; risk is highest on the affected premises and decreases with distance from those premises.

    The scientific evidence suggests the following actions will minimise the risk of further horses being affected:

    • Avoid abrupt changes in management and diet. It is tempting to move horses to a different pasture or to stable them after a positive diagnosis of EGS. There is no evidence that this is protective and plenty of evidence to suggest that dietary change increases the risk of EGS.  
    • Slowly, over the course of two to three weeks, introduce some supplementary forage (hay or haylage). Don't do this suddenly for the reason given above.
    • Be reassured that older horses and those who survive more than a month following the diagnosis of a case of EGS are likely to be very resistant to the disease.

    If your horse is stabled on "high risk" premises — those that have recently given rise to a case of EGS, the following advice will help minimise the chance of your horse getting EGS.

    • Avoid grazing young horses (less than seven years old) on high-risk premises if possible. Young horses are most at risk from EGS. Older horses are relatively resistant to the disease.
    • Avoid changes in management during the high-risk spring period (April-June). This includes moving horses to different pastures, buying new horses from other areas and changes in diet. Change in diet is one of the biggest risk factors for EGS.
    • Supplement the diet of horses "at risk" with extra hay or haylage throughout the spring.
    • Avoid digging up or otherwise disturbing the soil. Don't dig holes for fence posts, trenches for drains or foundations for buildings in the pastures of grazing horses. Mechanical removal of droppings is also best avoided.
    • Avoid the intensive use of worming drugs in the months leading up to the high-risk spring period. Worm control is obviously important, but the over-use of worming drugs may increase the risk of EGS. Talk to your vet about getting the balance right.
    • Don't worry about ploughing and re-seeding pasture or about the use of nitrogen fertilizer. Neither of these has been demonstrated to change the risk of EGS.

    Taken From Horse And Hound

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