Equine Grass Sickness

Grass Sickness is a disease that causes damage to parts of the nervous system which control involuntary functions. The main symptom seen with this is that gut movement stops. The cause of this condition is unknown, however, as the disease is seen almost exclusively in horses grazing, a toxin is considered most likely.

Grass sickness affects horses, ponies and donkeys of all breeds. There is some evidence that mares may be at slightly less risk than stallions and geldings although this is still being investigated.

The disease occurs in horses of all ages, however, most cases are seen in 2 to 7 year olds. The reason for this age group being most at risk is thought to be because the younger animals are not yet eating large quantities of grass and the older animals may have developed a degree of resistance to the disease.

Cases have been seen in every month of the year but most are seen between April and July. Grass sickness, as its name suggests, is strongly associated with grazing, however, horses don’t appear to need to be on pasture full-time to get the disease and the type of grazing does not seem to have any affect on the number of cases.

Animals undergoing stressful situations appear to be predisposed to the condition. These stresses include recent purchase, mixing with strange horses, breaking, castrating and travelling long distances.

Animals that are slightly overweight appear to also be predisposed.

Worming more frequently than is necessary has also been found to be a risk. Horses should still be wormed regularly and the wormer rotated each year. See section on worming.

Other risk factors include increased numbers of horses on the pasture, mechanical dropping removal, anything that causes soil disturbance, harrowing, and the presence of birds on fields.

Clinical Signs

Grass sickness occurs in three main forms, acute, subacute and chronic.

Acute

Affected animals will show signs of colic with distended, imotile small intestines and nasogastric reflux (foul smelling fluid pouring from the nose). These animals will often have a high heart rate patchy sweating and muscle tremors and will either die suddenly or have to be put to sleep immediately.

Sub-acute

The symptoms in the sub-acute form are similar those of the acute form but are less severe. Affected animals may show colic, trouble swallowing, increased heart rate, drooping eyelids, trembling, patchy sweating and rapid weight loss. Such cases may die or require to be put down within a week.

Chronic

The main sign in chronic grass sickness is rapid, severe weight loss, horses with this form of the disease have a typical ‘grey hound’ appearance. It was previously thought that nearly all cases died and that the few which survived made only a partial recovery and were subsequently useless for work. This is now known to be incorrect and attempts at treating the chronic form of the disease now made.

Diagnosis

The symptoms may seem quite clear-cut but unfortunately not all affected animals show all these signs and it can sometimes be very difficult for the veterinary surgeon to distinguish grass sickness from other causes of colic and weight loss.

At present the only reliable test to diagnose grass sickness is the microscopic examination of a section of small intestine. Because of this a diagnosis is usually only made once the animal has died and a post-mortem exam performed. The application of eye drops that reverse the drooping of the eyelids can be very useful as a test in the live patient, however, this can still only be used as a guide rather than a true diagnosis.

Treatment

The key to treating horses with grass sickness is carefully selecting cases where successful treatment is possible. Treatment should only be considered in chronic cases that are not in much pain, will still eat at least a small amount and are still interested in life. Animals that don’t meet these criteria including all acute and sub-acute cases will never make a full recovery. Treatment of chronic cases involves the feeding of palatable, easily swallowed food e.g. chopped vegetables, grass and high energy concentrates soaked in molasses. There are no rules in what to feed it is a case of feeding whatever high energy feed the horse seems interested in and varying the feed to find what this is must be done frequently.

Nursing is also vital and provides the mainstay of management. The patients require constant stimulation by human contact, frequent grooming to prevent them becoming scurfy and sticky with sweat. With careful selection of patients recovery rates of 70% can be seen in some specialist centres.

Prevention

Until the cause is known, exact methods of prevention will not be fully understood, however, many years of research have provided many guides in preventing the disease.

Reducing risk

  • Minimise exposure to pastures where previous cases have occurred.
  • Minimise any pasture/soil disturbance (e.g. harrowing/mechanical dropping removal/pipe-laying/construction work etc.).
  • Minimise soil exposure (e.g. close grazing/poaching of fields).
  • Avoid any sudden changes in diet.
  • Avoid the “over-use” wormers (e.g. rotate with other classes of wormers).
  • Preventative measures
  • Co-graze with cattle or sheep.
  • Regular grass cutting on pastures.
  • Hand removal of droppings.
  • Feeding hay/haylage to grazing animals.

These changes should be prioritized if necessary to cover animals at greater risk i.e. aged between 2 and 7, and at certain times of the year, especially spring and early summer.