EQUINE GRASS SICKNESS (EGS)
EGS is a disease which is characterised by damage to the parts of the nervous system that control the uptake and digestion of food. Unfortunately, EGS is often fatal, with a mortality rate of >90%.
Whilst the disease occurs throughout the UK, the true incidence is unknown. EGS does appears to be more prevalent in certain areas, with the highest incidence in the east of Scotland where it is believed that 1-2% of the grazing population is affected.
The cause of the disease remains unknown, but it is thought that a normal bacterium found in soil (called Clostrium botulinum) is involved. Under certain circumstances, this bacterium can produce a toxin which is responsible for the damage to the digestive tracts nervous system, resulting in its failure. It is believed that it is for this reason that the disease only affects grazing horses (either those grazing full or part time).
SYMPTOMS AND DIAGNOSIS
Cases that present suddenly (Acute Grass Sickness) are unfortunately invariably fatal. Whilst the horse may initially only appear ‘off colour’ or ‘hangy’, they will soon develop signs of abdominal pain (colic), caused by progressive intestinal distension as they become unable to digest food. Once diagnosed, unfortunately these cases can only be humanely euthanized (‘put to sleep’).
There is some hope for cases that present with the more insidious form of the condition (Chronic Grass Sickness). With intensive (and time consuming) nursing, 50-70% of these cases can survive, depending on the severity of the condition at initial presentation. Horses with Chronic Grass Sickness suffer a partial and gradual loss of normal digestive function. As such, they usually present with weight loss (which usually progresses very rapidly), varying degrees of anorexia, and low grade colic.
The diagnosis is generally made by a thorough veterinary examination, which picks up various clinical signs caused by the dysfunctioning digestive system. Absolute confirmation can only be made by analysis of biopsies taken from the horses bowl – this is generally a test performed post mortem.
SURVEILLANCE AND PREVENTION
It is not known exactly why certain individuals contract the disease, but it seems to be more common in certain areas / at certain premises. The disease also seems to mainly affect young horses (average age 4 years old), and the majority affected are in good or fat condition (‘good doers’).
A significant amount of the data available on the disease has been gathered through the Equine Grass Sickness Surveillance scheme. This scheme is aiming to collect as much data as possible about the disease so that, ultimately, a vaccination may be developed. Horse owners or vets who have experienced cases of EGS since 2000 can help provide more information at: www.equinegrasssickness.co.uk
By studying the data collected so far various associations / risk factors have been suggested, as well as advice for owners at a yard following an outbreak. This advice is summarised below, but should be taken on a case-by-case basis, and implemented where practical. Moreover, it would seem sensible to prioritise such strategies towards young adults, new arrivals and those in ‘show condition’, especially during spring/early summer and cool, dry periods.
- if possible, avoid previously affected premises, and avoid unnecessary pasture disturbance / exposure of soil
- try to minimise movements to new premises and changing feed / ratio unnecessarily
- consider reducing total time grazing, cutting excessive grass, or supplementing grass with forage feeding, or co-grazing with ruminants
- if removing droppings, do it by hand rather than with a mechanical dropping remover
- avoid overuse of wormers, particularly of the ivermectin class; instead base worming on regular faecal worm egg counts