EGS: where are we now?

Devastating for both horses and owners, equine grass sickness remains incompletely understood. Rosie Olley MRCVS explains what we do know

- Rosie Olley MRCVS

EQUINE GRASS SICKNESS (EGS) is a disease that predominantly affects the nerve supply to the gastrointestinal tract. But it also affects the involuntary (sympathetic and parasympathetic) nervous systems more widely and, to a lesser degree, the voluntary nervous system connections between the brainstem/spinal cord and the skeletal muscles.

The disease has been recognised in horses at grass for more than 100 years and eight in 10 horses that develop the disease will die or be euthanised as a consequence.

Clinical signs relate to the consequences of this nerve damage, with the most common problems involving the gastrointestinal tract - the tube through which food passes from the mouth to the rectum. Thus, there can be:

● difficulty swallowing

● abnormal small intestinal motility, resulting in reflux of bile from the stomach (via a nasogastric tube)

● impaction of the pelvic flexure of the large colon

● production of small, dried faecal balls with increased mucus content

These problems can cause a horse to show signs of colic (abdominal pain). Even in the absence of colic, horses often have minimal appetite and are dull in demeanour.

Other signs that can be seen include:

● a high heart rate (relative to a horse's pain or dehydration)

● mild fever

● drooping of the upper eyelids (ptosis)

● increased salivation

● patchy sweating

The posture is altered, with muscle fasciculations (twitching) and tremor often evident and, later on, this progresses to a tucked-up appearance to the abdomen and an "elephant on a drum" stance.

A distinctive sign, known as rhinitis sicca, involves a reduction in nasal secretions, so that horses develop dried crusting nasal discharge within the nasal passages. For those horses that can be treated, marked weight loss is inevitable.