Equine Grass Sickness: What You Need to Know

This horse with EGS exhibits weight loss and the characteristic "tucked up" abdomen.

Photo: Courtesy CISCO-ONIRIS

By Anne Couroucé-Malblanc, DVM, PhD, Dipl. ECEIM, WEVA Secretary


Equine grass sickness (EGS) is a condition that affects horses’ central as well as peripheral nervous systems. As the name implies, EGS almost exclusively affects grazing horses and results in a characteristic array of clinical signs, most of which can be attributed to neuronal degeneration in the autonomic and enteric nervous systems.

The disease takes three forms, based on the duration of the horse’s clinical signs:

  • Acute (one to two days);
  • Subacute (two to seven days); and
  • Chronic (more than seven days). 

Extensive neuronal degeneration, as seen in acute and subacute forms, causes intestinal dysmotility, which often results in death. In comparison, a proportion of horses with the chronic form, characterized by less severe neuronal degeneration, will survive. This disease was first reported in 1909 in eastern Scotland. Since then, veterinarians have reported it throughout the U.K. and in many northern European countries. Suspected cases have also been reported in the Falkland Islands and Australia, and EGS was recently described in a mule in the United States, a country largely considered free of the disease. A clinically and pathologically identical disease, mal seco, also occurs in South America, predominantly in Chile.

Clinical Signs

Horses affected by grass sickness show:

  • An elevated heart rate, or tachycardia (50 to 60 beats per minute [BPM] for chronic cases and above 70 BPM for subacute and acute cases)
  • Dysphagia (difficulty swallowing), which can be difficult to detect if animals are anorexic
  • Mucus-covered feces in the small colon and the rectum
  • Weakness
  • Abnormal salivation
  • Gastric reflux
  • Bilateral ptosis (drooping eyelids)
  • Abnormal sweating
  • Muscle fasciculations (twitching)
  • Weight loss and a characteristic "tucked up" abdomen

A number of these signs are similar to those associated with colic. So, in countries not familiar with the disease, EGS could be mistakenly diagnosed as "simple" acute colic.

What is the prognosis for these horses?

For acute and subacute cases, it is important to obtain a diagnosis quickly in order to euthanize the horse and reduce his suffering, as the mortality rate is 100%.

For chronic cases, the first sign is often weight loss with dysorexia (impaired appetite) or anorexia. About 40% of these cases can survive with good nursing. Criteria to decide if the horse’s condition might be treatable include:

  • Does he have a desire to eat and drink?
  • Can he swallow water and food?
  • Does he have signs of moderate to severe constant colic?

In chronic cases, the major obstacle to survival is anorexia. Thus, it is important to give affected horses very palatable foods.

This 4-year-old pony has a subacute form of EGS. This pony has ptosis on the right eyelid, while the left one is normal following phenylephrine instillation.

Photo: Courtesy CISCO-ONIRIS

How Do Veterinarians Diagnose EGS?

The only test veterinarians can perform on the live horse is to apply 0.5% topical phenylephrine to the eye’s cornea, which can result in temporary reversal of the ptosis and is suggestive of EGS.

Veterinarians can only gain a true diagnosis—a histopathologic analysis of a portion of ileum (last part of the small intestine) and/or the lymph nodes—post-mortem.

What are the risk factors?

Researchers have linked various risk factors for EGS to the horse, his environment, and how he’s managed:

  • Age: Young adults (2 to 7 years old) are most frequently affected.
  • Environment: A recent move to a new pasture and the time of year (with a peak incidence in the spring and early summer months) can play a role in EGS development. Climatic conditions—including cooler, dryer weather and irregular ground frosts—could also be related to disease onset.
  • Management: Feed changes (in quality and quantity) and the use (and overuse) of ivermectin-based dewormers have been linked to disease onset. Interestingly, pasture manure removal via mechanical means seems to be associated with an increased risk of disease recurrence on a premises, while poo-picking by hand is associated with a decreased risk of disease recurrence. Mowing pastures and co-grazing with ruminants are also associated with a decreased risk of disease recurrence.

The etiology (cause) of EGS remains unknown. However, the risk factors suggest the existence of a soil-borne agent which, in certain conditions, leads to the production and/or release of a neurotoxin.

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