Anatomy and Function of the Horse’s Stomach – And Why So Many Horses Suffer from Gastric Ulcers

The Anatomy of the Horse’s Stomach

The horse’s stomach is a relatively small, L-shaped organ with a capacity of only about 10–15 litres – surprisingly little for such a large animal. Food enters the stomach via the oesophagus, where it mixes with gastric juices and begins the digestive process. A unique feature is the two-part division of the stomach lining:

• Pars non glandularis (non-glandular part): This upper third does not produce gastric acid and has little protection against acid.
• Pars glandularis (glandular part): Produces stomach acid and is protected by mucus and bicarbonate.

Between the two sections lies the Margo plicatus, a key anatomical border and a common weak point for ulcer development.

Function of the Stomach

The stomach’s main tasks include:

• Mixing food with gastric juice via muscle contractions
• Acidifying feed with hydrochloric acid (HCl) to kill pathogens
• Breaking down proteins using the enzyme pepsin

Important to note: Unlike humans, the horse’s stomach produces gastric acid continuously – even when the horse isn’t eating.

Why Do Gastric Problems Occur? – Equine Gastric Ulcer Syndrome (EGUS)

EGUS is a common yet often underestimated condition in horses. It involves inflammation or ulcers of the stomach lining caused by an imbalance between acid production and protective mechanisms.

Causes:

1. Feed gaps over 4 hours
2. Insufficient roughage intake
3. High levels of concentrate feed (grains, sugars)
4. Stress (housing, training, stable changes)
   
5. Medications (especially NSAIDs, corticosteroids)
   
6. Poor feed quality

If feed intake is too low, saliva and fibrous material are missing as natural buffers. The acid then attacks the unprotected lining of the non-glandular part – leading to irritation, inflammation, and ulcers.

Feeding: A Central Role

Roughage as a Protective Factor
Forage such as hay or grass should always be available – or at least provided often enough to prevent feed gaps of more than 3–4 hours.

Benefits:

• Plenty of saliva = more bicarbonate for buffering
• Loose structure = good mixing with gastric juice
• Fast stomach passage = acid moves along quickly

Concentrate Feed as a Risk Factor
Grain-based feed is fermented in the stomach, producing lactic acid – another source of acid. Concentrates are also dense and poorly mixed with stomach acid, staying in the stomach longer – ideal conditions for acid-related damage.

Risky Feed for Sensitive Horses

Tip: Small amounts may be safe for healthy horses – but should be strictly avoided in “ulcer-prone” horses!

Medications as a Risk Factor

Anti-inflammatory medications (NSAIDs) like phenylbutazone inhibit production of protective prostaglandin E2. Without this, less mucus is formed – weakening the stomach’s defences. Other risk medications:

• Corticosteroids
• Mucolytics
  
• Long-term antibiotics

Stress – The Invisible Trigger

Stress reduces stomach blood flow and suppresses mucus production. Common stressors:

• Frequent transportation
• Competition-related pressure
  
• Unstable herd dynamics
• Food-related aggression or rank conflicts
• Stall housing with limited movement

Minimising stress through species-appropriate management is crucial for ulcer-prone horses.

Prevalence – An Underestimated Epidemic?

A study (McClure et al., 1999) revealed alarming figures:

• 93% of racehorses
• 63% of sport horses
  
• 50% of foals
• 37% of leisure horses

…showed gastric lesions during gastroscopy. The actual numbers may be even higher, as many symptoms are non-specific (e.g. poor performance, girthiness, dull coat, yawning, chewing without food, colic tendencies).