Just a few years ago, the most considered area of concern for equine gastric ulcer syndrome (EGUS) was related to squamous erosions. Since then, with the advent of endoscopes longer than 3 meters, it is apparent that glandular ulcer disease near the pylorus also has an impact on horse health and performance. The European College of Equine Internal Medicine noted in their Consensus Statement that there are really two different syndromes: ESGS (equine squamous gastric disease) and EGGS (equine glandular gastric disease).
While their etiologies differ, they are both still addressed pharmaceutically using omeprazole-type products to suppress gastric acid production. “No acid, no ulcer” is one treatment objective. The goal is to have the pH of the stomach above 4 for at least two-thirds of a 24-hour period.
ESGS is also addressed through diet and management modifications, whereas EGGS tends to respond favorably to modifications in exercise programs. Horses exercised 6-7 days per week are 3½ times more likely to develop EGGS relative to horses exercised less than or equal to five days a week. Providing at least two rest days per week is an important management tool to help heal glandular disease. Behavioral stress also seems to contribute to glandular gastric disease. A common clinical sign of EGGD is unexplained weight loss.
Efficacy of oral omeprazole treatment is impacted significantly by timing of feeding because bioavailability of omeprazole might be reduced by 50-66% by ad lib feeding of hay. This can also be in part due to binding of the medication to hay. Horses fed a high-grain and low-fiber diet with an overnight fast and given omeprazole 60-90 minutes prior to their morning feed achieved greater magnitude and duration of acid suppression than horses fed ad lib [Sykes, B.W. Courses for horses: Rethinking the use of proton pump inhibitors in the treatment of equine gastric ulcer syndrome. Equine Veterinary Education Feb 2018, pp. 1-6].
It is important to stimulate turn-on of the proton pumps for acid secretion so the drug can then turn them off. This is accomplished in part by withholding nighttime feed. When a horse is fed a meal after feed is withheld, gastrin production increases. Large quantities of roughage distend the stomach and better stimulate acid secretion than the effect seen with smaller grain meals. Proton pump inhibitors (PPI) convert to their active form in the presence of gastric acid; once the PPI is turned on, its pharmacological effect is to turn off the production of gastric acid.
Sykes presented these findings at the Purina Equine Veterinary Conference in St. Louis on October 12-14, 2018. He suggested that most stalled horses tend to sleep at night and normally eat very little after 10 p.m.; they start looking for food around 4 am. Then, following the nighttime fast, the horse is given omeprazole treatment and fed 60-90 minutes later for the best effect. Roughage can then be offered free choice during the rest of the day.
Furthermore, Sykes advised that once a day treatment with oral omeprazole does not maintain acid suppression for the full 24 hours, but rather suppression lasts about 10-14 hours, with only a 4-6 hour window in a 24-hour period when oral omeprazole achieves a pH >4. Ad lib hay keeps acid production active through stimulation of stomach acid secretion whenever the horse eats.
To counteract this dilemma, an intramuscular formulation of omeprazole—not yet available in the USA—holds promise for more efficacious treatment of EGUS. IM omeprazole is given weekly and is able to suppress acid within 24 hours for up to 7 days, holding the pH above 4 for 80% of that time. Decreased acid suppression results in more healing.
In a clinical trial, 2 mg/kg IM omeprazole was administered once weekly to 22 horses with ESGD and 12 horses with EGGD. Some horses had both lesions [Sykes B.W.; Kathawala, K.; Song, Y., et al. Preliminary investigations into a novel, long-acting, injectable, intramuscular formulation of omeprazole in the horse. Equine Veterinary Journal 2017, vol. 49; pp. 795-801]. By four weeks, there was 100% healing in horses with ESGS compared to 70% given oral omeprazole. Healing of EGGS with IM omeprazole occurred in 75% compared to 25% healing with oral omeprazole. All horses receiving the injectable IM omeprazole healed at least one grade.