Effects of Intra-Rectally Administered Omeprazole Paste on Gastric Juice pH in Adult Healthy Horses
ACVIM 2008
C.L Rand; S. Stanley; N. Pusterla
School of Veterinary Medicine, University of California
Davis, CA, USA

Gastric ulcers in horses have been associated with stress, prolonged periods without feed, and low gastric fluid pH. Omeprazole is a proven anti-ulcer medication used in veterinary medicine for treatment of equine gastric ulcer syndrome. Horses have previously been shown to have an increase in gastric fluid pH following administration of both the oral and intravenous formulations of omeprazole. However, use of the oral formulation of omeprazole is often not possible in horses suffering from gastric reflux or neurologic conditions which prevent them from swallowing, and the intravenous formulation of omeprazole is expensive and can only be administered in a hospital environment. The goal of this study was to evaluate the effects of a commercially available omeprazole paste given intra-rectally on the gastric juice pH of healthy adult horses.

Ten healthy adult horses were randomly assigned to one of two groups: 6 horses were assigned to the treatment group receiving 4.4 mg/kg omeprazole paste intra-rectally for 5 consecutive days and 4 horses were assigned to the control group. Horses were fasted for 12 hours prior to gastric fluid collection. Gastric fluid was collected from each of the horses via nasogastric tube prior to (day 0) and 4 hours following the last day (day 5) of omeprazole administration. The gastric fluid pH was analyzed immediately following collection via a pH meter. Nonparametric analysis was used to compare the two groups against one another, as well as against baseline (day 0) measurements. Levels of significance were set at p<0.05.

Gastric juice pH was low in all study horses on day 0 and varied from 1.46 to 4.52 (mean±SD=3.45±1.3) for the treatment group and 2.14 to 4.46 (3.52±1.0) for the control group. On day 5, the gastric juice pH remained low in 4 treated horses (range 1.8 to 2.66), while 2 horses showed an increase in gastric pH (pH>5.0) that persisted for 24 hours following final intra-rectal omeprazole paste administration. The gastric juice pH of all control horses remained low on day 5 (range 1.7 to 3.91, 3.0±1.0). No significant differences (p>0.05) in gastric juice pH were found when groups were compared against one another on day 0 and day 5 as well as against baseline.

In conclusion, the study has shown that omeprazole paste, when administered intra-rectally, causes an increase in gastric juice pH in only 33% of the treated horses. Reasons for this variability could be related to proximity of defecation following drug administration, the formulation used in this study or differing rates of absorption through the rectal mucosa. Further work is warranted at this time to determine if a different formulation of omeprazole would result in enhanced ability to suppress gastric acid secretion and increase gastric juice pH when administered intra-rectally.

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Courtney Rand

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