Induced diarrhoea in horses. Part 2: Response to administration of an oral rehydration solution.

Hydration status, electrolyte balance and acid-base balance were studied in four adult standardbred geldings with castor oil-induced diarrhoea. The horses received an oral rehydration solution (ORS) at a point when signs consistent with mild decreases in effective circulating fluid volume were first detected. Within 1.5 h of ORS administration, all horses exhibited a significant metabolic acidosis. At this time, mean values for venous blood pH, [HCO3], and standard base excess were 7.264 +/- 0.011, 17.7 +/- 0.3 mmol L-1, and -8.2 +/- 0.4 mmol L-1, respectively. Throughout the duration of the study, plasma volume did not change significantly, despite a decreasing trend, which tended to recover towards normal values 8 h after administration of the ORS. Signs of abdominal discomfort were observed in all horses following the last of three doses of ORS (8-10 L) administered at 30 min intervals. Faecal fluid sodium concentration increased significantly with diarrhoea, and reached values fourfold those in normal horses, while faecal dry matter sodium concentration increased exponentially following the onset of clinical signs. Despite this increase in sodium concentration, faecal fluid remained hypotonic at all stages. Our findings suggest that, while ORS can help restore systemic fluid balance, several factors influence their effectiveness. Two likely factors identified in this study were the ionic composition of the ORS as well as the rate of administration. We concluded that the electrolyte composition of current ORS may not be ideal to treat diarrhoea in horses and that administration of 8-10 of ORS every 30 min via nasogastric tube may result in too rapid small intestinal transit to allow sufficient time for absorption.