T he case was examined at the Easter Bush VeterinaryHospital, The University of Edinburgh. A 520 kg, 6-year-old Thoroughbred gelding was referred for evaluation of acute onset profound depression, head pressing, and lack of response to external stimuli. The horse had a history of moderate weight loss over several months despite adequate nutrition and a good appetite. Previous episodes of weakness had been observed following administration of anthelmintics, namely, alternate doses of pyrantel and ivermectin, administered every 8 weeks. In an attempt to eliminate a possible parasitic cause of weight loss, over the 7 days prior to presentation, the referring veterinary surgeon treated the horse with a 5-day course of fenbendazole a (7.5 mg/kg PO q24h) and moxidectin b (0.4 mg/kg PO), 2 days later. A 5-day course of prednisolone c (1 mg/kg PO q24h) was initiated on the same day as the fenbendazole. This dose was then tapered to 0.5 mg/kg PO q24h, and was still being administered at the time of presentation.On physical examination, the horse was thin, profoundly depressed, unresponsive to external stimuli, and had marked weakness and ataxia. There was bilateral mydriasis and sluggish pupillary light responses. Clinical examination was otherwise unremarkable.There was a peripheral blood neutrophilia (9.2 ϫ 10 9 /L; reference range ϭ 2.7-6.8 ϫ 10 9 /L), hypoglycemia (24 mg/ dL; reference range ϭ 50-100 mg/dL) and hypoinsulinemia (Ͻ2 U/mL; reference range ϭ 5-36 U/mL). Other blood analytes were within normal limits. Urinalysis revealed a specific gravity of 1.019 and pH of 8.0. The urine was negative for blood, glucose, protein, and bile pigments, contained no cells of renal origin, but contained numerous calcium carbonate and calcium oxalate crystals.Two liters of 5% glucose d were administered intravenously. The horse responded rapidly and, within 10 minutes, was markedly brighter, less ataxic, and able to eat succulent feed and molasses. Concentrates were provided at intervals of 4 hours and hay was provided ad libitum. The dose of prednisolone was reduced by 0.05 mg/kg daily. The following morning, the horse was dull and ate feed slowly, despite being normoglycemic (58 mg/dL). The horse remained relatively stable, with the exception of one episode of mild colic, until day 6, when it exhibited head pressing, mild focal seizures (facial twitching), and profound ataxia. Venous blood analyses revealed hypoglycemia and hypoinsulinemia (16 mg/dL and Ͻ2 U/mL, respectively). Intravenous administration of 2 L of 5% glucose resulted in a clinical improvement within 5 minutes. A further hypoglycemic episode occurred early in the morning of day 7. Although a blood sample was not taken on this occasion, the clinical signs resolved within 5 minutes after intravenous administration of 1 L of 5% glucose.No hypoglycemic episodes were observed between days 8 and 10, at that time, prednisolone treatment ceased. On day 12, rectal examination identified a smooth mass occupying a large proportion of the right side of the abdomen, cranial...
