An 11-month-old miniature Shetland Pony colt weighing 26 kg was presented to the University of Zurich Equine Hospital for evaluation of acute onset of generalized seizures, followed by unconsciousness and recumbency, 34 hours after a massive (>25-fold) overdose of deworming paste (140 mg ivermectin and 1.05 g praziquantel). Instead of the labeled quantities of 0.2 mg ivermectin/kg and 1.5 mg praziquantel/kg, the foal had received the entire tube, equivalent to 5.4 mg ivermectin/kg and 40.4 mg praziquantel/kg. At presentation, the colt was in a stuporous condition but responded to pain stimuli including jugular venous catheter placement. Heart rate was 26/min, respiratory rate was 20/min, and rectal temperature was 34.5°C (94.1°F). The peripheral pulses were very weak and not detectable by palpation at all superficial arteries. Both jugular veins were poorly filled and capillary refill time was prolonged at approximately 3 seconds. The distal limbs and ears were cold to the touch. Palpebral and pupillary reflexes as well as menace responses were absent. In contrast, the corneal reflex was preserved and both eyes showed spontaneous horizontal nystagmus. Muscular tone of the tongue was decreased, whereas the anal reflex appeared normal. The clinical signs were most consistent with ivermectin intoxication. Ivermectin overdoses trigger severe neurologic signs by opening c-aminobutyric acid (GABA)-gated chloride channels, which in turn causes membrane hyperpolarization and blockade of neuronal impulses. In adult horses, no signs of intoxication are observed up to an ivermectin dosage of 1.8 mg/kg, whereas ingestion of 2 mg/kg has been shown to cause lethargy, ataxia, and visual impairment. The praziquantel overdose was considered less relevant in this foal, because praziquantel has a wide margin of safety, and the present exposure would result in only mild neurological and gastrointestinal signs. Analyses of blood samples at admission identified a low blood glucose concentration (3.4 mmol/L; normal, 3.6–6.1 mmol/L), normal plasma protein concentration (70 g/L; normal, 57–80 g/L) with low albumin (11 g/L; normal, 22–37 g/L) and increased globulin concentrations (59 g/L; normal, 27–50 g/L), a total bilirubin concentration of 9 lmol/L (normal, 9 –39 lmol/L), increased c-glutamyl transferase activity (GGT, 53 IU/L; normal, 5–24 IU/L), and low total calcium concentration (2.40 mmol/L; normal, 2.88– 3.55 mmol/L). Blood urea nitrogen and creatinine concentrations, as well as plasma sodium, potassium, and chloride concentrations, were within reference ranges. The blood lactate concentration was 2.3 mmol/ L (normal, <3 mmol/L). A CBC disclosed normocytic, normochromic anemia (hematocrit, 19.3%; normal, 30–42%; hemoglobin concentration, 7.0 g/dL; normal, 10.8–14.9 g/dL) with increased white blood cell count (WBC, 11.2 9 10/lL; normal, 4.7–8.2 9 10/lL). Treatment was initiated with warmed lactated Ringer’s solution (LRS) supplemented with glucose to a concentration of 2.5%, administered through a jugular venous catheter at a rate of 60 mL/kg/h. After 10 minutes, the blood glucose concentration had reached 8.3 mmol/L and treatment was continued with LRS without glucose at the same rate. After 1 hour, the fluid was changed to a maintenance crystalloid solution containing 1.5% glucose and the infusion rate was decreased to 4 mL/kg/h. By that time, peripheral pulse quality, jugular vein filling, and capillary refill time had improved, and the patient urinated. The colt was placed under a heat lamp, on a forced-air heating From the Equine Department, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland (Bruenisholz, Schwarzwald); the Institute of Pharmacology and Toxicology, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland (Kupper, Muentener, Naegeli); and the Institute of Forensic Medicine, Faculty of Medicine, University of Zurich, Zurich, Switzerland (Dally, Kraemer). The analytical methods described herein were previously presented as a research abstract. Dally AM, Kupper JR, Bruenisholz H, et al. Determination of ivermectin in plasma of a mini Shetland Pony after poisoning using LC-MS/MS – effectiveness of intravenous lipid therapy. 12th International Congress of Therapeutic Drug Monitoring and Clinical Toxicology, Stuttgart, Germany, October 2–6, 2011. Corresponding authors: H. Naegeli, Prof. Dr. med. vet., Institute of Pharmacology and Toxicology; and C.C. Schwarzwald, PD Dr. med. vet., PhD, Dipl. ACVIM & ECEIM, Equine Department, Vetsuisse Faculty, Winterthurerstrasse 260, CH-8057 Zurich, Switzerland; e-mails: naegelih@vetpharm.uzh.ch and cschwarzwald@ vetclinics.uzh.ch. Submitted October 11, 2011; Revised November 4, 2011; Accepted November 29, 2011. Copyright © 2012 by the American College of Veterinary Internal Medicine 10.1111/j.1939-1676.2011.00865.x Abbreviations: