Confirmed 2,4-Dichlorophenoxyacetic AcidToxicosis in a Dog A 2-year-old, intact male Weimaraner was evaluated for episodic extensor rigidity and a stiff gait of 24 hours’ duration. Percussion of the proximal appendicular muscles with a reflex hammer resulted in formation of dimples consistent with myotonia. Electromyography identi- fied myotonic potentials. Residues of 2,4-dichlorophenoxyacetic acid (2,4-D) were detected in both serum and urine. The dog was treated with intravenous fluid therapy for 36 hours, and clinical signs improved dramatically.Toxicosis with 2,4-D should be considered a differential for acquired myotonia in dogs with or without systemic signs. Exposed dogs with only clinical signs of myotonia can have good clinical outcomes. A confirmed clinical case of 2,4-D toxico- sis in the dog has not previously been reported. J Am Anim Hosp Assoc 2010;46:43-47. * Multimedia Content * AnnieV.Chen,DVM, DiplomateACVIM (Neurology) Rodney S. Bagley, DVM, DiplomateACVIM (Neurology, Internal Medicine) PatriciaA.Talcott,DVM,MS, PhD,DiplomateABVT Introduction The chlorophenoxy herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), is commonly used in the agricultural industry and around the home. When ingested in people, early signs include vomiting, abdominal pain, diar- rhea, and gastrointestinal hemorrhage. Neurological signs such as ataxia, nystagmus, convulsions, paralysis, and coma are seen only with higher doses. Peripheral neuromuscular signs, such as loss of tendon reflexes, increased creatine kinase activity, myotonia, and peripheral neuropathy, have also been reported. 1-3 Reports of 2,4-D toxicosis in dogs have been primarily from experimental settings. Research dogs have been given var- ious doses of 2,4-D to evaluate acute, subchronic, and chronic signs of toxicity. These experimental studies indicate that 2,4-D toxicity in dogs can lead to gastrointestinal signs and subclinical and clinical manifesta- tions of myotonia. 4-11 The purpose of this report is to describe the pre- sentation, diagnostic plan, and treatment for a confirmed clinical case of 2,4-D toxicosis in a dog with myotonia as the only clinical manifestation of disease. Case Report A 2-year-old, 35.6-kg, intact male Weimaraner was referred for episodic extensor rigidity and abnormal gait in all limbs for the previous 24 hours. Prior to this event, the dog was normal to the owners and had no previous medical history of illness. The dog was housed in an outdoor kennel and had free roam within a fenced yard, which was in close proximity to a plant nursery. The dog had no travel history outside the state of Washington. Upon presentation, the dog episodically exhibited an extended posture in all limbs. This posture was most pronounced immediately upon stand- ing from a recumbent position. In some instances, the dog would fall, usually in a lateral direction, as he attempted to stand. Occasionally, the dog could stand but would immediately knuckle over in all paws and sub- sequently fall upon walking. While in lateral recumbency, periods of extensor rigidity lasted for 10 to 15 seconds, especially when the dog was stimulated either with physical touch or sound. When supported by the C JOURNAL of the American Animal Hospital Association 43 From the Department of Veterinary Clinical Sciences (Chen, Bagley) and the Department of Veterinary and ComparativeAnatomy, Pharmacology, andPhysiology(Talcott), College of Veterinary Medicine, Washington State University, 100GrimesWay, Pullman, Washington 99164.