Isoniazid toxicosis in dogs: 137 cases (2004-2014)

OBJECTIVE To establish the minimum toxic dose of isoniazid in dogs, characterize the clinical signs and outcomes for dogs following isoniazid ingestion, and determine whether IV administration of pyridoxine to dogs with isoniazid toxicosis is protective against death. DESIGN Retrospective case series. ANIMALS 137 dogs with isoniazid toxicosis. PROCEDURES The electronic database of the American Society for the Prevention of Cruelty to Animals Animal Poison Control Center was reviewed from January 2004 through December 2014 to identify dogs with isoniazid toxicosis. For each dog identified, information extracted from the medical record included signalment, estimated dose of isoniazid ingested, clinical signs, treatment, and outcome. Follow-up communication with pet owners or primary care veterinarians was performed when necessary to obtain missing information. RESULTS Clinical signs of isoniazid toxicosis were observed in 134 of 137 (98%) dogs and included seizures (n = 104), CNS signs without seizures (94), and gastrointestinal (41), cardiovascular (19), urogenital (4), and respiratory (1) abnormalities. Of the 87 dogs for which the outcome was available, 61 survived, 18 died, and 8 were euthanized. Probability of survival was positively associated with body weight and IV administration of pyridoxine and negatively associated with dose of isoniazid ingested and presence of seizures. Dogs that received pyridoxine IV were 29 times as likely to survive as dogs that did not receive pyridoxine IV. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated rapid diagnosis of isoniazid toxicosis and prompt treatment of affected dogs with pyridoxine and other supportive care were imperative for achieving a successful outcome.