QUARTERLY BEAT / APRIL 2025
REC
TOP POISONS THAT KILL Each year, the ASPCA Animal Poison Control Center manages hundreds of thousands of poisoning calls. The majority of toxicants affecting dogs and cats do not kill acutely 1 – it takes several days before end-stage effects from toxicosis occur (e.g., anuric renal failure, acute hepatic necrosis, etc.). Likewise, only a smaller percentage of toxicants result in acute death. 1 In this lecture, we will review the mechanism of toxicosis, clinical signs, and overall treatment of the deadly and most unusual drug overdoses seen in dogs and cats. The reader is advised to contact the ASPCA Animal Poison Control Center (888-426-4435) for lifesaving, 24/7 advice as needed. Justine Lee, DVM, DACVECC, DABT Director of Medicine, VETgirl ISONIAZID: Isoniazid (commonly known as INH) is a human medication used for tuberculosis. While it is used in veterinary medicine to treat Mycobacterium or Actinomyces, it has a narrow margin of safety in dogs and cats. 2,3 This drug works by blocking the synthesis of mycolic acid. INH depletes the CNS of pyridoxine and also decreases levels of GABA within the brain. Many assume that since this is an “antibiotic” that it is safe; however, when accidentally ingested in dogs (and rarely, cats), it can result in severe CNS signs (e.g., tremors, refractory seizures, coma, death). The LD50 in dogs is estimated to be as low as 50 mg/kg; 2,3 at this same dose, seizures can be seen. One 300 mg tablet can result in severe poisoning in a 10-pound dog. Other clinical signs include GI signs (e.g., hypersalivating, vomiting, diarrhea), acid-base disturbances (e.g., metabolic acidosis), hyperthermia (secondary to tremors or seizures), and organ injury (e.g., hepatic injury, acute kidney injury, etc.). Due to the rapid onset of clinical signs, it is often too late to decontaminate the patient. Gastric lavage under anesthesia may be necessary. Treatment also includes IV fluids, antiemetics, anticonvulsants, muscle relaxants, supportive care, and the antidote pyridoxine hydrochloride (typically available as 100 mg/ml) (Dose: suggested dose of 71 mg/kg IV, diluted to 5-10%, slow over 30-60 minutes). 2,3 Clinicopathologic monitoring should include a biochemistry panel and recheck hepatic panel 3-5 days later. 5-FLUOROURACIL (5-FU): The most life-threatening topical toxin to dogs and cats is 5-fluorouracil (5- FU). 5-FU, commonly known by the brand names Efudex®, Carac®, Adrucil®,
and Fluoroplex®, is a prescription anti-neoplastic medication that is often used for treatment of actinic keratosis or superficial basal cell carcinoma in humans. It is commonly sold in low concentration products (e.g., 0.5-5%), and works by inhibiting DNA and RNA synthesis and production, resulting in programmed cell death. 4-7 While IV administration of 5-FU is occasionally used as a chemotherapeutic agent in dogs (e.g., for mammary gland tumor, etc.), it is not recommended for use in cats. Decades ago, topical 5-FU was used in cats for the treatment of squamous cell carcinoma; however, it resulted in severe toxicosis and death due to its narrow margin of safety. Clinical signs of 5-FU toxicosis can often be seen within 30 minutes up to 6 hours; death has been reported as early as 7 hours. 4-7 Clinical signs include acute GI signs (e.g., hypersalivation, anorexia, vomiting, abdominal pain, diarrhea, bloody diarrhea, etc.), CNS signs (e.g., ataxia, tremors, seizures), and bone marrow suppression (e.g., anemia, leukopenia, thrombocytopenia). 4-7 The lowest reported toxic (oral) dose in dogs is 6 mg/ kg, while the minimal reported lethal dose is 20 mg/kg. One case report did have a dog survive ingestion of 46 mg/kg of 5-FU. 4-7 That said, the prognosis with 5-FU toxicosis is typically grave in cats and guarded in dogs (with a reported survival in dogs of approximately 25%). Death typically occurs due to secondary complications from the 5-FU such as sepsis (due to leukopenia), increased intracranial pressure (due to persistent seizures), intracranial hemorrhage (due to severe thrombocytopenia), or DIC (due to severe seizures). Unfortunately, most patients present with severe clinical signs, where it is too late to perform decontamination. Therefore, treatment should be aimed at symptomatic supportive care, anti-convulsant therapy, anti-emetics, anti-diarrheals, IV fluids (to help maintain perfusion), thermoregulation, broad-spectrum antibiotics, clinicopathologic monitoring, and symptomatic supportive care. If the patient is able to survive the acute In this VETgirl Webinar “Top Poisons That Kill” on December 18, 2024, Dr. Justine Lee, DACVECC, DABT reviews the top 5 poisons that have a high fatality rate, including isoniazid, 5-FU, ethylene glycol, organophosphates/carbamates, and bifenthrin. If you see any of these, you want to read on to make sure you’re aware of these top poisoning fatalities! In case you missed the webinar, watch it again HERE or read the cliff notes below!
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