Toxicity and efficacy of ivermectin in chelonians

Five red-footed tortoises (Geochelone carbonaria) treated for intestinal nematode parasitism with a single IM injection of ivermectin (0.4 mg/kg) were found in a state of extreme paresis or flaccid paralysis. One tortoise recovered normal motor function over the next 7 to 10 days. The remaining tortoises died within 3 days of receiving the ivermectin. The only consistent postmortem finding was a marked fatty change in the liver. Studies in the red-footed tortoise showed that some paresis will occur with dosages as low as 0.05 mg/kg. At least 3 other species of chelonians were found to be susceptible to ivermectin toxicosis at similar dosages (0.1 mg/kg or less). The leopard tortoise (Geochelone pardalis) appeared to be the most susceptible of the species tested, consistently developing mild paresis with a dosage of 0.025 mg/kg. Death occurred with dosages as low as 0.3 mg/kg. A dosage of 0.05 mg/kg was found to be safe in red-footed tortoises, provided that treatment was not repeated at intervals of less than 7 days. Shedding of nematode larvae and eggs in the feces of parasitized red-footed tortoises was prevented with 2 to 6 weekly IM injections of ivermectin at a dosage of 0.05 mg/kg, but limited necropsy findings indicated that elimination of adult nematodes from the intestines was incomplete.