Retinopathy associated with ivermectin toxicosis in five cats

Case Description-5 cats from the same household were examined because of a sudden onset of tremors, obtundation, blindness, and dilated pupils. Approximately 12 hours prior to evaluation, the owner had attempted to treat the cats for suspected ear mite (Otodectes cynotis) infestation by aural administration of a dose of an ivermectin paste intended for oral administration to horses (approx 22 mg/cat; half of the dose was administered into each ear canal). Clinical Findings-None of the cats had a menace response; all cats had dilated pupils and decreased pupillary light reflexes. Findings of fundic examination were unremarkable. Electroretinography was performed for 4 cats, and b-wave responses were identified as diminished. Toxicological assay results for serum samples from 2 cats confirmed the presence of ivermectin (450 and 610 mu g/L). Treatment and Outcome-All 5 cats made a complete recovery. Neurologic abnormalities resolved, electroretinographic responses improved, and vision was restored with no residual pathological changes detected during fundic examination. Clinical Relevance-To the authors' knowledge, the information reported here provided the first description of ophthalmic and electroretinographic findings in cats with ivermectin toxicosis resulting from transdermal administration. Clinical signs, including blindness, resolved with time, without additional medical intervention.