The owner of 5 farm cats had treated them for suspected ear mites with an equine oral ivermectin paste (22 mg/cat divided into each ear). All cats were presented for sudden onset of tremors, obtundation, blindness, and dilated pupils within 12 hours of application. The cats were hospitalized for 8 hours of observation with no treatment.
Toxicologic testing performed on 2 cats confirmed high levels of systemic ivermectin. Retinal responses to light, determined by electroretinography (ERG), were abnormal in all 4 cats tested. Clinical signs improved within 5 days; recheck ophthalmologic examinations after 1 month showed no residual effects.
Ivermectin is a broad-spectrum antiparasitic drug that can be used in cats in microdoses (eg, 0.1 mg/cat topically) for ear mites. The mechanism causing blindness secondary to ivermectin toxicosis is not fully understood. If ivermectin crosses the blood-retinal barrier and increases GABA release, retinal toxic effects could theoretically develop. This is supported by the findings of this report.