Ear mites (Otodectes cynotis) are a worldwide problem with increased prevalence in kittens and especially cats housed in dense populations. Ear mites are contagious and live not only in the ear canal but also on other areas of the body and in the environment. They can survive off the host for approximately 12 days. This study compared the speed of kill of two compounds known to have efficacy against O cynotis: ivermectin and selamectin. Rapid killing of mites could be important in crowded environments like animal shelters. This study was conducted at a busy urban shelter in central Colorado. Cats entering the study had to have clinical signs suggestive of ear mite infestation, be otherwise clinically healthy as well as at least 4 weeks of age, and have live O cynotis present in both ears verified by microscopic examination. Cats were housed individually during the study. One group of cats (n = 41) was given 0.5 mL of 0.01% ivermectin otic solution (Acarexx) in each ear as a single dose. The other group (n = 41) was treated with one dose of selamectin (Revolution; 15 mg total dose for cats ≤ 5 lb; 45 mg total dose for cats > 5 lb) applied to the skin. Microscopic examinations were made at approximately 6, 10, 24, 34, 48, or 72 hours after treatment. Both drugs appeared to kill O cynotis as early as 10 to 12 hours after a single administration. Use of 0.01% ivermectin significantly reduced the time required to kill 100% of mites as compared with selamectin. Study funded by Idexx Pharmaceuticals

Commentary: Although this study demonstrated that both 0.01% ivermectin otic suspension and topical selamectin have acaricidal properties after 48 hours, the role of ear cleaning in the treatment of Otodectes was not considered. Ear cleaning at the time of diagnosis followed by either otic or topical acaricidal treatment provides faster relief to the patient and removes the immature stages present in the ear debris.—Louis N. Gotthelf, DVM

Efficacy of a single dose of an otic ivermectin preparation or selamectin for the treatment of Otodectes cynotis infestation in naturally infected cats. Nunn-Brooks L, Michael R, Ravitz LB, et al. J FELINE MED SURG doi: 10.1016/j.jfms.2011.03.003.