Otodectes cynotis, a psoroptid mite, is the cause of a highly infectious and very irritating otitis externa in its host. Although this mite is not species-specific and may infest dogs, ferrets, foxes, and even humans (human infestation usually manifests as pruritus on the arms and trunk), the parasite is principally associated with otitis externa in cats, especially young cats. Approximately 50-80% of cases of otitis externa in felines are associated with these mites.
In addition to the commonly encountered otodectic otitis externa, in which these surface-feeding parasites cause irritation to the ear canal epithelium, increasing sebum production and causing bleeding, ear mites may actually cause a diversity of clinical signs. Some hosts may produce antibodies against mite antigens, resulting in hypersensitivity reactions. Ear mites are also not necessarily always confined to the ear canal. In some cats, otodectic mange can affect the face and neck, and even the entire trunk. Cats who scratch their pruritic ears may develop otodectic mange on the hind feet.
A low burden of Otodectes cynotis (two to three mites), which may be overlooked clinically and may be locally asymptomatic, can still induce type I or type III hypersensitivity reactions in veterinary patients. Moreover, inadequate treatment of all stages, locations, and all in-contact animals will generally lead to treatment failure. Many agents are not ovicidal; even though larvae and adult stages may be eliminated by short-acting topical products such as pyrethrins, organophosphates, and occlusive compounds, more eggs will continue to hatch, causing reinfestation.
Chronic, severe, and/or recurrent otodectic otitis externa can result in permanent and extensive damage to the epithelium of the external ear canal. Aural hematomata, miliary dermatitis, and alopecia may occur secondary to self-trauma. Secondary bacterial and fungal infections and even otitis media may also ensue. Stenosis, fibrosis, and mineralization of the external ear canal, and even the development of ceruminous gland adenomas or adenocarcinomas are potential long-term sequelae of untreated or inadequately treated ear mite infestations.
In this survey of peer-reviewed publications detailing 27 in vivo trials between 1978 and 2015, these authors systematically review the efficacy of pharmacological treatments for Otodectes cynotis infestation in cats, and provide recommendations for their use, as well as recommend methodologies for future clinical trials and research. In the 27 trials, 17 pharmacological agents were identified; two of the trials were randomized controlled trials (RCTs) and the other 25 were open, uncontrolled trials. Given that only two of the trials were RCTs, it was not possible to perform a meta-analysis of the pooled data.
Based on the data analyzed in this study, O. cynotis infestation in cats may be treated in any of three ways: spot-on application, subcutaneous injection, or otic preparation. After careful evaluation of the quality of each study design, overall efficacy, appropriate treatment follow-up period (> 21 days), and adverse effects reported in the studies evaluated, the authors recommend, based on fair evidence, the following treatments for ear mite infestations in cats: either spot-on 10% imidacloprid and 1% moxidectin, or selamectin, once or twice at 30-day intervals. Other medications that were reported in the papers evaluated by the authors to have high efficacy include Otomite® (pyrethrins/piperonyl butoxide), GAC®(neomycin/permethrin/amethocaine; made in the United Kingdom), and Oridermyl® (triamcinolone/neomycin/nystatin/permethrin; made in Canada and labeled for cats despite permethrin content), all otic preparations containing acaricides, but there is insufficient evidence for or against recommending these treatments. These medications were used as positive control for evaluating the efficacy of selamectin in one study.
Insufficient evidence was found for or against the use of Canaural® (prednisolone/fusidic acid/framycetin/nystatin), fipronil (either as otic drops or spot-on), ivermectin (either as otic suspension, subcutaneous injection, or spot-on), and other otic preparations including Oridermyl®(triamcinolone/neomycin/nystatin/permethrin), Surolan®(prednisolone/polymyxin B/miconazole), and Tresaderm®(dexamethasone/neomycin/thiabendazole). Also, treatments containing lindane, the use of which is controversial in cats, did not lead to complete recovery.
When choosing acaricidal treatments for cats, the age of the cat, severity of clinical signs, presence of secondary infections, numbers of animals affected, presence of ectopic mites, life cycle of the ear mite vis-á-vis timing of recheck, formulation, and owner's convenience and compliance need to be taken into account. Some products cannot be used in kittens and pregnant cats. Selamectin spot-on is labeled for use in kittens > 8 weeks of age, while the 10% imidacloprid/1% moxidectin spot-on is labeled for kittens >9 weeks of age, and Otomite® may be used in kittens > 12 weeks of age. As very young kittens (4-8 weeks of age) are often the ones with the problem, the unavailability of acaricidal medications that are safe, effective, and labeled for this age group is a source of frustration for both clinicians and owners alike.
Most of the otic medications containing glucocorticoids are not labeled for kittens and are contraindicated during pregnancy. In the studies reviewed, the only acaricidal treatment found to be both safe and effective in pregnant cats with otoacariasis is spot-on selamectin, although one case of abortion was found in a single study. The majority of the studies evaluated by the authors had no data regarding the use of treatments in pregnant cats. Products labeled for other species should not be used in cats of any age.
As O. cynotis is not species-specific, treatment of all cats and dogs in multi-pet households is recommended. Given that the life cycle of this parasite is 21 days, the follow-up period must be at least this long. Although extensive environmental control is generally not suggested, the housing/bedding, cages, and grooming equipment used by infested or potentially infested animals should be disinfected or kept vacant for at least 12 days, as the mite can survive off the host for 12 days.
These authors recommend that future studies regarding treatment of O. cynotis infestations in cats be in vivo double-blinded RCTs. Data on kittens less than one year of age should be reported separately to identify safety and adverse effects of the evaluated agents in young cats.