Parasitic Dermatosis
Otodectes cynotis
The most common parasite affecting the ear canals of dogs and cats, Otodectes cynotis is transmitted by direct contact. Transmission among and between dogs and cats is extremely common.
These ear mites are also present on other parts of the body. The sleeping habits of dogs and cats, where the head is in close contact with the tail, make the “tail-head region” a common site of infestation. The adult mites live on the skin surface of the horizontal ear canal, covered by a layer of debris. Mechanical irritation by these mites causes the production of a waxy, brown cerumen. The ceruminous glands become dilated with cerumen. Epidermal scales and debris combine with cerumen to form a favorable medium for the growth of secondary bacteria and Malassezia species. Otic discharge, complicated by secondary Malassezia or bacterial infection, is usually malodorous, and patients will present for the control of the odor rather than for the symptoms of otitis. Secondary bacterial and Malassezia infection can mask the primary underlying cause of the parasitic ceruminous otitis and thereby delay diagnosis.Toxic and allergic substances produced by Otodectes mites cause hypersensitivity reactions in dogs and cats. The saliva of mites contains potent allergens that are responsible for this immune reaction. Ceruminous otitis externa can also be a result of Otodectes infestation.
Clinical Signs. Head shaking is common in animals with Otodectes infestation. The head shaking often leads to the formation of auricular hematomas. Intense pruritus of the ears is common, and animals can present with a noticeable head tilt, circling, and sometimes, in chronic untreated infestation, convulsions.
Otic examination may or may not reveal mites moving on hair shafts. Especially in dogs, the inflamed ear canal is not a hospitable environment for mites, so the migration of mites to other parts of the body is common.
Ceruminous gland hyperplasia may make otic examination extremely difficult in cases of chronic Otodectes infestation. When there is hyperplasia of the ear canal, general anesthesia is recommended for otoscopic examination.Histologic findings include hyperplasia of the ceruminous glands in acute cases, along with epithelial parakeratosis and hyperplasia, an increase in inflammatory infiltrate, squamous metaplasia of apocrine ducts, and atrophy of hair follicles. Similar changes are present in chronic infections.
Diagnosis and Tireatment. Diagnosis is by finding mites in the ears on otoscopic examination, microscopic identification of the mites, determination of secondary bacterial or yeast identification, and response to treatment. The ears should be treated concurrently with the whole body. All animals that can come in contact with the patient should be concurrently treated. Various commercially prepared topical medications are available for the treatment of otic acariasis.
Sarcoptes scabiei
Although most commonly found on the pinnae of the ears, Sarcoptes scabiei mites can migrate into the external ear canal. The mechanical irritation and toxic substances produced by these mites lead to the hyperproliferation of the ceruminous glands of the horizontal ear canal. Ceruminous otitis caused by sarcoptic mites resolves when the primary problem and any secondary complications are treated.
Skin scrapes are not always positive for sarcoptic mites. If clinical signs and physical examination findings indicate sarcoptic infestation, it is best to treat the animal for the disorder and make a diagnosis by response to treatment.
Notoedres cati
The sarcoptic mite of cats, Notoedres cati, can also cause ceruminous otitis. Mechanical irritation, combined with the toxic substances and salivary antigens produced by the mites, results in glandular irritation and proliferation.
Diagnosis and treatment are pursued by identifying the mite and implementing proper treatment.
Sarcoptic mites are not always picked up on skin scrapings in the dog; in the cat, however, skin scrapings reveal numerous mites. In some cases, however, diagnosis is confirmed by response to treatment.Otobios megnini
Otobios megnini, the spinous ear tick, has been implicated in ceruminous otitis of dogs and cats. Although this condition is rare, the larval and nymphal stages of this mite feed on lymph and blood in the ear canal, resulting in irritation and subsequent hyperplasia of the ceruminous and sebaceous glands of the ear.
Diagnosis is made by identification of the larval or nymphal stages in the ear canal. Management of the tick population in the dog’s environment is important to a successful treatment plan.
Mechanical removal of ticks with forceps is recommended. Grasping the tick on the head next to the skin is ideal. Care should be taken not to rupture the tick, to prevent exposure to disease agents that may be present. If the tick is located deep in the ear canal, a video otoscopic removal may be necessary. Environmental treatment should be initiated and routinely maintained, especially in tick-infested areas.
Demodex
Some dogs with generalized demodicosis test positive for the Demodex canis mite on ear swabs. In some dogs with severe ceruminous otitis, Demodex mites may be identified in the cerumen even before truncal lesions are found. Some cats with ceruminous otitis have also been identified as having Demodex cati in the secretions. Microscopic examination of the exudates from ceruminous ears suspended in mineral oil reveals the mites. Identification of mites from these animals can aid in the treatment and management of demodicosis. The physical mechanical irritation and irritation from toxins produced by these mites result in the cystic dilation of the ceruminous glands of the ear. The result is hyperplasia of the ear canal in chronic conditions. The excessive cerumen produced by these glands is irritating and predisposes to secondary bacterial and Malassezia infections. The treatment of generalized demodicosis with topical solution (amitraz) should involve treatment of the ear canal for complete cure and to prevent recurrence.
The use of systemic medication is now widely used, although it is not approved by the U.S. Food and Drug Administration (FDA) for demodicosis. Ivermectin and milbemycin oxime, on an extra-label basis, are the systemic agents now being used for the treatment of this disease. When these systemic medications are used, treating the ear canal is not indicated. However, infected ears should be treated for secondary infections.
Other Parasites. Other parasites implicated in ceruminous otitis are Eutrombicula alfreddugesi and Cheyletiella species. Dermatophytes are also implicated.