Proliferative and necrotising otitis externa in a cat without pinnal involvement: video-otoscopic features

Case Report

Proliferative and necrotising otitis externa is a rare and recently described disease affecting the ear canals and concave pinnae of cats. ¹ So far, only five cases have been reported.^1,2 The aetiology is currently unknown, although an immunological basis is suspected.^1–3 In a recent case report, immunohistochemistry was used to show a closed association between caspase-3 stained keratinocytes and CD3+ T cells, consistent with keratinocyte apoptosis caused by epidermal-infiltrating T cells. ² So far, evidence of a viral aetiology has not been found: polymerase chain reaction for feline herpesvirus 1 and immunohistochemistry for papillomavirus have both been negative. ³ Typically seen in kittens, this disease must be considered among the differential diagnoses (ectoparasitic, allergic, immune-mediated and neoplastic diseases) of bilateral otitis externa in young cats.^1,2,4

The aim of this article is to describe a case of proliferative and necrotising otitis externa in a young-adult cat, which resolved completely with topical tacrolimus therapy. Confirmative diagnosis in this case was obtained with histopathological examination; however, video-otoscopy findings, not described previously, were very peculiar and may help clinicians to include proliferative and necrotising otitis in the list of differential diagnoses of feline otitis externa.

A 2-year-old castrated male domestic shorthair cat was presented for evaluation of chronic otitis externa. The owners reported that the cat had been pruritic at both ears for 7 months since it was found at the presumed age of 1.5 years. Since adoption, the cat lived indoors with no other pets and was vaccinated regularly, dewormed and fed commercial dry food.

Over the previous months, the cat had been treated with numerous topical preparations for otic parasites, yeast and bacterial infections, including thiabendazole/dexamethasone/neomycin sulfate (Tresaderm; Merial) for 1 week, miconazole nitrate/prednisolone/polimixin B (Surolan; Janssen Animal Health) for 5 days, polimixin B/lidocain/neomycin (Anauran; Zambon Italia) and PCMX/EDTA (Epi-Otic; Virbac) for 1 week. At the same time the cat was also treated with systemic drugs, including a single application of selamectin spot-on (Stronghold; Pfizer Animal Health) and enrofloxacin (Baytril; Bayer Animal Health) and itraconazole (Itrafungol; Janssen Animal Health) for 3 weeks. No response was observed with any of these treatments.

General physical examination was unremarkable. The inner surface of both pinnae showed a small amount of black and dry cerumen. Otoscopic examination showed abundant black and dry cerumen in both ear canals and the tympanic membranes (TM) could not be visualised. Initial differential diagnoses were otoacariasis, which may have not resolved with previous treatments, and chronic bacterial or yeast otitis externa secondary to otitis media or, less likely, to bilateral ear polyps. Microscopic examination of the ear cerumen was negative for otic parasites and cytological examination showed only keratinocytes. In order to completely rule out otoacariasis, given the dry appearance of the ear cerumen and because previous treatments had been administered for short courses, an otic preparation containing thiabendazole/dexamethasone/neomycin sulfate was dispensed to be applied twice daily into both ear canals for 3 weeks.

The cat was re-examined 4 months later, showing very similar clinical signs. Otoscopic examination revealed abundant wet and black exudate in both ear canals with a small amount of the same material visible on the inner aspect of both pinnae. The removal of the otic material from the vertical ear canal allowed the detection of underlying erythema and erosions; the TM could not be visualised on either side. Pruritus was still present. Differential diagnoses were chronic bacterial or yeast otitis externa secondarily to otitis media or, less likely, to bilateral ear polyps. A neoplastic process was unlikely owing to the age of the cat and the bilateral involvement of the ear canals. Bilateral cytological examination of the otic exudate revealed degenerated neutrophils and intra- and extracellular cocci, suggesting bacterial otitis externa.

The cat was premedicated with 5 µg/kg medetomidine (Domitor; Pfizer Italia) and 10 mg/kg ketamine (Imalgene; Merial Italia) intramuscularly and induced with 4 mg/kg propofol (Rapinovet; Schering Plough) intravenously, followed by maintenance with isoflurane in oxygen. The cat was positioned in lateral recumbency and both external ear canals were examined with a 2.7 mm 30° videoscope (Karl Storz GmbH, Tuttlingen, Germany) after flushing with saline to remove the otic exudate. Both right and left external ear canals were characterised by a proliferative appearance of the wall, which partially obstructed the scope passage. A digitally proliferative lesion, growing at 360° all around the ear canals and for the entire length of the canals could be seen, while it was not possible to visualise the normal vascularisation (Figure 1). The ear canal walls were found to be bleeding easily and inflamed. The TM could be examined completely at the end of the left ear canal, while in the right ear canal a dark, brown-to-black, vegetative plaque covered most of the TM. The plaque was removed with the aid of endoscopic clamps and a ruptured TM was observed. The proliferative disease protruding in the middle ear without affecting the tympanic cavity was detected. The tympanic cavity appeared normally covered by a pinkish and well-vascularised respiratory mucosa (Figure 2).

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Figure 1

Video-otoscopy of the external ear canal. A digitally proliferative disease extends the entire canal length

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Figure 2

Video-otoscopy of the middle ear. The proliferative disease protrudes into the middle ear without affecting the normal respiratory mucosa

The nasopharyngeal inspection did not reveal any anatomical or pathological modifications during the retroversion technique with a flexible 5 mm 0° videoscope (Olympus Italia).

Multiple biopsy samples were taken for histopathological examination with the aid of the endoscopic forceps. Biopsy samples were fixed in 10% buffered formalin, embedded in paraffin, sectioned at 5 µm and stained with haematoxylin and eosin. Histopathological examination showed markedly hyperplastic epidermis with moderate and focal spongiosis and lymphocytic exocytosis. Hyperplasia extended into the wall of follicular infundibula (Figure 3). There were a few dyskeratosic keratinocytes (Figure 4) and large and thick parakeratotic plaques with colonies of coccoid bacteria could be observed focally in one biopsy sample from the left external ear canal. The dermal inflammatory infiltrate was mixed with neutrophils, lymphocytes and other mononuclear cells. In one ear biopsy there were also a few eosinophils. Periodic-acid Schiff stain was performed to rule out Malassezia species overgrowth and dermatophytosis; the result was negative. Although keratinocyte dyskeratosis was very mild, the epidermal and follicular hyperplastic changes were striking and, together with the clinical lesions, were considered typical of proliferative and necrotising otitis externa. ⁴

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Figure 3

Histopathology of the ear canal. Marked hyperplasia of the follicular wall (*) accompanied by epidermal parakeratosis and erosion (haematoxylin and eosin, bar = 300 µm)

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Figure 4

Histopathology of the ear canal. Dyskeratotic keratinocyte in the hyperplastic follicular wall (arrow) (haematoxylin and eosin, bar = 100 µm)

Therapy was initiated with 10 g 0.1% tacrolimus ointment (Protopic; Astellas Pharma) diluted in 10 ml mineral oil to be applied into both ears once daily for 6 weeks. Clotrimazole/dexametasone/marbofloxacin ear drops (Aurizon; ATI Pets) were also prescribed to be applied once daily for 3 weeks to treat the bacterial otitis externa.

The cat was re-examined 6 weeks later, and both pinnae and ear canals appeared normal on otoscopic examination. A small amount of cerumen still obscured the TM on the left side, while the TM on the right side could be visualised normally. Cytological examination of the ear cerumen showed keratinocytes only.

Proliferative and necrotising otitis externa was reported initially as a disease that typically affected kittens; however, two of the five cases reported previously occurred in a 3-year-old and a 4-year-old cat, respectively. ¹ In this case, the age of onset is not defined clearly, as the affected cat was adopted at approximately 1 year of age and already showed bilateral ear pruritus.

The clinical presentation of this case is somewhat different to the ones reported in previous publications.^1,2 Feline proliferative and necrotising otitis externa usually involves both the pinnae and ear canals, with dark-coloured coalescing plaques covering the concave surface of the pinnae and external ear canals. Otoscopic examination is not always possible owing to the large amount of material that accumulates in the ear canals. Pruritus may be present or not and a secondary bacterial or yeast otitis externa is invariably present.^1,2 In this case, the concave aspects of the pinnae were not affected, with lesions similar to the ones reported affecting only the external ear canals. The cat was extremely pruritic and the primary disease was complicated by secondary bacterial otitis externa.

Otoscopic evaluation of the external ear canal and TM should always be performed in animals presenting with otitis externa.^5,6 The enhanced illumination and magnification of video-otoscopy provide the practitioner with more detailed information for diagnosis and prognosis. The configuration of the working channel facilitates sampling, improves efficacy of cleaning procedures, and decreases the risk of iatrogenic injuries to structures of the middle and inner ear. Direct examination of the ear canals revealed a specific pathological finding. The video-otoscopy findings observed in this cat were peculiar, with unusual, digitally-protruding lesions extending for the entire length of the canal and growing at 360° round the ear canal. Necrotising proliferative otitis externa is usually characterised by ulcerated, proliferative or sclerotic lesions. ⁷ Surprisingly, the right tympanic membrane was broken, with no middle ear involvement. Even if the procedure was performed under direct visualisation, the possibility of an iatrogenic lesion during plaque removal with the endoscopic clamps cannot be excluded.

The main histopatological alterations observed in the skin biopsies were the hyperplastic changes in the epidermis, as well as in the follicular walls. This lesion accounts for one of the terms used to describe the disease, namely proliferative otitis.^{1 –3} Necrotic changes, which account for the second term used to name the disease, were less prominent and dyskeratosis was only occasionally seen; nevertheless, other diseases were considered unlikely.^{1 –3} The main histopathological differential diagnosis would have been an adverse reaction to topical medications; however, in this condition apoptosis with satellitosis and less hyperplastic changes would have been expected. ³ Moreover, the application of topical ointments did not lead to worsening of the condition.

Tacrolimus is reported to be an effective therapy for this disease.^1,2 However, tacrolimus is marketed as an ointment, which may be difficult to apply deeply into the ear canals owing to its thickness. For this reason, it was decided to reformulate 0.1% tacrolimus ointment, diluting 10 g in 10 ml mineral oil. This made the ointment far more liquid and allowed the active ingredient to penetrate more deeply in the ear canals. Although the patient was a very cooperative cat, once daily application was preferred in order to overcome problems related to burning or itching sensations reported as adverse effects of topical application of tacrolimus. The concurrent once daily application of a topical preparation for otitis externa (Aurizon; ATI Pets) effectively resolved the secondary bacterial otitis externa. According to previous case reports^1,2 it is not clear whether dexamethasone acetate, contained in this topical preparation, might have a synergistic effect with tacrolimus in the treatment of proliferative and necrotising otitis externa.