Treating Otitis Externa in Dogs
Ear cytology is a quick way to confirm infection and the degree of inflammation for patients with suspected otitis externa.
Canine otitis externa is a common dermatologic problem in small animal practice. Affected ears are often painful and pruritic (e.g., head shaking, scratching); if the condition becomes chronic, it is more difficult to control. Therefore, the primary causes of the ear problem must be identified, eliminated, or managed long term.
Otitis externa is considered a multifactorial disorder; the most common primary cause is allergy, which is often secondarily accompanied by microbial infections (Malassezia and bacteria). Other predisposing and perpetuating factors also contribute to the disease process (TABLE 1).1,2
The diagnostic workup for otitis externa requires a detailed history, thorough physical and ear examinations, and cytologic evaluation. Bacterial and fungal cultures are rarely needed unless systemic antimicrobial therapy is necessary for otitis media. Diagnostic imaging (e.g., computed tomography scan, radiography) may be helpful for assessing the extent of mineralization of cartilage in the external ear canal or bone destruction in the middle ear, as well as soft tissue defects, and determining whether surgical intervention is needed. Radiographic findings may be falsely negative, but radiographs may also be a cost-effective option in general practice. Otoscopy will enable identification of the amount and consistency of ear wax or discharge, any foreign bodies and/or masses, and degree of inflammation and stenosis. Whenever possible, the tympanic membrane should be evaluated for any changes (e.g., opacity) or perforations. Ear cytology, with Diff-Quik staining, is an effective and quick in-house option to confirm and quantify any type of infection and degree of inflammation and should be performed for all dogs with otitis externa.
Most otitis externa cases can be sufficiently managed by topical therapy; however, otitis is likely to return after topical treatment if the underlying cause (e.g., allergy) is not addressed. Before beginning therapy, ensure that the amount of debris in the ear canal is minimal; excessive ear debris should be removed by a deep ear flush. If the ear canals are severely inflamed, the authors recommend pretreatment with systemic anti-inflammatory drugs (e.g., prednisolone) before flushing. A thorough ear flush will enable visualization of the tympanic membrane but will also prevent trapping and inactivation of ear medication in the debris. After successful treatment of an ear infection and inflammation, regular ear cleaning at home (every 7 to 14 days as needed) will also help reduce accumulation of otic material and recurrence of infections. Ear cleaners have different properties (e.g., drying, antiseptic, and/or cerumenolytic) and should be selected according to specific needs. TABLE 2 lists some of the ear cleaners available on the market.
TYPE OF OTITIS EXTERNA AND TREATMENTS
Acute Otitis Externa
Acute otitis externa typically appears as acute onset of increased head shaking/scratching and inflammation for several days. Some cases can be quite painful, and the ear canal can be edematous from the inflammation, which can make an otoscopic examination very difficult, even with the patient under sedation.
Chronic Otitis Externa
Ear diseases that have been present for at least 1 month or that recur frequently are considered chronic. Management of chronic cases is more challenging (BOX 1). If the underlying primary cause is not addressed appropriately, antimicrobial drug resistance and/or progressive, nonreversible pathologic changes (e.g., ear canal cartilage mineralization) can result. For up to 70% of dogs with chronic otitis externa, the driving factor for the chronic inflammation is allergy.1 Given increased moisture, increased temperature, and altered cerumen composition, chronic inflammation within the ear canal provides the ideal environment for microbial overgrowth and infections.
Although it is important to treat the infections, it is also important to address the inflammation and underlying problem. Depending on the severity of the inflammation and stenosis, oral glucocorticoids may be needed to reduce ear canal inflammation. Prednisolone (1 to 2 mg/kg q12h to q24h for 1 to 3 weeks) or dexamethasone (for more severe cases) is recommended. A pilot study of 5 dogs indicated that cyclosporine may be another potent anti-inflammatory drug and that it may effectively control chronic otitis, especially with long-term use.3 All of these drugs should be used at the lowest possible doses and frequencies to prevent recurrence of the otitis.4
Many commercially available topical ear medications contain a combination of antibiotics, antifungals, and glucocorticoids and are supplied in different formulations for daily or weekly applications (TABLE 3). Nevertheless, the authors’ clinical observation that many infected ears in atopic dogs contain Malassezia raises the question whether commercially available ear medications containing both antibiotics and antifungals should be used for patients with Malassezia otitis without bacterial infection. Considering frequent multidrug-resistant bacteria in today’s practice, limiting the use of antibiotics is wise.
Malassezia Otitis Externa
Malassezia yeast organisms are often secondarily present in patients with chronic ear disease. Malassezia otitis typically results in variable degrees of inflammation and excessive, malodorous, mostly brown to dark-brown, waxy discharge. Of note, however, the number of Malassezia organisms seen during cytologic examination does not always correlate with the severity of clinical signs. Malassezia organisms not only have the ability to increase hyperplasia and secretion by proteins and enzymes they produce but also may be able to trigger an immune response, resulting in severe inflammation and potentially hypersensitive reaction.5 The diagnostic test of choice to confirm Malassezia infection is cytology. Fungal culture is not recommended because Malassezia organisms are usually not resistant to antimicrobials such as azoles, the organism does not grow well on regular fungal culture media, and standardized susceptibility testing is not available.5
Uncomplicated Malassezia overgrowth can usually be controlled by regular at-home ear cleaning (e.g., Epiotic Advanced [Virbac, virbac.com]). For more severe and chronic cases, the authors prefer to use a topical product containing antifungal medication (e.g., TrizUltra+Keto Flush [Dechra, dechra.com]). Initially, the ear flush is performed daily until the yeast infection is under control (1 to 3 weeks), followed by once weekly for maintenance. If significant concurrent inflammation is present, dexamethasone SP can be added to TrizUltra+Keto Flush for a final dexamethasone concentration in the range of 0.1% to 0.25%. A recent preliminary in vitro study showed that the dexamethasone SP added to several commercially available ear flush products is stable for 21 to 90 days at room temperature.6 Note, however, that Malassezia otitis will relapse after the treatment is stopped unless underlying causes are addressed.
Bacterial Otitis Externa
The normal microflora within the ear canals is characterized by a diverse population of bacteria in phyla such as Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, and Fusobacteria. In patients with chronic otitis externa, the microbial populations become much less diverse and are dominated by organisms such as Staphylococcus and Pseudomonas, and to a lesser degree by Corynebacterium, Proteus, Lactobacillus, Streptococcus, Porphyromonas, and Enterococcus.7 Most bacterial ear infections, especially Pseudomonas infections, are characterized by purulent discharge and severe inflammation, some with ulcerations and pain (FIGURE 1).
As with Malassezia, bacterial ear infections are considered secondary problems; if bacterial infections are recurring, the patient should be thoroughly investigated for primary underlying diseases, such as allergies. The most effective diagnostic tool for identifying (cocci and/or rod shape) and quantitatively assessing bacteria is cytology. Because bacterial otitis externa is treated topically, bacterial culture and sensitivity is often not necessary because susceptibility testing cannot accurately determine resistance for topically applied antimicrobials. Minimum inhibitory concentration (MIC) is the lowest concentration of antibiotic that prevents bacterial growth and helps guide susceptibility testing; however, correctly applied topical antibiotics usually reach MICs of up to 100- to 1000-fold. Despite the high MICs, before treatment is started, any debris and purulent discharge must be removed and sufficient ear medication must be applied. The recommended amount is 0.5 to 1 mL (10 to 20 drops) per ear, depending on the size of the patient, and the recommended frequency is 1 to 2 times per day.
To increase client compliance with treatment, 1-mL syringes can be sent home with clients so that they can draw up the recommended amount of ear medication. Numerous commercially available ear products contain antibiotics or disinfectants with antibacterial activity (TABLE 3). Common antibiotics are aminoglycosides (gentamicin, neomycin), fluoroquinolones (ciprofloxacin, enrofloxacin, marbofloxacin, orbifloxacin), polymyxins (colistin sulfate, polymyxin B), fusidic acid, florfenicol, and silver sulfadiazine.8
Many chronic cases of bacterial otitis externa are associated with Pseudomonas organisms; typically effective antibiotics are fluoroquinolones, gentamicin, and polymyxin B. However, treatment can be complicated by development of multidrug resistance as well as the formation of biofilm.9 To enhance the effectiveness of antibiotics, chelating agents such as Tris-EDTA can be added. These agents weaken the bacterial cell wall and facilitate penetration of the antibiotic into the bacteria. This action has been demonstrated in vitro for multidrug-resistant Pseudomonas.10 Tris-EDTA should be instilled into the affected ear 15 to 30 minutes before application of the topical antibiotic ear medication. It is well tolerated and not ototoxic.
Biofilm has been recognized as a major virulence factor in chronic infections. It increases antimicrobial resistance by shielding bacteria from the immune system and topical medication. Biofilm is an extracellular matrix (i.e., slime) produced by bacteria, which contains extracellular polymeric substances such as extracellular polysaccharides, structural proteins, cell debris, and nucleic acids. Biofilm plays a major role, especially in Pseudomonas infections (FIGURES 2 AND 3). It can be found in up to 40% of Pseudomonas isolates and increases the antimicrobial MIC needed to treat the infection.9
Removing biofilm can be challenging; it needs to be broken down physically by thorough flushing and aspiration. In addition, combination of Tris-EDTA and N-acetylcysteine topical product can help to disrupt and liquify the biofilm.
Parasitic Otitis Externa
Various parasites (e.g., Otodectes, Sarcoptes, Notoedres, and harvest mites; ticks) can affect the external ear canal. Otodectes cynotis mites are seen in up to 50% of cats and 5% to 10% of dogs with otitis.11 Even in small numbers, ear mites may cause irritation in the ear canal because of a hypersensitivity reaction. In higher numbers, mites are typically associated with dark-brown, granular (coffee-ground) exudate. Chronic ceruminous otitis externa is sometimes caused by Demodex mites.
A few topical acaricidal ear preparations containing ivermectin (e.g., Acarexx Otic Suspension [Boehringer Ingelheim, bi-vetmedica.com]) or pyrethrins are available. However, because many mites and ticks commonly affect other body areas, pesticides with a more systemic or generalized effect are recommended, such as spot-on topicals (e.g., selamectin, fipronil, moxidectin) or oral isoxazoline (e.g., fluralaner, soralaner).
SURGICAL INTERVENTIONS FOR OTITIS EXTERNA
Although treating otitis externa can be challenging, most chronic and recurrent cases can be medically treated and successfully managed, especially when underlying causes are addressed. Under certain circumstances, a surgical intervention may be necessary, but it should be performed only after all primary causes have been identified and corrected. Indications for surgery are tumors/polyps, end-stage otitis with severe mineralization, or unsuccessful medical treatment. Because of the severity of chronic otitis and potential involvement of the middle ear, the most effective surgical procedure is total ear canal ablation combined with lateral bulla osteotomy; however, clients must be informed about the potential risks and postsurgical complications.
CLIENT EDUCATION ON OTITIS EXTERNA
It is very important to clearly and thoroughly discuss with clients the requirements for successfully managing chronic otitis. Discussions should address the primary causes (e.g., allergies), determine what the client is able to do at home (ear cleaning and treatment), determine how cooperative the patient is, and ensure that the client understands the instructions and is willing to return for follow-up visits every 2 to 4 weeks. To increase client compliance, show them how to clean and treat the ears, provide detailed written instructions, and schedule follow-up calls, all of which can be done by skilled veterinary nurses. To increase patient compliance, provide pain control and ensure that ear cleaning and treatment are gently performed.
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7. Borriello G, Paradiso R, Catozzi C, et al. Cerumen microbial community shifts between healthy and otitis affected dogs. PLoS ONE. 2020;15(11):e0241447. doi: 10.1371/journal.pone.0241447
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10. Buckley L, McEwan N, Nuttall T. Tris-EDTA significantly enhances antibiotic efficacy against multidrug-resistant Pseudomonas aeruginosa in vitro. Vet Dermatol. 2013;24(5):519–e122.
11. Glaze MB. Chapter 19: Diseases of eyelids, claws, anal sacs and ears. In: Miller WH, Griffin CE, Campbell KL, editors. Muller & Kirk’s Small Animal Dermatology. 7th ed. St. Louis, MO: Elsevier; 2013:724–773.