Definition and who is affected
Merck defines otitis externa as inflammation of the external ear from the base of the auricular cartilage to the tympanic membrane, and notes that the pinna may or may not be involved [1]. The Canadian Veterinary Journal review includes the pinna in its definition and defines chronic otitis as persistent or recurrent disease lasting 3 months or longer [2]. The Microorganisms review notes that the definition of chronic disease varies between authors [3].
Merck reports no recognized sex distribution and says young animals may be more often affected. It adds that breed predispositions mirror those for skin disease, such as allergies in retrievers and terriers [1]. The Microorganisms review cites a UK primary care study that diagnosed otitis externa in 7.3% of a random sample of 22,000 dogs over one year in 2016, second only to periodontal disease. Basset Hounds, Chinese Shar Pei and Labradoodles were predisposed in that study, and Chihuahuas were the least susceptible [3]. The review cites other published estimates of 5 to 20% [3].
Causes and contributing factors
Both core sources sort causes into categories rather than naming a single agent [1][2]. The Microorganisms review calls this the PSPP system, for primary, secondary, perpetuating and predisposing factors. It notes that the system is widely taught but that the separation between categories is "artificial" [3].
Primary causes start disease in a normal ear. Merck lists allergy (adverse food reaction, atopic dermatitis, contact dermatitis), mites including Otodectes cynotis, Demodex and Sarcoptes, autoimmune and immune-mediated disease, endocrine disease such as hypothyroidism and hyperadrenocorticism, epithelialization and glandular disorders, and juvenile cellulitis [1]. The Canadian Veterinary Journal adds foreign bodies and otic neoplasia [2]. The Microorganisms review cites figures that 70 to 80% of dogs with allergic skin disease have otitis externa, and it notes that some cases have no identifiable primary factor [3].
Secondary causes, according to Merck, are bacteria (Staphylococcus, Streptococcus, Enterococcus, Pseudomonas, Proteus and others) and Malassezia yeast [1]. The Canadian Veterinary Journal review names Staphylococcus as the most frequently isolated genus and notes that some dogs appear to develop an allergic response to Malassezia [2].
Merck describes predisposing factors as ones present before disease begins. Examples are pendulous or hairy pinnae, narrow canals, moisture from swimming, obstructive masses, and changes to the normal microbiota caused by treatment [1]. Perpetuating factors develop because of the inflammation, and include failed epithelial migration, canal stenosis and edema, eardrum rupture, glandular hyperplasia and calcification [1]. The Canadian Veterinary Journal article identifies perpetuating factors as often the main reason recurrent otitis fails treatment [2].
Clinical signs and classification
Merck lists head shaking, pruritus, pain, malodor, erythema, erosions, ulceration, edema and discharge among the clinical signs [1]. The Canadian Veterinary Journal describes possible pinnal alopecia, crusting and hyperpigmentation. In the canal it describes ulceration, ceruminous or suppurative discharge, masses, stenosis and glandular change, and it notes that affected ears usually show more than one abnormality [2].
Merck classifies the disease as infectious or noninfectious, acute or chronic, and unilateral or bilateral [1]. The Microorganisms review mentions a published scoring tool, OTIS-3, which combines erythema, swelling, erosion or ulceration, and exudate. The review notes that it is "used infrequently" [3]. None of the cited sources describes a formal staging system endorsed by a guideline panel.
Diagnosis
Merck recommends a full physical and dermatological examination first. The ears are then examined starting with the less affected, less painful side, with a clean cone for each ear [1]. Palpation can reveal swelling, pain and mineralization from chronic disease [1]. If the patient is in severe pain, Merck notes that sedation may be needed. As an alternative, many dogs with severe disease can receive glucocorticoids and topical therapy for 1 to 2 weeks before a full examination [1].
Cytology
Merck describes rolling a swab of exudate from the horizontal canal onto a slide and staining it with a modified Giemsa or Wright stain, without heat fixing. The slide is then reviewed at magnifications up to oil immersion [1]. Cocci are usually staphylococci or streptococci. Rods are usually Pseudomonas aeruginosa, Escherichia coli, Proteus mirabilis or Corynebacterium [1]. When many rods are seen, Merck suggests a Gram stain [1]. Neutrophils phagocytosing bacteria confirm that the organisms are pathogenic, because neutrophils are not found in healthy ears [1]. Merck also recommends a mineral oil preparation to look for mites. In cats, a dark "coffee-ground" exudate suggests Otodectes [1].
Testing pitfalls
The sources flag several traps:
- Merck states that samples should be taken before any cleaning [1].
- Exudate often hides the eardrum. The Canadian Veterinary Journal article says it is reasonable to defer eardrum assessment until active otitis is controlled [2].
- According to the Microorganisms review, cytology is not sensitive and may miss low numbers of organisms. Culture grows commensals regardless of whether they are in excess, so results need clinical context [3].
- Merck notes that susceptibility reports may understate the effect of topical drugs [1]. The Microorganisms review makes the same point, that topical antibiotics are used at many times the MIC [3].
- Radiographs are normal in many otitis media cases, per Merck, and some clinically normal dogs show middle ear fluid on CT or MRI [1].
- The Microorganisms review cautions that DNA sequencing does not distinguish live from dead bacteria [3].
Merck says CT or MRI is indicated for severe chronic otitis that fails appropriate treatment [1]. The Canadian Veterinary Journal article reports that CT is more sensitive and specific than radiography for middle ear disease [2]. Merck says biopsy is mainly indicated for a mass obstructing the canal [1].
Treatment
Merck lists the core elements as owner compliance, control of inflammation, directed antimicrobial therapy and investigation of the underlying cause [1]. The Canadian Veterinary Journal describes topical therapy as the mainstay. It says systemic antibiotics for otitis externa are typically discouraged [2]. Merck says systemic antimicrobials are not required for otitis externa alone but should be used when otitis media is suspected, chosen on cytology or culture [1].
Glucocorticoids reduce pain and swelling, which makes cleaning and medicating easier, according to both core sources [1][2]. Merck names oral prednisone and triamcinolone as the most commonly used [1]. For chronic disease with severely stenotic canals, Merck cites triamcinolone "dosages equivalent to 0.8 mg/kg every 24 hours" until remission, followed by tapering. It warns that this dose cannot be maintained long term because of adverse effects [1]. The Canadian Veterinary Journal discourages long glucocorticoid courses unless they are necessary [2].
Merck recommends an initial cleaning in the clinic with the owner watching, using a cleaner with a neutral pH and a pleasant odor [1]. In its account, waxy ears may need a ceruminolytic such as carbamide peroxide or dioctyl sodium sulfosuccinate 2 to 3 times weekly, and purulent ears may need cleaning once or twice daily [1]. Merck advises a tris-EDTA cleaner when rods are seen. If the eardrum is ruptured, detergents and dioctyl sodium sulfosuccinate are contraindicated, and saline or tris-EDTA are the milder options [1]. Merck also advises against home remedies, vinegar dilutions and powders [1].
Most prescription topical products combine an antimicrobial, an antifungal and a glucocorticoid. Merck advises choosing among them based on cytology [1]. Merck stresses confirming an intact eardrum before applying topical drugs, because agents such as aminoglycosides can be ototoxic [1]. The Pharmaceutics review names polymyxin B and neomycin as ototoxic antibiotics that require extreme care [4]. Merck notes that yeast otitis usually responds to a topical antifungal plus steroid [1].
For managing recurrent otitis, the Pharmaceutics review cites published guidance that glucocorticoids are preferred for reversing chronic changes and that ciclosporin may help in continuing care [4].
Pseudomonas and resistant infections
The Microorganisms review describes Pseudomonas as the most common bacterium in chronic otitis, affecting up to 35% of cases, and reports a typical interval of 10 to 28 months from the onset of primary disease to Pseudomonas infection [3]. Merck states that when a Gram stain is not available, ulceration, a slimy green discharge and rods alone on cytology are highly suggestive of Pseudomonas infection [1].
The Microorganisms review reports biofilm formation in 40 to 95% of P. aeruginosa isolates from canine otitis [3]. Across the studies it summarizes, resistance ranged from 27 to 68% for enrofloxacin and 3 to 43% for gentamicin, with multidrug resistance in 13 to 35% of isolates [3]. Merck notes that polymyxin B and fluoroquinolones have been most successful where resistance showed up as clinical failure [1]. The Microorganisms review reports in vitro evidence that tris-EDTA works synergistically with several antibiotics [3]. Merck lists the steps for resistant cases as managing the primary cause, irrigating the canal, treating any otitis media, and choosing drugs based on culture and susceptibility [1].
Alternative therapies remain early. The Microorganisms and Pharmaceutics reviews both describe a single-dose bacteriophage cocktail given to 10 dogs with resistant Pseudomonas otitis [3][4]. The Pharmaceutics authors rate the evidence as low quality because the trial was small and had no control group [4].
Monitoring, prognosis and client communication
Merck recommends physical examination and cytology weekly to every other week until no infection is seen. Most acute cases take 2 to 4 weeks, while chronic cases can take months, and some need indefinite maintenance [1]. When otitis does not resolve despite appropriate therapy and good compliance, Merck says otitis media should be considered [1]. The Canadian Veterinary Journal article says referral to a veterinary dermatologist should be considered for recurrent or chronic otitis or otitis media [2].
Repeated inflammation can lead to fibrosis, stenosis and calcification, which the Canadian Veterinary Journal describes as end-stage ear disease that early treatment can usually prevent [2]. When medical management fails, the Microorganisms review states that total ear canal ablation, with or without bulla osteotomy, resolves disease in most cases [3]. The Canadian Veterinary Journal also reports that hearing loss can occur and that cleaning exudate produces measurable hearing improvement [2].
For client education, Merck stresses reasonable expectations and warns that many owners stop treatment once the ear looks better [1]. It describes a cotton-ball cleaning method for dogs that resist cleaner squeezed into the canal [1]. When the eardrum cannot be seen, Merck says owners should watch for decreased hearing, head tilt or vertigo [1]. A 42-dog study cited in the Pharmaceutics review suggested better compliance with a once-daily pump product than with a dropper product. Owners in the dropper group forgot doses or miscounted drops [4]. For maintenance, Merck recommends keeping canals dry and notes that cleaning frequency usually decreases over time to once or twice weekly [1].
