Endoscopic visualization and irrigation are essential in management of the recalcitrant discharging canine ear

The discharging ear is a common presentation in the dog, and in some cases difficult to manage. In a large retrospective audit of UK primary veterinary practice, otitis externa was recorded as the underlying diagnosis in 7% (1631/22 333) of all canine consultations (1).

There are many causes of a discharging ear, but in most cases bacteria, fungi, or parasites are present. In a case series from Greece (2) of 100 dogs diagnosed as having otitis externa, 11% had parasites, and 38% cultured bacterial cocci, 22% bacterial rods, and 66% the yeast Malassezia. Similarly, in a review of 149 cases in Israel (3) 75% cultured bacterial cocci, 52% bacterial rods, and 72% Malassezia. Other series had comparable findings (4–8).

Factors increasing the risk of dogs developing infective otitis externa are debated. Ingress and trapping of water in the ear canal is a precedent in some, or possibly most cases, and a likely explanation for the increased incidence reported in breeds with pendulous rather that erect pinnae (2,3,8–10). Persistent moisture supports growth of micro-organisms, but grooming practices that wash the ear canal may also introduce soaps and chemicals that can irritate the skin. In addition, compromised ear canal skin barrier function e.g., due to dermatitis, can predispose to super-added infection.

Initial assessment and management

In an ideal world, all dogs with a discharging ear would be comprehensively examined with otoscopy at first presentation to detect an underlying cause. In reality that may not be easy, because ear canal swelling or copious ear discharge (otorrhoea) inhibits a view of the deep ear canal or tympanic membrane, because a dog is uncooperative with examination, or because the treating veterinarian does not have equipment (or experience) to reach a conclusive diagnosis. Given that micro-organisms are present in most cases of otitis externa, it may be reasonable to treat first presentations with polyvalent topical products containing an antimicrobial, an antifungal, and possibly a steroid (11).

Most consultations for canine otitis externa in UK primary care are a single visit (12), implying that initial treatment is often effective (although we do not know how many may be treatment failures that did not return for follow-up). However, in some cases, ear discharge becomes persistent or recurrent. Of 100 cases of otitis externa in dogs seen at a specialist practice in Greece (2), 63% (63/100) were chronic or recurrent, 39% (39/100) had previously been treated, and 90% (173/193 ears) had bilateral exudate. In another series of 320 dogs treated for otitis externa at a practice in Canada, 24% (78/320) recurred within 1 mo. Persistent or recurrent cases should be referred to a specialist (13,14).

If ear discharge persists, it is critical to clear the ear canal of debris and inspect the deep ear canal and tympanic membrane. This is best undertaken using an irrigation pump and endoscope, because the optics on a modern endoscope are far superior to an otoscope and naked eye. Irrigation or endoscope examination requires patient cooperation and may be uncomfortable, and so in many dogs should be performed under general anesthesia.

Why is this so important? As detailed below, cleaning and inspecting the ear is critical to diagnosis and management.

Managing the recalcitrant discharging ear: Is this persistent infection?

One cause of recalcitrance, and indeed possibly the most likely cause, is an infection that has been inadequately treated. This may be because the organisms causing the infection were not sensitive to the treatment previously given. For example, fungus was present, but antifungals were not used, or bacteria were present but resistant to antibiotics given. In a series from a specialist center in the UK (15), 78% (46/59) of dogs with Malassezia infection had been treated in primary care with steroids alone, and not anti-fungal medication. In a national analysis in France, there was multidrug antibiotic resistance in 12 to 16% of bacteria cultured from dogs with otitis (16). So, in recalcitrant cases it would seem wise to consider sending ear discharge for microbial culture and antibiotic sensitivity testing, as a means to guiding future therapy.

Perhaps a more important cause for recalcitrance is continued presence of infected debris, which (if copious) will hamper efficacy of topical medications. If debris is cleared by irrigation, then antibiotics delivered topically often achieve high target concentrations, high enough even to exceed the minimum inhibitory concentration that defines resistance, and so kill apparently antibiotic-resistant organisms (11). Hence, effective clearing of debris from the ear canal must be considered mandatory if persistent infected ear discharge is present. Furthermore, in fungal infection, microscopic spores can exist for weeks after visible infection has resolved, and so risk re-infection. In such cases, as well as clearing debris, anti-fungal treatments should probably continue for at least 2 wk after apparent resolution (17).

These ideas parallel established concepts in the management of infective otitis externa in human medicine. Based on UK data, human otitis externa is common (affecting 1% of all adults each year); although most are effectively treated with topical therapies in primary care, 3% of cases are referred to secondary care (18). Human disease does differ, however, in that it is likely to present earlier, and specialist care is more readily available. In high-income countries, hospital otolaryngology departments typically offer emergency appointments for management of recalcitrant or severe otitis externa, and so comprehensive cleaning of ear debris is usually available within days of symptom onset. Moreover, it is not unusual for these specialists to see patients once or twice weekly, because frequent and thorough clearing of debris is key to resolution of infection. In the most persistent cases, patients may be seen 4 or 5 times a week. The same concepts should be translated to recalcitrant infective otitis externa in dogs: thorough cleaning to maximize probability of resolution and repeated as necessary. In the UK series of dogs with recalcitrant otitis externa due to Malassezia, 12% (8/68) of ears needed more than one irrigation (15). However, few veterinarians are specialized in otology, limiting availability of such specialist treatment.

Managing the recalcitrant discharging ear: Is there undiagnosed underlying pathology?

Ear discharge is not always due only to infection; therefore, clearing debris and visualizing the ear canal and tympanic membrane are important for identifying potential underlying pathology. In fact, over a third of cases in the audit of 100 dogs with otitis externa in Greece (2) had underlying pathology: 11% (22/193) of ears had a grass seed awn (foxtail) foreign body, 22% (43/193) had a perforated tympanic membrane, 5% (9/193) had myringitis, and 1 ear (1/193) had a tumor. Other series from the UK (19) and USA (20) reported 8% (16/197) and 24% (11/46) of ears with apparent chronic otitis externa had a perforated tympanic membrane.

Similarly, in many cases referred to us with recalcitrant ear discharge, cleaning revealed pathology including a grass seed awn, perforated tympanic membrane, or (rarely) a tumor (Figure 1). In such cases, treating the underlying cause is critical. Grass awns can be removed under endoscopic guidance using grasping forceps. Such removal (combined with topical antibiotics) is often curative — even in cases that have been ongoing for months (although sometimes there is an associated tympanic membrane perforation, which places the patient at risk of infection recurrence). Tumors of the ear are rare, but if identified can either be removed or managed conservatively, with that decision informed by radiological imaging and biopsy.

Figure 1.

There may be underlying pathology in the chronically discharging ear that is only visible once the ear canal is cleaned. Shown are examples from our practice of a grass seed awn (foxtail), middle ear tympanokeratoma/cholesteatoma, and a tumor.

Perforation of the tympanic membrane signifies that middle ear disease was the cause of ear discharge, and that any otitis externa developed secondarily. Some in the veterinary community have opined the opposite view, arguing that chronic otitis externa causes tympanic perforation (21). However, this is incongruent with vast experience in human medicine, in which even the most severe and persistent cases of otitis externa have not been known to perforate the tympanic membrane (22). In some cases of otitis externa, there may be radiological evidence of middle ear inflammation (19), but if not associated with endoscopic evidence of infection or tympanic perforation, such findings are of uncertain clinical significance.

In the dog, tympanic perforation is often due to middle ear tympanokeratoma (cholesteatoma), which can be treated by accessing the bulla to remove or debulk disease. Traditionally this is performed using an external surgical approach, but we have recently described a less invasive endoscopic approach (23). If the tympanokeratoma is excised or de-bulked, the outer ear inflammation and associated discharge usually resolves, or substantially improves.

Managing the recalcitrant discharging ear: Is there another cause of chronic dermatitis?

There are some cases of chronic otitis externa in which skin erythema and irritation persist, even after all debris has been cleared away, topical antibiotic and antifungal treatments have been given, other underlying causes have been excluded, and microbiological cultures or cytological analyses are unremarkable. There may still be an identifiable cause. Perhaps there is hypersensitivity to some of the topical treatments being used to treat infection; if so, stopping all medications may allow things to settle. Alternatively, the ear may be inflamed as part of a generalized dermatitis, and if so, the rest of the skin must be examined. Such widespread dermatitis may be of uncertain etiology, but topical or even systemic steroids may help.

In many cases, however, there is isolated inflammation of the ear canal skin without identifiable cause. This is best termed idiopathic chronic otitis externa, a condition also recognized in human medicine (24). The disorder is not well understood, but we should recall that when tissues are chronically inflamed, remodeling may mean inflammation becomes irreversible (25), even if the initial trigger (e.g., an infection) has been eliminated.

Some authors suggest a large proportion of cases of idiopathic chronic otitis externa are caused by food allergy (2,3,21), and so prescribe elimination diets. The current literature does not detail clear diagnostic criteria for dermatitis due to food allergy, and a review by the American College of Veterinary Dermatologists reported no evidence in dogs of a link between atopic dermatitis and food allergy (26). Studies of changes in serum immunoglobulins in dogs exposed to food challenge have conflicting results that are of doubtful clinical relevance (27). We believe food allergy is a rare or non-existent cause of localized dermatitis of ear canal skin. Although we are aware that some will trial dietary modifications or restrictions for the persistently inflamed ear, this is not our practice.

Idiopathic chronic otitis externa is often a difficult condition to treat. In human medicine, steroids and tacrolimus have been used, but often provide only temporary benefit, and disease may progress to fibrosis (24). We treat disease in dogs in a similar manner, with topical anti-inflammatory medication such as steroids, but sometimes with medication applied to a pack sutured into the ear canal to enable prolonged therapy; this is removed after a few days. We accept that the cause may never be known that the disease may be irreversible, and that symptoms and signs may be recurrent or persistent. In cases causing substantial distress, surgical excision of the external ear is an option (28), although such surgery has risks and we preference medical management wherever possible.

Conclusion

The discharging ear is a common presentation in the dog. In recalcitrant cases, unresponsive to topical treatment, specialist assessment is advised and that should include endoscopic irrigation to remove infected debris and to diagnose any underlying cause. However, some cases are idiopathic and may become persistent, irreversible, and difficult to manage.