Rosanna Marsella, DVM
Otitis externa is one of the most frequent reasons for owners to seek veterinarians help. The prevalence of otitis externa in dogs has been reported to be between 10-20 percent, although in more tropical climates it is probably closer to 30-40 percent. Unfortunately, the term otitis does not refer to a specific disease but to an inflammation of the external ear canal. It is a symptom of many diseases and not a specific diagnosis. The actual underlying causes of otitis are numerous. The purpose of this lecture is to review the general principles of ear care and the most important causes of otitis externa.
Physiology of the Ear Canal
The ear canal in the dog and cat can be divided into a vertical segment (which is continuous with the pinnae) and a horizontal segment that abuts the tympanic membrane. The canal is almost entirely surrounded by cartilage that offers stability to the structure. Besides the obvious auditory function of the external meatus, the canal also offers protection of the tympanic membrane and the middle ear from direct injury.
Preventative Ear Care and Ear Cleaning
Preventative ear care begins with a complete history and thorough physical examination.
Historical information and physical findings are necessary to identify patients at risk. Specific information about previous and concurrent medical disorders is essential, because ear disease may co-exist with other disease or be secondary to systemic diseases.
Routine cleaning of the ear canal is not necessary and may be contraindicated in the healthy dog and cat. Most dogs do not require cleaning of the ears. Mild to moderate amount of wax is normal. Cerumen has antibacterial properties that help to reduce the over population of bacteria and yeast.
Cleaning, when necessary, should be complete and non-irritating. A mixture of vinegar/water (1/10) is a good degreasing solution to remove wax and dry the excessive moisture in the ear canal. The liquid should be gently applied in the canal, the ear massaged to allow breakage of the cerumen and cotton balls used to remove the cerumen and wipe out the excess of liquid. Extreme care should be used when mechanically cleaning the ears. The use of cotton applicators should be avoided or limited as they may cause rupture of the tympanic membrane. Also powders should not be applied in the canal as they build up predisposing to the development of secondary infections.
Accumulations of cellular debris and exudates indicate the presence of ear disease. Swabs of this material should be collected and the canal should be cleaned. The color, texture and odor of the exudates from a diseased ear can provide clues regarding the underlying primary cause of the otitis and the perpetuating factors that may be involved. Dark brown or black, granular, dry (like coffee grounds) exudates characterizes infestations due to ear mites. A moist brown discharge tends to be associated with bacteria (cocci) and yeast infections. Purulent creamy to yellow exudates are most often seen with bacteria such as Pseudomonas. Waxy, greasy, yellow to tan debris is typical of a ceruminous otitis.
Thorough cleaning of the ear canals is vitally important for successful management of otitis for several reasons. Examination of the external ear canal and the tympanum cannot be complete until the canal is cleaned. Wax, oil and cellular debris may be irritating, prevent medication from contacting the canal epithelium, and produce a favorable environment for microorganisms to proliferate and inactivate certain antibiotics. Several products are available on the market and they should be used as directed by a veterinarian as some of them may interfere with the efficacy of the topical medications. Also some of them may be irritating if not completely removed thus appropriate flushing by a veterinarian might be required. These products are usually classified as either ceruminolytic or drying agents.
1. Ceruminolytic agents (e.g. Cerumene) emulsify the waxes and lipids to help flush them more readily from the ear canal. They contain surfactants and detergents (e.g. diotyl sodium sulfosuccinate or DSS, squalene, carbamide). In general such products should be applied 5-15 minutes prior to cleaning. General massage improves their effect. Most of these products are contraindicated with ruptured tympanum. However, frequently the condition of the tympanum cannot be determined until after the canal has been cleaned. In those cases the probability of ototoxicity may be decreased by flushing with water after the application of such agents. In a recent study several cerumuminolytic agents were applied in the middle ear and squalene was the only one that did not cause any damage.1 However, it should be realized that there is no completely safe solution for cleaning the middle ear. Even water can cause ototoxicity.
Some disinfectant cleansers, such as chlorhexidine, are contraindicated with ruptured tympanums.
2. Drying agents (e.g. Epi-Otic) are applied after the ear has been cleaned and is relatively dry. Most contain alcohol and one or more of the following: boric acid, benzoic acid and acetic acid. Some products are a combination and they tend to have less drying agents and mildly ceruminolytic than the standard desiccants (e.g. Epiotic, Oticlens).
When flushing an ear with a ruptured tympanum the use of saline of 1:1 or 1:3 dilutions of five percent acetic acid (white vinegar) are recommended. The fluid is discarded with every flush and suck cycle and the canal is filled again with clean saline. This is repeated multiple times using a fair amount of saline. The best results for deep ear cleaning or flushing are obtained with the patient under general anesthesia.
Cleaning cannot be done on very swollen, stenotic, ulcerated or painful ears. Such cases need to be treated symptomatically initially and cleaned at a later date when the inflammation has been reduced and the canals have opened.
Causes of Otitis Externa
Otitis may have numerous causes and a common classification is to break them down into predisposing, primary and perpetuating. Predisposing factors are those that place a patient at risk but by themselves are not able to cause otitis externa. Primary causes are usually the actual inciting agent that directly causes otitis externa. Perpetuating factors are those that prevent the resolution of otitis externa once the problem has been established.
Predisposition Factors and Risk Assessment
The most successful management requires that these factors are recognized and, whenever possible, controlled. Early detection may prevent unnecessary pain/hearing loss and reduce the prevalence of chronic and refractory disorders.
1. Breed Predisposition and Anatomic Conformation
Otitis occurs more frequently in breeds of dogs that have pendulous ears (e.g. Cocker Spaniel) and those with hair growth in the ear canal. Originally this difference was thought to be secondary to variations in the temperature and/or humidity of the ear’s microenvironment in dogs with different ear types, however no difference in temperature was found between ear types. It is becoming more and more evident that variations in the anatomy and the number of glands may predispose certain breeds to otitis externa.
Dogs with longhair coats and pendulous ears should have the hair clipped frequently around the auricular orifice and the concave surface of the pinnae. Hair in the ear canal should be removed with a forceps and twisting (rather than plucking, which is more painful) the hairs out by twirling the forceps to improve ventilation in the canal. Great care should be used when removing these hairs as excessive trauma to the area may predispose to an infection.
Stenosis of the ear canal (e.g. Shar-Pei) is another variation in the anatomy that can predispose dogs to otitis externa. In Shar-Peis the stenotic canal and the conformation of the pinna that is tightly folded over the external orifice increases the risk of otitis externa. Stenosis of the canal can also be acquired (e.g. abscess, neoplasm).
2. Climatic variations
In a recent study monthly variation in ambient temperature, rainfall, and relative humidity correlate positively with increases in the number of first-time otitis externa cases seen.
3. Life Style
Dogs used for activities that involve exposure to field are at increased risk of ear disease.
Foreign bodies, especially plant material, often become trapped in the canal. These animals should be examined frequently.
4. Maceration of the Ear Canal
Any increase in the moisture of the ear canal can lead to maceration. Moisture in the canal, whether introduced by swimming, bathing, or inappropriate treatment may cause otitis externa of inflammation of the external part of the ear canal. A combination of water retention, epidermal maceration, increased ceruminous gland activity and secondary infections may be responsible for disease. Dogs that swim may benefit from prophylactic treatment with a drying agent (e.g. acetic acid).
5. Excessive Ear Cleaning
Mechanical trauma of the ear canal through vigorous hair plucking and the use of cotton swabs or other objects to remove wax, as well as the use of irritant topical solutions and excessive cleaning that alters the normal micro-flora, are all factors that predispose to the development of infections.
Primary Causes of Otitis Externa
When discussing the primary causes of otitis externa, it is important to remember that the epithelium of the external ear canal is simply an extension of the rest of the skin. Most causes of otitis externa are associated with generalized dermatologic conditions. A complete dermatologic history and work up may therefore be necessary in the diagnosis of many primary otitis externa cases. The most common causes seen in dermatology are atopy (inhalant allergies), food allergy, diseases of keratinization (e.g. primary seborrhea of Cocker Spaniels), and ear mites. It is critical to long-term management of otitis externa that a primary cause can be found.
The ear mite (e.g. Otodectes) is the most common mite, being responsible for up to 50 percent of the cases of otitis externa in cats; in dogs the incidence is controversial but most authors agree that it is responsible for 5-10 percent of cases. They are most commonly found in the external ear canal, but can survive for some time on the surface of the skin, typically of the head and neck. In the ear the mites are protected by desiccation by a typical dark brown crust. In recurrent cases, it is possible that other in contact animals can act as asymptomatic carriers.
Other mites that can be responsible for otitis include Sarcoptes (mite that causes scabies),
Demodex (mite that causes the so-called red mange), Eutrombicula (chiggers) and Otobius (the spinous ear tick of dogs).
Dermatophytes (which cause â€œringwormâ€) are a relatively common cause of disease of the pinna and in rare occasions may cause otitis externa. Bacteria are most commonly perpetuating factors.
Allergies are the most common underlying cause for otitis externa in dogs. They include inhalant allergy (also called atopy), food allergy and contact allergy.
Inhalant allergy is extremely common in dogs and cats and is the most common underlying cause for recurrent otitis externa in dogs. At least 50 percent of atopic dogs have bilateral otitis externa. In up to five percent of cases, otitis may be the only complaint. Atopic dogs tend to have itchy feet (e.g. they lick and chew their feet), itchy face (e.g. they rub their face against the carpet or pieces of furniture) and itchy ears. They are predisposed to secondary skin and ear infections that tend to recur after treatment unless the underlying allergy is well controlled. A familial history is present in most cases and strong breed predilection has been reported (e.g.Dalmatians, Terriers, Golden Retrievers). Clinical signs are initially seasonal. Progressive worsening with time is also typical. Diagnosis is based on history, clinical signs, exclusion of other diseases and intradermal skin test.
Food allergy is not as common as the inhalant allergy, but over 20 percent of these cases start with just otitis externa, and ear disease is present in 80 percent of the cases. Food allergy should be considered as a top differential for otitis externa in any young dog (less than one year of age). Food allergy is diagnosed by appropriate food trial (a novel source of protein is selected based on the individual history and used for a minimum of two months).
Contact allergy can result from medications used to treat otitis externa. Whenever a case of otitis externa fails to improve with therapy or worsens after therapy, a contact dermatitis should be suspected.
4. Foreign bodies
Plant material (fox tails), dirt, sand, impacted wax, loose hair and dried medications are frequently responsible for otitis externa. In most cases this is a unilateral otitis.
5. Diseases of keratinization (e.g. primary seborrhea of Cocker Spaniels)
Excessive and abnormal composition of cerumen in these cases is responsible for the development of otitis externa and secondary infections of skin and ears. It is usually seen in young animals.
6. Endocrine disorders
Hypothyroidism (decreased production of thyroid hormone) and Cushing’s disease (disease associated with excessive production of steroid hormones) are the most common endocrine diseases that can cause otitis externa. If a middle-aged dog keeps relapsing with otitis externa and is not itchy, then endocrine diseases should be considered as possible underlying causes.
7. Autoimmune disorders
Pemphigus (disease in which the organism produces antibodies against component of its own skin) affects the pinna and may extend to the ear canal causing otitis. Lupus (other autoimmune disease in which the organism produces antibodies against various components of the body) can also cause ear disease.
They include anything that prevents the resolution of an already present otitis externa.
Perpetuating factors are a major reason for poor response to therapy regardless of the predisposing factors and the primary cause. In early cases treating the primary cause might be sufficient to resolve the otitis, while in more chronic cases perpetuating factors have to be addressed to resolve the case.
In most normal ear canals a variety of bacteria can be cultured. Once predisposing and primary factors cause alterations in the ear canal environment, these bacteria may proliferate and perpetuate an inflammatory reaction. In most cases of chronic otitis externa bacteria such as Staphylococcus and Pseudomonas are present. Aggressive treatment is warranted as resistance to antibiotic may easily occur, especially in cases when Pseudomonas is cultured. Although bacteria are not a primary cause of otitis, once the infection is established, they can cause significant inflammation and damage. These dogs often present with purulent discharge in the ears. Pain on palpation of the ears is quite common and a strong odor is usually present.
Diagnosis is based on cytology and culture. Initial topical therapy for a case of otitis externa is based on the results obtained from the cytology of the exudates, while in chronic cases is best based on results of culture and sensitivity.
Pseudomonas-related infections are extremely frustrating and difficult to treat. Most effective treatments include topical Polimixin B and systemic enrofloxacin or ciprofloxacin. Doses that are used are higher than the ones suggested on the label as resistance occurs rapidly. A commonly used dose for these drugs is eleven mg/kg twice daily. Dogs with OM frequently require two months of systemic antibiotic. As Polimixin B is rapidly inactivated by the exudates, aggressive cleaning is an essential part of therapy. Other topical treatments used for Pseudomonas include acetic acid (vinegar/water 1:1) and silver sulfadiazine (one gram of silver sulfadiazine is mixed with 100 ml of sterile water and 0.5 ml of the mixture is applied twice daily). Also pre-soaking the ear with edetate trisodium (tris-EDTA) 15 minutes prior to application of the antibiotic increases the efficacy. Finally, topical enrofloxcin (Baytril otic) can be used in Pseudomonas infections. In cases where Staphylococcus is the cause of otitis, other antibiotics are usually used including cephalexin (22 mg/kg twice daily) and trimethoprim-sulfa (25 mg/mk twice daily).
Malassezia is the most common perpetuating yeast that contributes to otitis externa. It is a budding organism with the shape of a peanut and is part of the normal flora (both skin and ears) of dogs and cats. It is a common complication with allergic otitis (80 percent of cases) and may result as a super-infection following antibiotic therapy. Grossly the discharge is thick, dark, and sweet smelling. Diagnosis is usually based on the physical findings and microscopic examination of the exudates (cytology). Topical therapy is usually sufficient and miconazole and clortrimazole (e.g. Conofite, Lotrimin) are the most commonly used ingredients. In rare cases of otitis media due to Malassezia, systemic treatment is necessary and ketoconazole (Nizoral tablets) is used at five mg/kg twice daily for three to four weeks. Side effects include anorexia, vomiting and diarrhea.
3. Progressive pathological changes
Chronic inflammation stimulates the proliferation of the skin lining the ear canal. As a consequence, thickening of the canal occurs leading to stenosis of the canal. More importantly the skin is thrown into numerous folds, which inhibits effective cleaning and the application of medications. These folds act as a site for the perpetuation and protection of secondary microorganisms (e.g. bacteria). Laser surgery has been used successfully to correct excessive stenosis and thickening of the canal.
4. Otitis media
Otitis media (inflammation of the middle ear) results from chronic inflammation of the external part of the ear canal, rupture of the tympanic membrane, and establishment of infection in the middle part of the ear. Exudate in the tympanic cavity is difficult to treat with topical therapy and often remains as a source for infection. Otitis media is usually bacterial in origin. Clinical signs suggestive of otitis externa include head shyness and pain on palpation of the ears. Some cases of otitis media might present with head tilt, circling and dry eyes, but the vast majority does not have neurological abnormalities. As the tympanic membrane quickly grows back after rupture, otitis media may also be present even if an intact membrane is seen on otoscopic examination. Radiography cannot be used to completely rule out the presence of otitis media since 25 percent of confirmed cases had no radiographic evidence of the disease. In a study otitis media was present in 80 percent of cases of chronic, relapsing otitis externa therefore it must be considered as a possible cause of any refractory or relapsing otitis externa. Treatment of otitis media is based on bacterial culture/sensitivity results. Most cases require long-term antibiotic therapy (minimum of two months) and aggressive topical therapy.
Otitis externa is a very common clinical presentation in small animals.
A good history is very important in every case of otitis externa to identify predisposing and primary factors.
Aggressive treatment is needed in most cases of relapsing otitis externa as an infection of the middle ear might be present.
Mansfield PD, et al. The effects of four commercial ceruminolytic agents on the middle ear. JAAHA 97; 33: 479-86.
1Griffin CE, Kwochka KW, MacDonald JM. Current Veterinary Dermatology: the art and science of therapy.Mosby Year Book, St. Louis, 1993.
Dog Owners and Breeders Symposium
July 27, 2002
University of Florida
College of Veterinary Medicine