Diseases of the Ear in Dogs & Cats

Otitis Externa

Inflammation of the external ear canal (external to the eardrum), otitis externa, is a symptom of many diseases and not a disease or diagnosis in itself. As many as ten to twenty percent of the dogs brought to the hospital have ear infections. In cats, it is much less common and usually involves parasites (ear mites). In fifty percent of the chronic cases, the eardrum is ruptured and extension of the infection into the middle ear is expected. This causes otitis media (infection of the middle ear). This middle ear infection becomes the source for otitis externa recurrence. Otitis media is also a common cause for failure of lateral ear re-section surgery to be successful.

Dog Ear Disease Cause

The many causes of ear disease may be categorized as follows:

1. Predisposing Factors
2. Primary Causes
3. Perpetuating Factors

Predisposing Factors for Ear Disease

These factors increase the risk for a patient to have otitis – inflammation of the ear. The most common are:

1. Hair in the ear canal (poodles, schnauzers)
2. Pendulous ears (cocker spaniels)
3. Moisture (Labradors)
4. Inappropriate treatment (clients, health care team and groomers)
5. Certain breeds fit the most commonly seen predisposing problems. Chinese Shar-Pei’s and English bulldogs have small ear canals, cockers have pendulous ears, and retrievers like to swim

Primary Causes of Ear Disease in Animals

A primary cause is an actual inciting agent that causes ear disease by itself without predisposing or perpetuating causes.

1. Parasites— Common parasites found in ears include: Ear mites (Otodectes cynotis), Demodex mites (Demodex canis, Demodex cati), Sarcoptes mites (Sarcoptes scabiei), Notoedres mites (Notoedres cati), and ticks. Ear mites are the most common cause of ear disease being responsible for over fifty percent of the cases in cats and five to ten percent of the cases of otitis in dogs. Many mite infestations may only have a few mites. There is recent evidence that an allergic reaction to the mites is the primary reason for the itching found in these animals with few mites. Therefore, few mites may be found in dogs that have severe reactions. Also, mite infections are highly contagious, and there is the possibility that asymptomatic carrier dogs may be re-infecting the patient with persistent ear disease. If inflammation is severe, the mites will leave the ear canal. Cats with chronic, waxy ears have been found to have Demodex cati infections.
2. Micro-organisms— In most cases, bacteria and yeast are perpetuating causes and not primary causes of otitis externa. Ringworm (dermatophyte) is a common cause of disease of the earflap (pinna). Bacteria will only become pathogenic (disease causing) when inoculated into the ear with liquid media. (A good example is when a dog goes swimming in nasty water.) Yeast infection, likewise, is pathogenic only when inoculated with a broth type media. This can happen in a grooming salon with poor hygiene. The message here is that the true problem is not necessarily the “bugs” but the conditions for bug growth. The ears are warm, moist, and dark – this is a good place for bacteria and yeast to grow. Therefore, changing the conditions of the ear canal may be more important than killing the bacteria or yeast.
3. Hypersensitivity (Allergies)— Allergies to pollens in the air (atopy), food allergy, and contact-allergy can all cause otitis. Because it is more common, atopy is more frequently associated with ear disease. A common indication that atopy is the cause of otitis is that the pinna and vertical canal will be red and inflamed while the horizontal canal is normal. Another indication of allergic disease is that both ears will be similarly affected. Contact allergic dermatitis of the ear canal can occur from medications used in the ear and developing hypersensitivity to them.
4. Seborrhea disorders are common in dogs and denote a defect in the normal maturation cycle of the skin as well as excessive oil production. Breeds prone to seborrhea such as the cocker spaniel, Irish setter, Beagle and Basset hound tend to have otitis. This otitis is of the ceruminous variety (ceruminous meaning excessive secretion of earwax). Other causes of seborrhea include hormonal disorders such as hypothyroidism, male feminizing syndrome, Sertoli cell tumors and some ovarian imbalances. Hypothyroidism is the most common cause of ear related hormonal dysfunction.
5. Foreign Bodies— Plant material, dirt, sand, and dried medication are common causes of ear infections.
6. Auto Immune Disease— This is a group of rare diseases characterized by immune dysfunction. The immune system attacks normal tissues causing inflammation and ulceration. The ears may be involved in this type of reaction.

Perpetuating Factors of Ear Disease

Perpetuating factors are those that prevent the resolution of the ear disease. In all chronic cases, one or more of these factors will be present. Perpetuating factors may be the reason for poor response to therapy, regardless of the predisposing factors or primary cause.

1. 1. Bacteria are rarely primary causes of ear disease, therefore, a diagnosis of bacterial otitis is not a complete diagnosis. Pseudomonas spp are commonly transmitted by exposure to contaminated water, therefore, a combination of excessive moisture and Pseudomonas spp is the correct approach to the diagnosis. Other commonly isolated bacterial infections include Staphylococcus intermedius and the gram-negative infections Proteus spp, Escherichia coli, and Klebsiella spp. Once these bacteria establish infections, they cause a self-perpetuating course of inflammation and damage as diagrammed above.
   2. Yeast— Malassezia pachydermatis is the most common perpetuating yeast infection. It is a budding organism that is peanut, bottle-shaped or footprint-shaped. It is found in one-third of the normal dogs’ ears. It becomes pathogenic by itself when fluid is placed in the ear.
   3. Progressive Changes— Chronic inflammation stimulates the lining of the ear canal to undergo several pathological changes resulting in a perpetuating disease process. These changes include increased production of ear secretions (oils, waxes, sebum) and excessive production of skin that becomes folded, thickened and hyperplastic. This thickened skin leads to narrowing of the ear canal. The folds that the skin forms prevent effective cleaning and medicating of the ear canal. These folds also act as sites of perpetuating bacterial and yeast growth. Inflammatory debris accumulates in the ear canal including dead, exfoliated cells and the waxy secretions. This combination feeds growing numbers of bacteria and yeast. These organisms produce toxins as metabolic by-products that further initiate inflammatory changes seen in chronic otitis.

1. Middle Ear Infection (Otitis Media)— Otitis media is the term for inflammation of the middle ear. Anything past the tympanic membrane (eardrum) is considered the middle ear. Infection and pus within the tympanic cavity is difficult to treat with topical therapy and often remains the source for infection and perpetuating inflammation to the outer ear. Disease of the middle ear commonly results in calcification of the ear canal as well as accumulations of plugs of debris in the middle ear and bulla. These plugs must be removed by surgery. This inflammatory reaction may extend to affect the temporomandibular joint (jaw joint) causing pain when chewing or pain when opening the mouth or pain when palpating this area. See the relationship of the jaw joint to the ear in the image below.