Diagnosis and Treatment of Pruritic Otitis

Most cases of acute otitis externa are pruritic. The pet is presented to the veterinarian for scratching at the ear, erythematous pinnae, shaking of the head, or apparent pain when the ears are manipulated. The differential diagnosis for pruritus is extensive and is dealt with in textbooks on dermatology.

This chapter entails a "rapid-fire" clinical trial approach to the treatment of pruritus in a pet with or without otitis externa. Such an approach addresses treatment for several different causes of pruritus at once. This regimen not only promises the best chance to achieve fast and lasting relief from pruritus but also allows the veterinarian to make a rapid clinical diagnosis based on the response to the various elements involved in the trial. The chapter concludes with a program of treatment for otitis.

Perhaps the most common cause of acute otitis externa is hypersensitivity. The most common hypersensitivities encountered in small animal practices are flea allergy, food allergy, and atopy. Dogs with atopic dermatitis are presented to the veterinarian with a clinical history of pruritus of the feet, face, axilla, and ears. It has been estimated that as many as 70% of dogs with atopy have otitis externa and pruritic ears. In dogs and cats with food hypersensitivity, otic pruritus may be the only sign of disease. Flea allergy may contribute to overall pruritus but rarely causes otic pruritus singularly.

If the clinician suspects atopy, a pretreatment blood sample for in vitro allergy testing should be obtained prior to the start of this pruritic clinical trial, which involves the use of some corticosteroids. The blood should be spun and the serum frozen. Frozen serum can be submitted for in vitro allergy testing as long as 60 to 90 days after sampling without affecting the results. Testing and therapy for atopy are both tedious and expensive; the treatment is life long. The clinical trial described here may provide the client proof that the pet is suffering from hypersensitivity, making the clients decision to proceed with testing and hypersensitization easier.

Of course, a good physical examination and observational skills are not to be neglected in the attempt to sort out the etiology of dermatological disease. It is prudent to perform skin scraping and analysis, fecal analysis, heart worm testing, fungal culture, skin and ear cytological evaluations, and other tests as indicated by clinical common sense during the initial examination.

In addition, recheck visits and the observational skills of the client are crucial to securing an accurate diagnosis. The veterinarian should have the client keep a daily diary of the pet's response, using a scale of 1 through 10 to quantify the intensity of pruritus, erythma, pain, any discharge from the ear, and head shaking.

This clinical trial for pruritus should be performed in conjunction with topical ear treatment (see later). Start all actions on Day 1.

Of the estimated 116 million dogs and cats in the United States, 12% to 20% have allergy-induced skin problems that require frequent bathing, preferably with "Hypoallergenic" shampoos.

Shampoo therapy has moved to the forefront in the treatment of all but the rarest skin disorders. It involves the use of cleansing, moisturizing, antiseborrheic, degreasing, antiparasitic, antibacterial, antifungal, and antipruritic shampoos. Specific products and protocols usually are selected on the basis of the presenting morphological characteristics, such as dryness, oiliness, scaling, inflammation, and associated pyoderma. Generally, the use of a milder, more elegant product before a coarser, more potent one increases the pet owner's compliance with the regimen and reduces the risk of side effects, such as irritation.

Various issues must be considered in the selection of therapeutic shampoos to relieve pets' specific symptoms.

Cleansing and moisturizing shampoos are designed to do just what their names imply. The mechanical process of bathing (even with water alone) helps remove scales, crusts, organisms, dander, loose hair, and other debris. All such shampoos should be pH-adjusted for dogs, which have the highest skin pH of any mammal (6.2 to 7.2), including humans.

Moisturizing agents, such as bath oils, conditioners, emollients, and humectants, may be applied after bathing and rinsing to soften, lubricate, and rehydrate the skin. They can be used on a more regular basis for dryness.

Seborrhea is the term used for any skin disease involving dry (sicca) or greasy (oleosa) scaling. The term also encompasses disorders in the formation of keratin, a complex protein unique to the skin, hair follicles, and nails. Today, many experts prefer the term disorders of keratinization. This subject is covered in detail elsewhere in this book.

The epidermis is completely replaced every 22 days in the normal dog. Epidermal cell turnover time in dogs suffering from idiopathic seborrhea, which is more common among Cocker spaniels. May be as short as 3 to 6 days. This fast turnover creates a defect in the normal protective barrier, which may result in dry or greasy scales, comedones, alopecia, inflammation crusts, pyoderma, and pruritus. Any one of these conditions, in turn, may lead to further skin damage. In these cases, it is important to slow the turnover process and treat the secondary problems.

Bathe the pet daily in a hypoallergenic shampoo. If the pet remains itchy after these baths, an oatmeal shampoo or conditioner may help resolve mild pruritus. If pyoderma, Malassezia dermatitis, or seborrhea oleosa is present, degreasing and antiseptic shampoo should be used every 2 to 3 days. The author prefers a shampoo with acetic acid and boric acid (MalAcetic, DermaPet) followed by an acetic acid-boric acid conditioner. This unique combination kills bacteria and yeast on the skin.

Shampoo therapy should be used as a component of the clinical trial for 3 weeks. If the pet improves symptomatically, the owner should continue the bathing.

Supplementing the pet's diet with an omega-3/6 fatty acid supplement should reduce the inflammation associated with pruritus. Antioxidants such as vitamin E and vitamin C should be included in the supplement because they are depleted more rapidly when there is fish oil (omega-3) in the diet. In some breeds, zinc is also a useful supplement when a zinc responsive dermatosis is present.

Conduct a strict food elimination diet for a minimum of 21 days; in some food-allergic dogs, 60 to 90 days may be required to show beneficial effect. Many diets that can be used for the food elimination trial are commercially available. They contain either uncommon, novel protein sources (venison, rabbit, duck, fish) or purified low-molecular-weight polypeptides. The author prefers to use a home-cooked vegetarian diet (see the box).

All Vegetable Hypoallergenic Diet

Vegetable Puree (multiple batch)

Ingredients:

Three undrained #1 cans of: carrots, peas, green beans, tomatoes, and greens (kale, dock, spinach, or mustard).

One 10 oz. Package of chopped, frozen, broccoli.
Instructions:

Boil the broccoli in 2 cups of water until tender. Combine with other vege-tables and freeze.

Rice (prepare as required)

Ingredients:

2 1/2 cups rice

5 cups water

1/2 cup sunflower oil

1 tsp. salt
Instructions:

In a large kettle, mix and puree until smooth. Fill 18 one pint plastic containers

Mix ingredients, bring to boil, reduce heat and simmer until water is absorbed. Allow to cool

Mix Vegetables and Rice

Thaw 1 pint of vegetable puree and add to rice, mix thoroughly. Feed 1/2 - 3/4 cups per 10 lbs body weight twice daily. Monitor weight weekly. Do not add meat supplements.

*From Byrne K. Food allergy. In Skin Diseases of Dogs and Cats. DermaPet Inc, Potomac, MD 20854; 1994.

Administer a low-dose corticosteroid for 12 days (e.g., prednisone at 0.5 mg/lb bid). Use a tapering dosage regimen that ends with 0.5 mg per pound every other day. The rationale is to break the pruritic cycle and to observe whether the pruritus is responsive to corticosteroids.

If the pet has pyoderma, use an antibiotic such as cephalexin at 10-15 mg per pound bid for a minimum of 21 days. If the ear is also infected with bacteria, prescribe a fluoroquinolone such as enrofloxacin at 5 mg/lb bid or 10 mg/lb once daily.

If Malassezia dermatitis is present, bathing with the acetic acid-boric acid shampoo as described in step 1 and antibiotic therapy will usually control the pruritus. On rare occasions, or when there is very deep involvement of the skin with Malassezia, oral ketoconazole at 10 mg/kg once daily or itraconizole at 5 mg/kg once daily may be used. The duration of the treatment varies, but it should be at least 2 weeks.

Treat scabies, other ectoparasites, and some endoparasites with a trial of ivermectin. In all breeds except collies or their mixes, use ivermectin (Bovine Ivomec, MSD/AgVet) at 0.1mL/10lb body weight. Use every 7-10 days for 4 treatments. The extra label use of this drug requires informed consent of the client.

If the clinical signs stop while the prednisone and ivermectin are being given and never return, the diagnosis is ectoparasites, most likely scabies, or ear mite hypersensitivity. The ivermectin killed the parasites, and the prednisone reduced the pruritus.

If the response is more gradual and the pruritus does not return after bathing with the acetic acid-boric acid (MalAcetic^TM) shampoo and/or ketoconazole or itraconazole, cutaneous Malassezia infection is the likely cause. Unfortunately, Malassezia infection is often a secondary complication that perpetuates otic pruritus in many atopic dogs. An acetic acid-boric acid ear cleaner (Dermapet) may need to be used on a maintenance basis once or twice a week to control otic Malassezia yeast.

If the pruritus returns between day 12 and day 21, a cortisone responsive hypersensitivity is most likely, and allergy testing should be done. The frozen pretreatment serum sample can then be submitted for in vitro allergy testing.

If itching remains controlled comfortably after the original pretreatment diet is re-instituted, the possibility that the animal has a fatty acid or shampoo therapy-responsive atopy is high. Primary food allergy should be suspected if the itching returns within 72 hours after the hypoallergenic diet is withdrawn and the pet is returned to the pretreatment diet.

Bacterial hypersensitivity may be suspected if the itching returns within 30 days after the antibiotic is stopped, provided that the pruritus was controlled during antibiotic therapy. If the itching disappears after a second course of antibiotic therapy, the diagnosis is confirmed.

Keratinization disorders, including hypothyroidism, skin neoplasia, and other less common primary skin diseases, prevent the itch from fully resolving. Incomplete treatment of Malassezia dermatitis or the failure to treat all contact animals for scabies may cause persistent pruritus. A biopsy of the skin may be helpful in many circumstances to identify the etiology of pruritic skin diseases.

In many cases of acute otitis externa without progressive pathological changes, resolution of pruritic skin disease also decreases otic pruritus. However, the skin of the ear is treated differently form the skin of the trunk, primarily with concentrated topical medications.

The following is a simple, logical, progressive method of providing ear therapy. Prior to initiating these steps, appropriate specimens for cytological evaluation or culture should be obtained. The clinician should perform the steps in the order given (no skipping); if one step does not resolve the problem, therapy should continue to the next step.

Clean the ear thoroughly using a nonototoxic ear cleaner in the hospital so that a thorough otic examination can be performed, including assessment of the tympanic membrane. The author prefers an acetic acid-boric acid ear cleaner by Dermapet. Problems may arise when detergents and alcohols are used as an ear cleaner in an ear in which the eardrum may be ruptured. Use anesthesia when appropriate.

Acetic acid and boric acids in combination have been shown to be an effective combination in vivo to treat Malassezia otitis (Gotthelf, 1997). In many cases, continued daily ear cleansing by the owner at home for 1 week is all that is needed. Twice-weekly maintenance cleaning may prevent recurrence of Malassezia infection. Cleaning the ear is important in removing surface debris from the affected ear canal epithelium. Apply medications, if needed after cleaning.

In dry or irritated ears with little debris and wax, the cotton ball may be irritating. In these cases, a tiny bulb syringe or Water Pik can be used and the frequency of cleaning may need to be increased to one to three times daily.

It is important to remember that infected ears are very acidic, a condition that inactivates some antibiotics commonly used on the ear, such as gentamicin and amikacin. Many ear cleaners are also acidic. Waiting 30 seconds after cleaning to apply these antibiotics may be warranted.

One nonototoxic alkalinizing agent that the literature reports to have primary antimicrobial properties is ethylenediaminetetraacetic acid with TrizEDTA^TM . Follow the preceding instructions on how to clean an ear, substituting the alkalinizing TrizEDTA^TM for the acidic ear cleaner. It appears that pretreatment with this agent increases the antimicrobial activity of many antibiotics used to treat ear disease. It is especially useful in stubborn cases of Pseudomonas otitis. Gentamicin or enrofloxacin may be added to the TrizEDTA^TM solution.

Logical otic therapy is based on the results of otic examination and ctyological evaluation. In bacterial infections, antibiotics should be used. In yeast infection, use an anti-yeast therapy. In inflammation (neutrophils on the cytological specimen) or erythma, use a topical corticosteroids. If ear mites are present, use topical insecticide or topical ivermectin. After the ear has been cleaned, apply the topical formula (see details in box).

Topical Ear Formula or TrizEDTA^TM

12 ml. acetic acid-boric acid ear cleaner
2 ml Enrofloxacin injectable
6 mg Dexamethasone sodium phosphate (for greater solubility)

Also Add as Appropriate

0.5-1ml Medical-grade dimethyl sulfoxide (DMSO)
0.5 ml ivermectin

Steps 3a and 3b may be performed separately or concurrently. They are the logical next step after ear cleaning and use of a topical formula fail to cause remission of disease. Steps 1 and 2 are continued, and Step 3a and/or 3b is added.

When steps 1 through 3 do not cause remission of disease after 2 weeks, re-examine the ears. For stenosis and strangulation of the canal, use a 1.5-inch, 20-gauge needle to inject approximately 0.5 ml methylprednisone acetate (Depo-Medrol) between the epithelium and cartilage as distally in the canal as possible.

If the eardrum is bulging, myringotomy may be performed to relieve the pressure and pain and allow collection of a culture specimen from the middle ear. If the eardrum is ruptured, infuse a combination of 1-2 mg of dexamethasone sodium phosphate and 10-20 mg of enrofloxacin directly into the tympanic bulla.

The use of corticosteroids is multifunctional. Although we are most familiar with the anti-inflammatory properties of such agents, their ability to reduce viscosity of the secretions and exudate is a noteworthy goal in cases of otitis media.

Many dermatologists recommend a culture specimen from each ear in difficult cases to assess the bacterial microflora. It is possible for a pet to have different bacteria in each ear and for otitis media to be complicated by bacteria that are not normally found in the external ear. Topical treatment with antibiotics achieves much higher tissue levels than the minimum inhibitory concentration (MIC) reported by the laboratory, so bacteria resistant according to culture results may be susceptible to higher topical doses of the same drug.

In stubborn bacterial otitis externa or otitis media, use TrizEDTA^TM prior to the instillation of the antimicrobial. Clean the ear thoroughly for 7 to 21 days. TrizEDTA^TM is an alkaline solution with a pH of 8.0. There are reports in the literature that this agent has primary microbiocidal properties, particularly against Pseudomonas. There is also evidence that TrizEDTA^TM potentiates the action of antibiotics that are inactivated by other acidifying ear cleansers. If Pseudomonas is suspected from cytological evaluation or found by culture, or use the appropriate systemic and topical antibiotics after a15-minute pretreatment with TrizEDTA^TM.

If (1) corticosteroid injection into the ear canal epithelium fails to open up an inflamed, swollen, occluded ear canal (Step 4), (2)permanent pathological changes are evident, or (3) there is severe calcification of the ear canal, surgery of the ear canal is indicated for relief of pain and to allow for drainage.

Gotthelf L, Young S: A new treatment for canine otitis externa. Vet Forum. Aug 1997.