Last in a Two-Part Series: Uncovering the pruritic dog takes more than scratching the surface

Last in a Two-Part Series: Uncovering the pruritic dog takes more than scratching the surface

Apr 01, 2002

An example of a focal pericular deep pyoderma.
As mentioned in the first article (Feb. 2002) of this series, the presentation of the pruritic dog can be frustrating for the veterinarian because of the number of possible differential diagnoses.

It cannot be emphasized enough that a detailed history and thorough physical examination is essential in determining the cause of the pruritus. A detailed history includes age, breed and sex of the affected animal as well as duration of the pruritus, areas of the body involved, and response to any medications either topical or systemic that have been administered.

A thorough physical examination should include dermatologic procedures such as combings, skin scrapings and cytology of any lesions present. Depending upon the outcome of these in-house tests, other laboratory procedures such as complete blood counts, serum profiles, thyroid panels, tests for Cushing's disease, allergy testing or performing skin biopsies may be necessary. In part one, differentials of pruritus that included ectoparasites were discussed. Now that in-house testing and/or therapy to rule out this category have been performed, the next step is to rule out bacterial pyoderma (primary or secondary), atopy and/or food allergy as the possible cause of pruritus.

Labrador Retriever with inhalant allergy. The other differential would be scabies because of the ear-edge involvement.
Bacterial pyoderma In the canine, bacterial pyoderma, the majority of the time, is caused by Staphlococcus intermedius. That is why many veterinarians do not feel the need to routinely culture a bacterial pyoderma. However, the time when a culture and sensitivity is necessary is when the patient is not responding to an appropriate antibiotic (assuming antibiotic only is used, not in conjunction with steroids) or when a deep pyoderma is present, particularly, when demodicosis is involved.

In the latter, many types of bacteria may be present including Pseudomonas and Proteus species. The diagnosis of bacterial pyoderma is usually made by clinical presentation, i.e. the appearance of pustules or epidermal collarettes.

When intact pustules are present, cytology should be performed revealing results consistent with bacterial pyoderma such as degenerative neutrophils with or without intracellular cocci. The main differential for bacterial pyoderma is pemphigus foliaceus which, at times, may be difficult to differentiate for both the clinician and pathologist. In older patients, epitheliotropic lymphoma may appear clinically similar to bacterial pyoderma. A skin biopsy is necessary to differentiate between the two diseases.

Antibiotic therapies In a "first time" bacterial pyoderma, antibiotics such as sulfa derivatives (use with caution in patients with keratoconjunctivitis sicca (KCS), those breeds prone to KCS, or in Dobermans which can show a possible genetic predisposition for adverse reactions to sulfas), cephalosporins, macrolides such as lincomycin, or b-lactamase potentiated amoxicillin (Clavamox) should be administered until total clearing of the pyoderma plus an additional week past clearing. Antibacterial bathing should also be performed.

Periocular alopecia and erythema in an atopic Boston Terrier.
Tetracyclines and penicillins normally are not effective in treating canine staph pyoderma. Steroids should be avoided even if the patient is pruritic as they may cause immunosuppression thereby "undoing" the effects of the antibiotic. Normally 21 to 30 days of antibiotic is initially dispensed with a recheck at the end of that period of time to be sure the pyoderma is resolving, then additional antibiotic is dispensed to complete the "one week past clearing" process. Antibacterial shampoos such as chlorhexidine, ethyl lactate (Etiderm), benzoyl peroxide or sulfa/salicylic acid combinations can be helpful adjunctive therapy. In a recurrent pyoderma, factors to consider include: Was the pyoderma treated with an appropriate antibiotic without steroids until one week past clearing? Is there a potentiating underlying disease such as demodicosis, hypothyroidism, Cushing's disease, diabetes mellitus, atopy or food allergy? Treatment of a recurrent pyoderma involves antibiotics, antibacterial bathing and addressing the underlying disease perpetuating the pyoderma.

If no underlying disease is found, then "pulse dosing" antibiotics i.e. one week on/one week off is performed or immune stimulants such as Staphage Lysate are administered.

Dietary troubles Food allergy can manifest with many symptoms including a recurrent bacterial pyoderma, otitis, Malassezia dermatitis and pruritus without lesions. Dogs of any age, breed or sex can be affected. Some dermatologists classify food allergic dogs as having symptoms involving "ears and rears".

Nondermatologic manifestations of food allergy can include gastrointestinal problems such as vomiting, diarrhea, and flatulence, respiratory problems or neurologic symptoms including seizures. Food allergic patients usually have nonseasonal problems since they are eating the same food all of the time (or variations of the same food) and are nonresponsive to antipruritic doses of steroid (however some patients may get temporary relief from anti-inflammatory or immunosuppressive doses of steroid).

Since blood or skin testing for food allergy has not been proven to be accurate, the best way to assess food allergy in the dog is the feeding of a hypoallergenic diet either cooked or commercially prepared for at least eight to 10 weeks and perhaps longer. The favored method is a home-cooked diet consisting of a novel protein and single carbohydrate source.

Since this is impractical for most large breed dog owners, commercially available prescription foods containing novel proteins such as venison, rabbit, duck, fish, kangaroo or a protein hydrolysate diet are available. The concept of what one is trying to accomplish must be explained to the owner so that the diet is performed correctly.

Erythemic alopecia caudal in an atopic patient.
The point is to remove the pet from all ingredients to which they have been exposed to in previous foods or treats e.g. corn, wheat, egg, beef, chicken, soy and dairy. No other foods, bones, rawhides, treats or flavored heartworm preventatives should be administered during the diet period.

Some owners elect to administer distilled water as opposed to tap water to further control what the animal is receiving. It may be difficult to undertake a hypoallergenic diet for owners with young children since theoretically a dietary indiscretion can set the patient back one to two weeks. Since there are two categories of hypoallergenic diets, the single allergen diet and the protein hydrolysate diets, a patient may respond to one type of diet and not the other. If you have undertaken a hypoallergenic diet of one particular type without success, be sure to feed one of the other category to fully rule out food allergy. Be extremely inquisitive about what is administered to the patient i.e. medications cannot be given with cheese, lunch meats or peanut butter (contains corn syrup) which will negate the effect of the diet.