The diagnostic approach to the pruritic dog (Sponsored by Pfizer Animal Health)

In veterinary dermatology, the most common species evaluated is dogs and pruritus is the most common owner complaint. In order to treat pruritus effectively, veterinarians should try to make a diagnosis in addition to treating the clinical signs.

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Obtaining a diagnosis is less difficult if it is approached methodically. First, determine if the pruritus is caused by an infectious condition that could be cured or if it is caused by an allergy, in which case the condition needs to be managed, possibly for life. Allergic conditions can be secondarily infected, so these infections need to be controlled prior to starting the diagnostic process for a suspected allergic disease. Some "infections," such as sarcoptic mange, can cause pruritus through both an allergic mechanism and the infestation itself. Figure 1 demonstrates how the common pruritic conditions of the dog can be organized and where they can overlap.

Figure 1: Is the dog pruritic because it is infected, allergic, or both?

Canine pruritus: Diagnostic approach to the pruritic dog

Infectious dermatoses

1. Rule out scabies by history, physical examination, negative skin scraping, and a lack of response to treatment:

• Selamectin at initial examination; repeat 30 days after

- Off label alternatives: Ivermectin—three treatments at 14-day intervals; lime sulfur—three treatments at 14-day intervals; fipronil spray—three treatments at 14-day intervals; milbemycin; organophosphate rinses

• Treat all in-contact dogs.

2.Rule out Malassezia colonization with physical examination, cutaneous cytology, and lack of response to treatment:

• Ketoconazole 10 mg/kg every day for 30 days

- Alternatives: itraconazole, fluconazole or terbenifine

• Antifungal shampoo—PRN, 10 minutes of contact time (chlorhexidine, miconazole, ketoconazole, selenium)

• Leave-on rinse (25% vinegar, miconazole, ketoconazole).

3.Rule out Staphylococcus colonization with physical examination, cutaneous cytology, and lack of response to treatment:

• Identify and treat the underlying cause

- Skin scrape to rule out demodicosis

- Check thyroid status

- Weight loss if infection involves intertriginous areas

• Systemic antibiotic for minimum of 14 days for superficial infections

- Cefovecin, cefpodoxime, cephalexin, clindamycin, lincomycin, ormetoprim/sulfadimethoxine

• Antibacterial shampoo-PRN, 10 minutes of contact time (benzoyl peroxide, ethyl lactate, chlorhexidine, triclosan).

Allergic dermatoses

4. Rule out flea bite hypersensitivity with history, physical examination, identification of fleas, flea feces, and intradermal test with flea allergen:

• Treat or prevent fleas

- Selamectin, fipronil, imidacloprid, or other adulticide

• Treat secondary infections with antibiotics or anti-yeast products

• Treat pruritus with a short course of corticosteroids.

5. Rule out an adverse reaction to foods with history, physical examination, and a minimum of one month of a novel protein and carbohydrate diet trial.

6. Diagnose atopic dermatitis with history, physical examination, positive response to treatment with corticosteroids, and exclusion of all other causes of pruritus:

• Treat with safe doses of corticosteroids

• Control infections

• Try antihistamines, omega-3 fatty acids or cyclosporine

• Intradermal test or allergen-specific IgE serology to select allergens for allergen-specific immunotherapy.