The pruritic patient (Proceedings)

Dogs and cats are often presented with a complaint of scratching and chewing resulting self-inflicted trauma. Pruritus is a very unspecific clinical sign which can be caused by many different problems such as various ectoparasites, microbial infections and a wide range of hypersensitivities. Besides a thorough diagnostic work-up, veterinarians must make sure that the pet owner understand the complexity of most of these cases and is on board with you with whatever treatment protocol you are going to suggest. With this statement, it is obvious that a patient with a pruritic skin problem cannot be thoroughly worked-up within a 15-30 minute appointment.

History

A detailed history is crucial for every dermatological problem presented to a clinician and can lead in up to 30% of cases to a preliminary diagnosis by reducing the list of differentials. Most veterinary dermatologists have questionnaires which the pet owner has to fill out before the patient is seen. Here a few example questions with comments specifically related to pruritus:

     • What breed? Many different breeds are predisposed for atopy. e.g. Beagle, Terriers, Shar-Pei, etc.

     • Age? Demodex and dermatophytosis in young animals

     • Seasonality? Seasonal-atopy (plants, insects); non-seasonal: atopy (dust mites), food adverse reaction

     • Other animals in the household affected? If yes consider ectoparasites and dermatophytosis

     • Travel history? Boarding, contact with other animals. etc. If yes consider something contagious

     • Where did the itch and lesions started? Face/ears: Demodex, dermatophytosis, scabies; feet: atopy, food adverse reaction, Demodex, secondary pyoderma; tail base and hind legs: fleas

     • Previous treatments and did they work? Good response to steroids: atopy, flea bite hypersensitivity; got worse on steroids: Demodex, dermatophytosis. Good response to antibiotic: pyoderma

     • Is the dog on monthly flea prevention? If no, suspect fleas

     • Is the dog on flavored heartworm prevention? Be aware these are commonly beef flavored and can potentially interfere with a hypoallergenic diet

Diagnostic work-up

The primary differentials which should be ruled out are ectoparasites. Various diagnostic tools are available to collect and identify these parasites as follows:

     • Flea combing: collects not only fleas and flea feces. Lice and Cheyletiella can be found with this method

     • Paper towel test: helps to confirm the presence of flea dirt and if positive, fleas should be considered

     • Superficial skin scraping: Sarcoptes, Cheyletiella, chigger mite. Ruling out mites such as Sarcoptes is important in patients with suspected allergies, because these mites will react with dust mites on allergy testing and so resulting in a potentially false positive reaction

     • Deep skin scraping: Demodex

The next step is skin cytology

     • Helps to identify Malassezia, Staph. pseudointermedius commonly found in dogs as a secondary problem

     • identifies specific inflammatory cells especially in cats: high number of eosinophils indicate hypersensitivity or parasites

At this point, treat what you see if:

     • Infections and parasites are ruled out and the dog or cat is still pruritic, consider fungal culture, especially in cats to rule out dermatophytosis

     • Demodicosis and dermatophytosis are ruled out consider a short trial treatment with an anti-inflammatory dose of steroids (atopy is usually highly responsive to low concentrations of steroids)

     • Pruritus is strictly seasonal (e.g. every year from April to September) consider allergy testing

     • Pruritus is non-seasonal start a STRICT hypoallergenic diet trial

     • No response consider second strict hypoallergenic diet trial or allergy testing

Be aware that the patient may suffer from different skin problems at the same time which should be addressed at the same time. Once parasites, secondary bacterial and yeast skin infections, dermatophytosis, and food adverse reactions are ruled out (no or only partial decrease in pruritus a tentative diagnosis of atopic dermatitis can be made. The next step would be to perform an allergy skin test to confirm your tentative diagnosis and to identify the allergen(s) which may be relevant for the pats pruritic skin condition. Allergy tests are not considered screening tests and should only be used to confirm your preliminary diagnosis of atopic dermatitis.

Treatment

Resolving the pet's pruritus does not only entail the identification of the cause(s), but also the proper treatment of the disease(s). It is very important that all parasitic conditions and secondary skin infections are treated at the same time and treated until completely elimination. This requires that the pet is rechecked every 2-4 weeks and treatment continued until the repeated tests (e.g. cytology, skin scrapes) are negative. If the pruritus is still present or only partially improved than you should proceed to allergy work-up.

What follows is a short list of current and common treatment recommendations for the diseases listed above causing pruritus:

     • Fleas: many different flea preventatives (Frontline, Advantage, Advantix, Advantage Multi, Revolution, Promeris, Comfortis, Vectra 3D) are commercially available. It is important that these preventatives are applied once monthly. In heavily infested environment it may take several months until the flea population significantly declines. Flea shampoos or collars are not effective.

     • Pyoderma: most commonly caused by Staph. pseudointermedius. First choice antibiotics are: oral Simplicef (Cefpodoxime; Pfizer) or injectable Convenia (Cefovecin; Pfizer) 2-4 weeks. Cytology has to be negative and skin lesions resolved before therapy is stopped

     • Malassezia dermatitis: less severe cases can be treated topically with once to twice weekly baths with medicated shampoos (e.g. KetoChlor, Virbac). In more severe cases oral antifungal drugs such as ketoconazole or itraconazole can be used.

     • Demodicosis: weekly amitraz dips or daily oral ivermectin. Repeat skin scrapings once monthly and treat one month beyond 1-2 consecutive negative skin scrapes

     • Sarcoptes and Cheyletiella: weekly amitraz (Mitaban; Pfizer) or lime sulfur dips for 4 weeks, or biweekly application of selamectin (Revolution; Pfizer) or moxidectin (Advantage Multi; Bayer) for 3 treatments

     • Dermatophytosis: weekly lime sulfur dips or daily oral itraconazole. Fungal cultures must be repeated once monthly and therapy continued one month beyond negative fungal culture

     • Hypoallergenic diet trial: Various forms of diets are available. Most commonly used are commercially available prescription diets (novel protein diets, hydrolyzed protein diets) or home-cooked diets with selected ingredients (e.g. pork or venison & potato). Duration of the STRICT diet should be 8-12 weeks. Symptomatic therapy may be necessary initially, but must be discontinued at least 2 weeks before the diet trial is completed