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The pruritic patient (Proceedings)

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Apr 01, 2010

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.