Itch is defined as an uncomfortable sensation on the skin that causes the desire to scratch. The desire to scratch, however, may be induced from a plethora of causes—from some seemingly innocuous environmental triggers, to even include infectious ones. Thus, a pragmatic and systematic approach to a pruritic canine patient with a final end result/diagnosis of canine atopic dermatitis (CAD) is very important in order that a correct diagnosis may be made consistently and the quality of life quickly restored to the patient. CAD is defined as a genetically predisposed, inflammatory and pruritic, allergic skin disease with characteristic clinical features, most commonly associated with IgE antibodies to environmental allergens.1
Since the first diagnostic criteria for CAD was published in 1986 by Dr. Willemse, who directly transposed from the human criteria as set forth by Hanifin and Rajka, it set in motion a collective effort by veterinary dermatologists, immunologists and researchers to further define the disease, refine its diagnosis and treatment whilst broadening our understanding of this complex, multifactorial disease.2,3 Approximately 10 years later, Dr. Prélaud and colleagues proposed a new set of five criteria, where fulfilment of three criteria yielded a sensitivity of 79% and a specificity of 81% for the diagnosis of CAD.4 Most recently in 2010, Favrot and colleagues proposed a new criteria where a fulfilment of five criteria from the first set had a sensitivity of 85% and a specificity of 79%,5 an improvement from the previous. This increase in sensitivity could be explained by the added exclusion criteria of affected ear margins (suspicious of sarcoptic mange) and also an affected dorsolumbar area (suspicious of flea allergic dermatitis). Thus setting the tone for our approach in a pruritic canine patient where the diagnostic end point is the diagnosis of CAD, which is a clinical diagnosis based on the exclusion of other pruritic disease.
The approach to a pruritic canine patient begins with the history and clinical signs of the patient. From the age of onset, seasonality, acute/chronic itch, presence of alesional pruritus to response to previous treatment/s are important, indispensable clues to aid identification of the aetiological pruritic agent. Puppies are more prone to develop contagious pruritic diseases such as sarcoptic mange, dermatophytosis or demodicosis. Patients with onset of pruritus that begins less than 12 months of age is highly suspicious of cutaneous adverse food reaction (CAFR), whereas CAD patients are typically presented between 12–36 months of age.5,6 Seasonal pruritus is often associated with flea allergic dermatitis or canine atopic dermatitis. A typical canine atopic patient is often presented with chronic alesional pruritus that responded previously to steroids.5 From clinical signs, sarcoptic mange infestation often manifests itself on the ear tip margins, whereas it is the dorsal lumbar area for flea allergic dermatitis. The typical anatomical sites that are involved in canine atopic dermatitis are the forepaws and also the concave ear pinnae.5 Needless to say, patients are often pruritic, manifested clinically as either scratching, licking or having excoriation marks at these respective sites. If there is evidence of a parasitic infestation that could cause itching, it should be addressed accordingly and patient re-evaluated for persistence of pruritus on a later date. The new class of insecticides—isoxazolines such as fluralaner, sarolaner, afoxolaner and lotilaner—offers a safe, efficacious and even sustained efficacy for up to 90 days in one product.7-10 Understanding the pharmacokinetics and pharmacodynamics of this or other classes of antiparasiticals would aid in its assimilation to the veterinarian’s approach to the pruritic patient, potentially making it more flexible and robust.
For every pruritic patient, it is important for the attending veterinarian to evaluate the presence of superficial staphylococcal pyoderma and likewise important to determine what portion of the overall itch is related to staphylococcal infection. This is the same for Malassezia dermatitis. This can be done rather easily with routine cytology, collected from representative regions with typical associated clinical signs. The attending veterinarian must remember that recurrent pyoderma or recurrent Malassezia dermatitis is often associated with an underlying allergic disease.11 Due to this high rate of recurrent pyoderma, it is often that patients are frequently exposed to antibiotics which adds selective pressure that culminates in antimicrobial resistance.12,13 Thus, a robust approach to a pruritic patient and antimicrobial stewardship is indispensable that goes hand-in-hand in modern-day practice. Antibiotics are certainly not candies we should freely dispense. After the likelihood of an infestation and/or infection has been ruled out, the possibility of a CAFR must be investigated. Despite its clinical signs that are virtually indistinguishable from canine atopic dermatitis, CAFR does have its own particularities. CAFR is often thought to occur in younger patients, less than one year of age with non-seasonal pruritus, mostly widespread and generalized.5,6 This pruritus is usually severe, constant with high variability in its amelioration of pruritus with oral or parenteral glucocorticoids, with results ranging from poor to good.5,14 The only effective method to distinguish CAFR and CAD is to conduct a food trial either with a home-cooked elimination diet with novel sources of protein and carbohydrate or a commercial hydrolysed diet for 9–10 weeks. It is only with the lack of response from a food trial that we can finally arrive at the diagnosis of CAD.
If a systematic approach to a pruritic patient has been conducted and CAD confirmed, it not only allows the attending veterinarian to appreciate the atopic patient’s itch clinical threshold but also allows the identification of flare factors associated with it. The itch clinical threshold is defined as the point where a pet begins to scratch due to a summation of events, where each contributes independently to the itch cascade. As pruritic thresholds are unique for each animal, a systematic approach allows attending veterinarians to build a clinical pruritic impression for the patients under his/her care. Flare factors are biological or environmental factors that induce the exacerbation of atopic dermatitis. Current recognized flare factors in CAD include staphylococcal or yeast infections, flea infestation, dietary indiscretion and also environmental triggers.15
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