Historical and physical clues identified during client discussion can narrow the diagnosis (see Clinical Features of Pruritus: A Mental Chalkboard). Age and nature of onset can be helpful; although parasites or food allergy can occur at any age, environmental allergy often presents in young dogs and does not commonly begin in dogs older than 3 to 4 years of age. Evidence of contagion suggests parasite infestation or dermatophytosis (especially in cats). Concomitant GI signs may indicate food allergy.
Distribution is often (although not always) characteristic. Dorsal lumbosacral distribution suggests flea allergy dermatitis until proven otherwise. If distribution includes feet, face, ventrum, and/or pinnae (in any combination), environmental or food allergy should be considered. If ear margins, elbows, or ventrum are affected, scabies should be considered.
Basic in-house diagnostic procedures (eg, skin scrapings, skin cytology) can help rule out common and obvious causes. Fleas and Cheyletiella spp mites can be identified by brushing or combing the hair coat. Superficial and deep scrapings and hair pluckings are all required when looking for mites. Samples from lesions should be obtained by impression smear, scotch tape, or other collection methods to confirm staphylococcal or yeast infection on cytology.
Related Article: Sublingual Immunotherapy for Atopic Dermatitis