Pruritus can be a complicating factor or major feature of diseases, including flea infestations (FI), flea bite hypersensitivity (FBH), and canine atopic dermatitis (CAD); in many cases, pruritus may be the initial presenting clinical finding. Records from 346 dogs (91 CAD, 110 FI, 145 FBH) were analyzed determine if unique characteristics of the pruritus pattern were helpful in diagnosis. Pruritus was a sensitive diagnostic finding for dogs with CAD (84%) and FBH (81%) but not flea infestations (61%). In addition to exhibiting behaviors more frequently associated with pruritus (eg, chewing, licking, rubbing), dogs with CAD and FBH were more intensely pruritic than dogs with FI. The most common locations of the pruritus varied among FI (ventral abdomen and/or medial thigh, radius and/or carpus and/or tarsus), FBH (back, dorsolumbosacral area, tail), and CAD (face and/or neck, lateral thorax and/or axillary areas, paws).

Commentary
Several flow charts are available for working up patients with pruritus but, in reality, diagnostic suggestions stem from history and signs. Flow charts are guides, not mandates. This information emphasized that there are patterns of pruritus in dogs with no apparent skin lesions (or evidence of fleas). From a practical perspective, scabies is worth mentioning; the severity of the pruritus from scabies far exceeds that of FI, FBH, or CAD; only combined yeast (severe) and bacterial infections and the rare case of advanced epitheliotropic
lymphoma come close to this degree of pruritus.

Scabies is traditionally described as a disease with ventral distribution, but by the time it is seen, the lesions are usually generalized. Other differentiating factors include a purely papular eruption and a dog that is intensely pruritic to the point of ignoring everything else. Using the study, it is relatively easy to prioritize the differential diagnoses of dogs with apparent pruritus as the sole presenting complaint.—Karen A. Moriello, DVM, DACVD

Source
Characterization of pruritus in canine atopic dermatitis, flea bite hypersensitivity and flea infestation and its role in diagnosis. Bruet V, Bourdeau PJ, Roussel A, et al. VET DERMATOL 23:487-E93, 2012.