Flea hypersensitivity dermatitis (HD) usually presents with one of the following clinical reaction patterns: head and neck excoriations, symmetric self-induced alopecia, miliary dermatitis (MD), and/or eosinophilic dermatitis. However, once flea HD has been ruled out, differentiation of non–flea-induced hypersensitivity dermatitis (NFHD) is difficult because of overlapping clinical signs.

In this study, historical and clinical data from 501 cats were mathematically analyzed to look for differentiating criteria. Findings that increased the likelihood of NFHD included presence of more than 1 affected body site and multiple concurrent reaction patterns, such as symmetric self-induced alopecia, eosinophilic dermatitis, and MD. Presence of symmetric alopecia or head and neck erosions alone was strongly associated with NFHD. Of note, MD (particularly if not found on the dorsum) was a valid criterion for NFHD. Once fleas were excluded as a cause, the following findings were strongly associated with NFHD: symmetric alopecia localized on the abdomen or a dominant pattern of eosinophilic lesions. Criteria differentiating food allergy HD from NFHD were not found.

Commentary
This is one of several research papers attempting to clearly define populations of cats with hypersensitivity. In a previous study, the most common cause of feline pruritus (39%) was skin parasites (fleas or mites),1 which is strong evidence to treat for fleas and other parasites before investing in diagnostic testing. In this study, once flea hypersensitivity was eliminated, food or non-flea, non-food HD was the most likely cause. Unfortunately, the mathematical models could not identify any differences between food allergy HD and NFHD. Of interest was the finding on MD. In most dermatology textbooks, flea allergy is considered a major cause of MD; however, in this study MD not found on the dorsum was strongly associated with food allergy or non-flea, non-food HD (ie, atopic dermatitis).—Karen Moriello, DVM, DACVD

Source
Establishment of diagnostic criteria for feline nonflea-induced hypersensitivity dermatitis. Favrot C, Steffan G, Seewald W, et al. VET DERMATOL 23:45-e11, 2012.

1. Clinical characteristics and causes of pruritus in cats: A multicentre study on feline hypersensitivity-associated dermatoses. Hobi S, Linek M, Marignac G, et al. Vet Dermatol 22:406-413, 2011.