Intradermal skin test reactivity to histamine and substance P is blunted in dogs with atopic dermatitis

Skin reactivity to intradermal injections (0.1, 0.5 and 1 nM) of substance P (SP) was evaluated in 20 clinically normal dogs and 20 dogs with atopic dermatitis (AD). Saline and histamine were used as negative and positive controls, respectively. Wheal diameters were measured. Reactions were evaluated for erythema and induration and a subjective score, on a scale from 0 to 4+, was given. Evaluations were performed at 3, 5, 10, 15 and 30 min after the injections. Wheal diameters for histamine and SP injections were significantly smaller in dogs with AD compared with clinically normal dogs. In both groups, reactions to the various concentrations of SP were not significantly different from each other and were always smaller than histamine reactions. Erythema was not seen with SP injections. In addition, subjective scores for SP injections were significantly lower in dogs with AD compared with controls. The results of this study are similar to those reported in human medicine, where a role for SP in AD is proposed and desensitization of receptors to both SP and histamine is hypothesized. Further studies are needed to investigate the role of SP in the pathogenesis of canine AD.