Clinical, histopathological and immunological effects of exposure of canine skin to Malassezia pachydermatis.

The effects of the daily application for 7 days of suspensions of Malassezia pachydermatis to normal canine skin were evaluated in 10 beagle dogs. Four out of six dogs challenged without occlusion developed transient lesions generally characterized clinically by mild erythema with papules and histologically by mild epidermal hyperplasia and a superficial perivascular dermatitis. Saline-treated control sites showed no clinical signs. In four dogs challenged with occlusion, skin lesions occurred at both yeast and saline-treated sites; erythema and papules were more severe at the yeast-treated sites in three dogs. Occlusion induced more persistent lesions, which resolved within 24 days. Population densities of the yeast were highest at day 8 and declined rapidly following cessation of application. Peripheral blood mononuclear cell proliferation indices following M. pachydermatis exposure in vitro and serum concentrations of M. pachydermatis-specific IgG antibodies did not vary significantly during the study. Delayed (24 h) intradermal test reactivity to M. pachydermatis antigens developed in all eight dogs with clinical signs following yeast exposure. This study suggests that the resistance of healthy canine skin to infection by M. pachydermatis is mediated by local delayed hypersensitivity responses and, or innate epidermal immune mechanisms.