Experimental bovine Trichophyton verrucosum infection. The cellular responses in primary lesions of the skin resulting from surface or intradermal inoculation.

The sequential perivascular cell response occurring during stages of primary experimental infection of the keratinized portions of bovine skin with T. verrucosum differed from that resulting from intradermal inoculation with viable units of the same agent. Early infection of keratinized structures was first accompanied by infiltration with mononuclear cells, and some polymorphonuclear leucocytes. Progressive injection of the majority of hair follicles was accompanied by early inflammatory and allergic changes that resembled those seen in cutaneous chemical contact sensitivity. These coincided with the systemic development of delayed hypersensitivity to products of the fungus. Marked histological changes were observed in the local lymph node draining the site of infection that indicated a mixed response involving both cellular and humoral immunological mechanisms. Maximum serous and cellular exudation from blood vessels into the dermis and epidermis was associated with mast cell degranulation and was preceded by a systemic leucocytosis with neutrophilia. Marked hyper- and parakeratosis of the stratified squamous epithelium and the breakdown of follicular microabscesses was accompanied by massive accumulations of pyroninophilic cells in the region of capillaries and lymphatics. Large numbers of eosinophils were often seen in these areas during resolution of lesions. Allergic and inflammatory changes were most rapid and were well demarcated in older cattle. The cellular response to intradermally inoculated T. verrucosum involved a marked polymorphonuclear reaction, with necrosis and rupture of the epidermis over the inoculated area. Intense macrophage activation resulted in the destruction of fungal elements without marked lympho-proliferative changes and in healing with fibrosis. Invasion of keratinized portions of the skin by T. verrucosum did not occur.