Cellular responses in experimental chronic and acute dermatophilosis infections of sheep

The cellular infiltrate into the dermis in dermatophilosis lesions is composed of a range of cell types. The aim of this study was to establish if the composition of the cellular infiltrate in chronic lesions was different from that in healing lesions. Experimental Dermatophilus congolensis infections of sheep were used to study the sequential changes in cell types during the course of chronic and acute infections. Infestations of adult Amblyomma variegatum ticks were used to produce chronic lesions on infected sheep, infections of tick-free sheep provided acute lesions. Histopathology and immunohistochemistry were used to stain and label cell types in the dermis of infection sites. Neutrophils dominated the early response and were present in larger numbers in chronic lesions. Plasma cells were present in both types of lesion, however they persisted in chronic lesions but disappeared from the skin at acute lesion sites after the lesions had resolved. There were 2-3 times as many mononuclear cells in chronic than acute lesions from as early as 4 days post infection and these cells persisted in the chronic lesions. In the chronic lesions the mononuclear cell population was composed of T-helper and T-cytotoxic/suppressor lymphocytes in equal proportions whereas in acute lesions at 14 days post infection, when lesion resolution is underway, there were greater numbers of T-helper cells than T-cytotoxic/suppressor cells.