Melophagus Ovinus (Diptera: Hippoboscidae): Evidence of Local Mediation in Acquired Resistance of Sheep to Keds

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Abstract
The skin of 4 lambs was exposed to the feeding of Melophagus ovinus within areas restricted by cloth bags glued to the skin. Sequentially, when restricted areas became resistant to keds, bags were started on new areas on each sheep. As each area became resistant, it was tested for resistance by allowing 10 keds to attempt feeding for 1 h; this was compared to a control, previously unexposed, area. All previously resistant areas were retested when a new area became resistant. Punch biopsies of skin were taken from every area tested; these were processed and read for histopathology and quantitative assessment of eosinophils and mast cells. As judged by feeding success in test areas of skin, development of the resistant state in 1 area did not produce resistance anywhere else. Once acquired, the resistance in any area was gradually lost over ensuing weeks of nonexposure to keds. For each animal, the number of days taken for a new area to become resistant decreased with each sequential exposure. Numbers of eosinophils infiltrating the skin were related both to the onset and to the degree of resistance. It was concluded that acquired resistance was locally mediated.