Experiments on the relationship between feeding of the tick Amblyomma variegatum (Acari: Ixodidae) and dermatophilosis skin disease in sheep.

Experiments on the systemic effect of feeding adult Amblyomma variegatum (F.) on dermatophilosis caused by Dermatophilus congolenis in six sheep were performed by feeding ticks at sites separate from sites of experimental infection with D. congolensis. Comparisons were made with D. congolensis infections on six control sheep without ticks. Tick-infested sheep developed chronic dermatophilosis lesions; tick-free sheep healed rapidly. On two tick-infested and two control sheep a chronological sequence of D. congolensis infection sites were made for histological comparisons. Dermatophilosis lesions on tick-infested sheep were characterized by chronic cellular infiltration and accumulation of plasma cells. Lesions on control sheep healed in association with transient cellular infiltration including plasma cells. In neither test nor control sheep was there evidence of effective phagocytosis of D. congolensis. It was concluded that feeding of these ticks had a systemic effect on the host such that dermatophilosis was aggravated via an effect on immunological reactions in the skin. Control of dermatophilosis by tick control is advocated.