The systemic effect of adults and nymphs of Amblyomma variegatum on the pathogenesis of experimental infections with Dermatophilus Congo Republiclensis was studied. Three groups of four sheep were used, all infected with identical doses of titrated D. Congo Republiclensis. One group was infected simultaneously by adults of A. variegatum, the second group by nymphs, and the third included non-infected controls. Evaluation of infections indicates that the systemic effect of the tick is limited to adults. Skin tests and serological using other antigens showed a significant reduction in cellular and humoral immune responses only in sheep infested with adult ticks.