| Summary: | The relationship between the feeding of <i>Amblyomma variegatum</i> ticks and <i>Dermatophilus congolensis</i> infections has been studied under laboratory conditions. The local effects of hypersensitive or inflammatory reactions to larval and nymphal <i>A.variegatum</i> on subsequent <i>D.congolensis</i> infections were investigated using rabbits and sheep respectively. Multiple or single infestations of ticks were used to produce hypersensitivity or inflammatory reactions respectively. These reactions were confirmed by histological assessment of the tick attachment sites. Identical titrated doses of <i>D.congolensis</i> were applied to the tick attachment sites after the ticks had detached, a control titration was set up on skin with no previous exposure to ticks. The progression of the resulting lesions was assessed using a non-parametric ranking system. There was no significant difference (P > 0.05) between the severity or duration of the three groups of dermatophilosis lesions, either on the sheep or the rabbits. Therefore it was concluded that the local effects of the feeding of immature instars of this tick do not affect the pathogenesis of subsequent <i>D.congolensis</i> infections. The local effect of hypersensitive or inflammatory reactions to <i>A.variegatum</i> nymphs on simultaneous <i>D.congolensis</i> infections on rabbits was also studied. There was an increase in the initial severity of the dermatophilosis lesions and a positive correlation between inflammatory tick attachment sites and dermatophilosis foci. However, the local effects of the feed of nymphal <i>A.variegatum</i> did not result in the development of chronic dermatophilosis lesions. The systemic effect of adult and nymphal <i>A.variegatum</i> on simultaneous dermatophilosis lesions was compared, using sheep as the experimental hosts. <i>Dermatophilosis congolensis</i> infections on sheep infested with adult <i>A.variegatum</i> developed into chronic lesions which persisted for several months. Serological and skin tests revealed significantly (P < 0.01) reduced humoral and cellular immune responses in sheep infested with adult <i>A.variegatum</i> compared with sheep infested with nymphs or control sheep not exposed to ticks.
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