The relationship between the feeding of Amblyomma variegatum ticks and the skin disease dermatophilosis

The relationship between the feeding of Amblyomma variegatum ticks and Dermatophilus congolensis infections has been studied under laboratory conditions.

The local effects of hypersensitive or inflammatory reactions to larval and nymphal A.variegatum on subsequent D.congolensis 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 D. congolensis 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 D.congolensis infections.

The local effect of hypersensitive or inflammatory reactions to A.variegatum nymphs on simultaneous D.congolensis 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 feeding of nymphal A.variegatum did not result in the development of chronic dermatophilosis lesions.

The systemic effect of adult and nymphal A.variegatum on simultaneous dermatophilosis lesions was compared, using sheep as the experimental hosts. Dermatophilus congolensis infections on sheep infested with adult A.variegatum 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 A.variegatum compared with sheep infested with nymphs or control sheep not exposed to ticks.

Comparative studies of whole salivary glands from all three instars of A.variegatum revealed significantly greater (P < 0.01) proportions of type-2 acini filled with cl secretory granules in the salivary glands from the adult ticks compared with the salivary glands from the immature ticks. There were also significantly (P < 0.05) greater proportions of individual type-3 acini filled with e secretory granules in the salivary glands from the adult ticks compared with the salivary glands from the larval ticks. Gel electrophoresis revealed an 11% dissimilarity between salivary glands from adult A.variegatum compared with both of the immature instars. Similar analysis of artificially induced saliva revealed a 48% dissimilarity between the adult and immature ticks.