Local reactions to tick bites.

A retrospective histological and immunohistochemical study has been carried out in 25 cases of tick bites recorded in our Departments. The samples that included an attached tick showed a cement cone anchoring the mouthparts to the skin and a blood-soaked, spongiform appearance of the superficial dermis, with a mild neutrophilic and eosinophilic infiltration. The vessels displayed a loose multilayered endothelial proliferation, with plump endothelia, permeated with erythrocytes. A few of them were severed, allowing copious blood extravasation. The established lesions included the following: erythema chronicum migrans-like cases, foreign body granulomas-sometimes containing remnants of the mouthparts-cutaneous lymphoid hyperplasia, either of the T-cell or the B-cell type, and tick-bite alopecia. In both the T-cell and B-cell pseudolymphomas, several vessels showed concentric endothelial and perithelial proliferation similar to that seen in the acute lesions. In the tick-bite alopecia, a lymphocytic infiltrate attacked the permanent portion of the hair follicles, whose reaction was a noticeable hyperplasia of the fibrous sheaths, although only a minority of the hairs was destroyed. The observed alterations are specific in the acute lesions and in the alopecia, where they directly arise as a result of the interactions between the host's tissues and the antihemostatic, antiinflammatory, and immunomodulatory chemicals contained in the tick saliva. In the other lesions, the changes seem less characteristic, although the fragments of mouthparts and the special vascular changes provide a clue to their etiology.