Type-I cryoglobulinemia-like histopathologic changes in tick bites: a useful clue for tissue diagnosis in the absence of tick parts.

BACKGROUND: The histopathologic findings of localized reactions to tick bites may present as diagnostic dilemmas, especially if there is no history of a tick bite, or if the tick's mouthparts are not present in the biopsied skin.
OBJECTIVE: Skin biopsies of patients with a clinical history of a tick bite were selected and reviewed with the aim of detecting a common histopathologic denominator which could serve as a useful clue to the diagnosis, especially when the tick's mouthparts are absent.
METHODS: Hematoxylin and eosin-stained slides of 15 skin biopsies of tick bites were retrieved from three dermatopathology and pathology laboratories. Where additional paraffin-embedded tissue was available, additional sections were also stained with periodic acid-Schiff (PAS) and phosphotungstic acid-hematoxylin (PTAH).
RESULTS: In every case in which adequate tissue was available (13/ 15 biopsies), the capillaries and postcapillary venules of the superficial and deep vascular plexi adjacent to the attachment's site were filled with thrombi. Fibrin thrombi were seen in association with other more numerous thrombi characterized by homogeneous eosinophilic hyaline material similar to the cryoprecipitate present in type I (monoclonal) cryoglobulinemia. All thrombi were positive for PAS and PTAH; however, the latter staining was minimally present in the hyaline thrombi. In most cases, the site of the tick bite showed ulceration, with an underlying wedge-shaped superficial and deep perivascular and occasionally interstitial mixed lymphohistiocytic infiltrate. In addition, there were eosinophils, numerous neutrophils and extravasated erythrocytes. Other findings included suppurative necrosis (7/15) cases, giant-cell reaction (one case), fat necrosis (one case) and eccrine gland necrosis (one case).
CONCLUSIONS: Vascular eosinophilic hyaline thrombi were found to be a frequent histologic manifestation of a tick bite. This finding may be related to the secretory products of the tick's saliva during inoculation. We believe that a tick bite should be suspected when focal intravascular hyaline occlusion is observed, and that it should be included in the differential diagnosis of type I (monoclonal) cryoglobulinemia, even if there is no history of a tick bite or if tick parts are not present in the skin biopsy specimen.