[444] Histologic Features of Acute Ixodidae Tick Bites: Clues to Pathogenesis of Disease Transmission

MM Chan, L Zhao, SR Tahan. Beth Israel Deaconess Medical Center, Boston, MA, Venezuela; Harvard Medical School, Boston, MA

Background: It is well-known that tick bites, as those from the Iodidae tick, facilitate the transmission of clinically important diseases including Lyme disease, Rocky Mountain Spotted Fever and Bebesiosis. The ability of the tick to remain attached to the skin for days, however, has yet to be elucidated. By comparing the histologic features of acute and remote tick bites, we propose a mechanism by which the tick seemingly evades the host immune response during its prolonged feed.
Design: Skin specimens from fourteen patients who sustained tick bites were examined. Eight histologic sections showed retained tick mouth parts within the epidermis and were, thus, designated as acute tick bites. Remote tick bites lacked visible tick parts, however, patients reported a history of a tick bite greater than 2 months prior.
Results: Dramatic histologic differences were observed between skin biopsies from patients with acute versus remote tick bites. In all acute tick bite cases (n=8/8), a necrobiotic collagen core is observed surrounding the tick mouthpart that extends from the epidermis into the deep reticular dermis. The adjacent epidermis shows only mild reactive spongiotic changes. Interstitial neutrophils are seen scattered throughout the reticular dermis, yet are absent near and within the necrobiotic core. Extensive eosinophilic hyaline thrombi are seen in dermal and subcutaneous arterioles. In contrast, the skin sections of remote tick bites (n=6) show a prominent superficial to deep dermal and subcutaneous lymphocytic infiltrate with abundant admixed eosinophils suggesting a prolonged dermal hypersensitivity reaction. Two of these cases exhibited marked reactive lymphoid hyperplasia with germinal center formation. Necrobiotic collagen and eosinophilic hyaline thrombi were not seen in these remote tick bites.
Conclusions: This data represents findings from one of the largest reported series of acute tick bites to date. The tick appears to induce collagen necrobiosis which surrounds the tick mouth parts and prevents access by inflammatory cells. In addition, through induction of a localized coagulopathy, arteriole thrombi are formed that block leukocyte margination and chemotaxis. The pathologic mechanism of these changes is not understood, however the the tick appears to create an immune-protected environment capable of enhancing disease transmission.
Category: Dermatopathology

Tuesday, March 10, 2009 1:00 PM

Poster Session IV # 71, Tuesday Afternoon