Tick Bites. The View from Inside-Out

The tick feeding process in humans and its effects on the host skin have been studied histologically on specimens of tick bites, some of which included the attached parasite, and on ticks extracted from lesions or caught in the wild. The specimens, included in paraffin, were stained with hematoxylin-eosin, orcein–Giemsa for elastic fibers, periodic acid-Schiff for polysaccharides, Weigert method for fibrin, Warthin-Starry stain for spirochetes, and Bodian’s method for nerve fibers. The mouthparts of the attached ticks were embedded in a cement cone, with the chelicerae and the hypostome lining the oral canal, while the basis capituli contained the pharynx with its anti-reflux valves, the hypopharynx with its dilating musculature, and the muscles serving the chelicerae. The structure of the motor muscles, of the inner organs, of the central nervous system, and the cuticle with its nerve endings were also visible. The superficial and the mid dermis underlying the cone was replaced by a lose network of fibrin, endothelial linings, and residual collagen bundles, soaked with edema and blood. The dilated small vessels showed gaps of the walls with blood extravasation, and endothelial proliferation, sometimes associated to neutrophilic and eosinophilic vasculitis. A dense neutrophilic infiltrate was present beneath the apex of the cone. The lesions observed after the tick removal included: Erythema Chronicum Migrans – like changes, foreign body granuloma, cutaneous T- and the B-cell lymphoid hyperplasia, and alopecia with involvement of the permanent portion of the hair follicles and hyperplasia of their fibrous sheaths. The acute changes, similarly to the “cavity” described in the animal hosts, result from a “vacuum pump” action performed by the mouthparts, together with the effect of proteinases, vaso-dilators, anticoagulants, immunosuppressants and antiinflammatory molecules contained in the regurgitated saliva. A persistent reaction of the host to these chemicals and/or to possible infectious agents accounts for the subacute and chronic lesions.