Abstract

An otherwise healthy 93-year-old woman presented with fever and altered mental status, following one month of fatigue and poor appetite. On examination, at the anterolateral right hip and the right posterior shoulder were several mildly pruritic pink plaques with bullae. Initial differential diagnoses included bullous cellulitis, immunobullous disease, and allergic contact dermatitis. Upon further questioning, she recalled a recent tick bite, leading to concerns for an unusual presentation of a tick-borne illness. Serologies were positive for Borrelia burgdorferi IgG but not IgM, indicating past infection, and negative for Babesia microti. A peripheral blood smear showed cytoplasmic inclusions in leukocytes, but initial antibody testing was negative for both Ehrlichia chaffeensis and Anaplasma phagocytophilum. Biopsies were performed and showed a predominance of marginating and perivascular neutrophils, and the same cytoplasmic inclusions were seen within these neutrophils. The patient was preliminarily diagnosed with anaplasmosis, and she was treated with a 14-day course of doxycycline, leading to resolution of symptoms. Notably, repeat serologies two weeks later were positive for A. phagocytophilum IgM and IgG. Histologically, the morulae of A. phagocytophilum are endosomes, each essentially a microcolony filled with replicating bacteria, and these morulae are found in neutrophils, whereas E. chaffeensis infects monocytes and macrophages. The vector is the Ixodes scapularis tick, which can also transmit Lyme disease and babesiosis, so co-infections can occur. While Lyme disease is frequently accompanied by the classic erythema migrans rash, anaplasmosis rarely presents with an ill-defined rash, and the histologic features are not well-established. To our knowledge, we present the first case where morulae were visualized by H&E within dermal neutrophils in the setting of an atypical bullous rash of anaplasmosis.

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