The Overlooked Winter Presentation: A Case Series of Two Patients with Human Granulocytic Anaplasmosis

Background: Human granulocytic anaplasmosis (HGA) is a tick-borne zoonotic infection caused by Anaplasma phagocytophilum and transmitted by Ixodes species. In temperate regions, HGA is considered seasonal, with most cases occurring during late spring and summer. We describe two cases of HGA diagnosed in January during a winter period with episodic temperatures exceeding thresholds for tick activity, highlighting atypical seasonal presentation and diagnostic challenges. Methods: This report details the clinical course, diagnostic reasoning, and management of two patients evaluated at a tertiary care hospital in Suffolk County, New York. Data were derived from direct clinical care and the electronic health record. The institutional review board determined this work did not constitute human subject research. Written informed consent was obtained from both patients. Results: Both patients presented with acute febrile illness and characteristic laboratory abnormalities. Due to winter season, tick-borne infection was not initially suspected, resulting in delayed consideration. PCR testing confirmed A. phagocytophilum infection in Case 1, meeting CDC criteria for confirmed HGA. Case 2 met CDC criteria for probable HGA based on serologic testing showing elevated IgG (1:320) in the appropriate clinical context. Treatment with doxycycline led to rapid clinical improvement and complete recovery. Conclusions: These cases demonstrate that HGA can be diagnosed during winter months in endemic regions. Although the precise timing of infection cannot be determined, these observations occurred during a period when episodic temperatures exceeded thresholds for tick activity. The cases highlight limitations of season-based diagnostic assumptions and suggest maintaining clinical suspicion for anaplasmosis year-round in endemic areas.