An imported case of human granulocytic anaplasmosis presenting with altered mental status: A case report.

Human granulocytic anaplasmosis (HGA) is a tick-borne disease endemic to the northeastern United States but remains extremely rare in Japan. We report an imported case of HGA in a 77-year-old man who had recently traveled from the United States. He presented with a 5-day history of fever, lethargy, and loss of appetite, followed by altered consciousness. Laboratory findings revealed thrombocytopenia, elevated transaminases, and increased creatine kinase levels. Computed tomography was unremarkable except for mild splenomegaly. Considering his recent stay in Bemidji, Minnesota-a region with a high risk of tick exposure-empirical doxycycline therapy was promptly initiated for suspected tick-borne illness. Although indirect immunofluorescence antibody (IFA) assays for IgM and IgG against Anaplasma phagocytophilum were negative at admission, polymerase chain reaction (PCR) testing of whole blood targeting the groEL gene yielded a positive result. Sequence analysis revealed complete (100%) identity of the amplified 16S rRNA, groEL, and p44 fragments with reference A. phagocytophilum sequences (CP000235). These findings confirmed the diagnosis of HGA. The patient responded rapidly to doxycycline therapy and was discharged on hospital day 5. This case highlights the diagnostic value of molecular methods during the early phase of infection, when serology may remain negative, and underscores the importance of considering travel and tick exposure history when evaluating febrile patients.