Babesia microti (Babesiidae, Piroplasmida) infection in a Chinese traveler returning from the United States of America.

Abstract

Background: Human babesiosis, caused by intraerythrocytic protozoa of the genus Babesia (Piroplasmida, Babesiidae), is a globally emerging zoonosis transmitted primarily through Ixodes spp. ticks. Babesia microti, which is endemic particularly in the northeastern and midwestern United States, accounts for the majority of globally reported human cases. Recent studies highlight its spread to non-traditional regions and cross-border transmission, driven by climate change, blood transfusions and increased human mobility. Despite increasing reports of autochthonous B. microti infections in certain areas of China, imported cases remain critically underrecognized due to overlapping clinical manifestations with malaria and limited diagnostic awareness.

Case presentation: We report a diagnostically challenging case of acute B. microti infection in a 52-year-old Chinese woman, presenting with a sudden recurrent fever (39.0-41.0 °C), hemolytic anemia (hemoglobin 104 g/L), thrombocytopenia (platelet 78 × 10⁹ /L) and splenic hypodense lesions on July 11, 2023, seven days after returning from a 14-day visit to rural Wisconsin, United States. Peripheral blood smears demonstrated characteristic intraerythrocytic ring forms (parasitemia: 7800 organisms/μl) and pathognomonic "Maltese cross" tetrads. Polymerase chain reaction (PCR) targeting the 18S rRNA gene confirmed B. microti infection (GenBank No. PP087232), showing 99.8% identity with the US-type strain Gray (AY693840) and the sequence obtained from a US travel-acquired case in Singapore (MK609547). The patient received intravenous clindamycin (600 mg twice daily), oral dihydroartemisinin (80 mg twice daily), packed red blood cell transfusions, and supportive care, ultimately achieving full recovery after 17 days.

Conclusions: This study documented the first imported cases of human babesiosis in China, emphasizing the need for heightened clinical and public health vigilance. Screening travelers from endemic areas presenting with fever or hemolytic anemia for Babesia, bolstering molecular diagnosis, improving transfusion safety, and intensifying regional surveillance are crucial in reducing underdiagnosis and preventing transmission. These measures are essential for controlling babesiosis in China.