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.

Keywords: Babesia microti; Hemophagocytosis; Human babesiosis; Travel-related diseases.

Fig. 1

Peripheral blood smear photomicrographs depicting erythrocytes infected with Babesia spp. from both the patient and inoculated mice, complemented by molecular identification of the infecting parasite strain in the patient. A, B Initial presentation revealed typical ring-form trophozoites (a) and tetrameric (c) structures in the blood smear of the patient; C electrophoresis results displaying partial 18S rRNA target gene of Babesia spp. from human whole blood (Lanes 1–3). Lane 1, Lanes 2 and 3 show the amplification results on 12, 14 and 26 July, respectively. M means marker; D nested PCR product electrophoresis of covering the entire 18S rRNA gene from B. microti. Lane 1, Lanes 2 and 3 show the amplification results on 12, 14 and 26 July, respectively; E, F Giemsa-stained blood smears observed ring-form trophozoites (a) at day 3 post-inoculation and paired pyriform merozoites (b) at day 9; G Real-time fluorescence quantitative PCR depicting changes in B. microti load. Each sample was replicated three times, denoted by 1, 2, 3; H Comparative analysis of the 1667 base-pair nucleotide sequence alignment of the Babesia 18S rRNA target gene. The black icon indicates the sequence of this study. Phylogenetic analysis was performed using the neighbor-joining method and trees were tested by bootstrapping (1000 replicates)