A Set of Diagnostic Tests for Detection of Active Babesia duncani Infection

Human babesiosis is a rapidly emerging and potentially fatal tick-borne disease caused by intraerythrocytic apicomplexan parasites of the Babesia genus. Among the various species of Babesia that infect humans, B. duncani has been found to cause severe and life-threatening infections. Detection of active B. duncani infection is critical for accurate diagnosis and effective management of the disease. While molecular assays for the detection of B. duncani infection in blood are available, a reliable strategy to detect biomarkers of active infection has not yet been developed. Here, we report the development of the first B. duncani antigen capture assays that rely on the detection of two B. duncani -exported immunodominant antigens, BdV234 and BdV38. The assays were validated using blood samples from cultured parasites in human erythrocytes and B. duncani -infected laboratory mice at different parasitemia levels and following therapy. The assays display high specificity with no cross-reactivity with B. microti , B. divergens , Babesia MO1, or P. falciparum. The assay also demonstrates high sensitivity, detecting as low as 115 infected erythrocytes/µl of blood. Screening of 1,731 blood samples from diverse biorepositories, including previously identified Lyme and/or B. microti positive human samples and new specimens from field mice, showed no evidence of B. duncani infection in these samples. The assays could be useful in diverse diagnostic scenarios, including point-of-care testing for early B. duncani infection detection in patients, field tests for screening reservoir hosts, and high-throughput screening such as blood collected for transfusion.

Short summary

We developed two ELISA-based assays, BdACA38 and BdACA234, for detecting B. duncani , a potentially fatal tick-borne parasite causing human babesiosis. The assays target two immunodominant antigens, BdV234 and BdV38, demonstrating high specificity (no cross-reactivity with other Babesia species or Plasmodium falciparum ) and sensitivity (detecting as low as 115 infected erythrocytes/µl). The assays were validated using in vitro-cultured parasites and infected mice. Screening diverse blood samples showed no evidence of B. duncani active infection among 1,731 human and field mice blood samples collected from the north-eastern, midwestern, and western US. These assays offer potential in diverse diagnostic scenarios, including early patient detection, reservoir animal screening, and transfusion-transmitted babesiosis prevention.

Nested qPCR assay to detect Babesia duncani infection in hamsters and humans

Human babesiosis is caused by Babesia duncani that is transmitted through tick bites, blood transfusions, and transplacental transmission. Despite its health burden, diagnostic assays for this pathogen are either unsuitable for clinical applications or have a low detection efficiency; therefore, it remains undetected during transfusion and utilization of blood and blood-component transfusions. This study used a molecular approach via nested quantitative polymerase chain reaction (qPCR) by designing primers and probes corresponding to the variable regions of B. duncani 18S rRNA gene to specifically detect B. duncani DNA in experimentally infected LVG Golden Syrian hamster (n = 70) and human (n = 492; tick bite patients from Gansu Province, China) blood samples. Moreover, comparative analyses of this technique with previously reported nested PCR and microscopy were conducted. The newly optimized diagnostic technique exhibited no cross-reactivity with genomic DNA or plasmids containing the 18S rRNA gene of other zoonotically important Babesia spp., including B. microti, B. divergens, B. crassa, and B. motasi Hebei. The detection limit of nested qPCR was approximately one plasmid copy in 20 μL or one infected red blood cell in 200 μL whole blood. The specificity and sensitivity of the method were 100% and 98.6%, respectively. Comparative analyses revealed that nested qPCR detected B. duncani had relatively higher efficacy and specificity than microscopic examination and nested PCR. The 492 human blood samples were negative for B. duncani infection. Thus, the present study provides an improved diagnostic assay for the efficient and effective detection and analysis of B. duncani infections and its prevalence in infection-prone areas.

Establishment of a Transient and Stable Transfection System for Babesia duncani Using a Homologous Recombination Strategy

Genetic modification provides an invaluable molecular tool to dissect the biology and pathogenesis of pathogens. However, no report is available about the genetic modification of Babesia duncani, a pathogen responsible for human babesiosis that is widespread in North America, suggesting the necessity to develop a genetic manipulation method to improve the strategies for studying and understanding the biology of protozoan pathogens. The establishment of a genetic modification method requires promoters, selectable markers, and reporter genes. Here, the double-copy gene elongation factor-1α (ef-1α) and its promoters were amplified by conventional PCR and confirmed by sequencing. We established a transient transfection system by using the ef-1αB promoter and the reporter gene mCherry and achieved stable transfection through homologous recombination to integrate the selection marker hDHFR-eGFP into the parasite genome. The potential of this genetic modification method was tested by knocking out the thioredoxin peroxidase-1 (TPX-1) gene, and under the drug pressure of 5 nM WR99210, 96.3% of the parasites were observed to express green fluorescence protein (eGFP) by flow cytometry at day 7 post-transfection. Additionally, the clone line of the TPX-1 knockout parasite was successfully obtained by the limiting dilution method. This study provided a transfection method for B. duncani, which may facilitate gene function research and vaccine development of B. duncani.

Development of a Multiplex Bead Assay To Detect Immunoglobulin G Antibodies to Babesia duncani in Human Serum

Babesia duncani is the causative agent of babesiosis in the western United States. The indirect fluorescent antibody (IFA) assay is the diagnostic test of choice for detection of B. duncani-specific antibodies. However, this test requires parasitized red blood cells harvested from infected hamsters, and test results are often difficult to interpret. To simplify serological testing for B. duncani, a proteomics approach was employed to identify candidate immunodiagnostic antigens. Several proteins were identified by electrospray ionization mass spectrometric analysis, and four recombinant protein constructs were expressed and used in a multiplex bead assay (MBA) to detect B. duncani-specific antibodies. Two antigens, AAY83295.1 and AAY83296.1, performed well with high sensitivities and specificities. AAY83295.1 had a higher sensitivity (100%) but lower specificity (89%) than AAY83296.1, which had a sensitivity of 90% and a specificity of 96%. Combining these two antigens did not improve the performance of the assay. This MBA could be useful for diagnosis, serosurveillance, and blood donor screening for B. duncani infection.

Combined Immunofluorescence (IFA) and Fluorescence In Situ Hybridization (FISH) Assays for Diagnosing Babesiosis in Patients from the USA, Europe and Australia

Apicomplexan parasites of the genus Babesia cause babesiosis in humans and animals worldwide. Human babesiosis is a predominantly zoonotic disease transmitted by hard ticks that is of increasing health concern in the USA and many other countries. Microscopic examination of stained blood smears, detection of serum antibodies by immunoassays and identification of parasite nucleic acid in blood by qPCR and fluorescence in situ hybridization (FISH) are some methods available for diagnosing babesiosis. This study investigated the use of a Babesia genus-specific FISH test for detecting Babesia parasites in blood smears and immunofluorescence assay (IFA) for detecting serum antibodies to Babesia duncani and Babesia microti, two common species that cause human babesiosis in the USA. The findings with clinical samples originating from USA, Australia, Europe and elsewhere demonstrate that the parallel use of Babesia genus-specific FISH and IFA tests for B. duncani and B. microti provides more useful diagnostic information in babesiosis and that B. duncani infections are more widespread globally than presently recognized.

In vitro cultivation of Babesia duncani (Apicomplexa: Babesiidae), a zoonotic hemoprotozoan, using infected blood from Syrian hamsters (Mesocricetus auratus)

Human babesiosis, a tick-borne disease similar to malaria, is most often caused by the hemoprotozoans Babesia divergens in Europe, and Babesia microti and Babesia duncani in North America. Babesia microti is the best documented and causes more cases of human babesiosis annually than all other agents combined. Although the agents that cause human babesiosis are considered high-risk pathogens in transfusion medicine, federally licensed diagnostics are lacking for B. duncani in both the USA and Canada. Thus, there has been a need to develop and validate diagnostics specifically for this pathogen. In this study, B. duncani (WA1 isolate) was cultivated in vitro from Syrian hamster (Mesocricetus auratus) infected blood. We hypothesized HL-1 media with supplements would result in B. duncani propagating at higher levels in culture than supplemented M199 similar to the medium the parasite was originally cultivated with in 1994. We were unable to recreate Thomford's cultivation results with the M199 medium but supplemented HL-1 medium was able to successfully establish continuous culture. We further hypothesized that RBC from species other than hamsters would support B. duncani in vitro. However, rat, mouse, horse, and cow RBC did not support continuous culture of the parasite. Culture stocks of B. duncani were deposited at BEI Resources and are now commercially available to the scientific community to further research. The cultured parasite developed in this study was instrumental in the adaptation of B. duncani continuous culture to human RBC.

Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus

Babesiosis is a potentially fatal tick-borne zoonotic disease caused by a species complex of blood parasites that can infect a variety of vertebrates, particularly dogs, cattle, and humans. In the United States, human babesiosis is caused by two distinct parasites, Babesia microti and Babesia duncani. The enzootic cycle of B. microti, endemic in the northeastern and upper midwestern regions, has been well characterised. In the western United States, however, the natural reservoir host and tick vector have not been identified for B. duncani, greatly impeding efforts to understand and manage this zoonotic disease. Two and a half decades after B. duncani was first described in a human patient in Washington State, USA, we provide evidence that the enzootic tick vector is the winter tick, Dermacentor albipictus, and the reservoir host is likely the mule deer, Odocoileus hemionus. The broad, overlapping ranges of these two species covers a large portion of far-western North America, and is consistent with confirmed cases of B. duncani in the far-western United States.