BmGPAC: an antigen capture assay for detection of active infection.

Human babesiosis is an emerging zoonotic infectious disease caused by intraerythrocytic protozoan parasites of the genus Babesia. Most cases of human babesiosis are caused by Babesia microti and often manifest in individuals over the age of 50 years or in patients with a compromised immune system. Patients who develop symptomatic B. microti infections usually experience months of asymptomatic infection after the acute infection has resolved. About one-fifth of B. microti-infected adults never develop symptoms. These asymptomatically infected individuals sometimes donate blood and thus can transmit B. microti through blood transfusion. Current assays for detection of active B. microti infections can be used to screen donor blood prior to transfusion, but they rely primarily on microscopy or PCR methods, which have sensitivity and technical limitations. Here we report the development of an antigen capture enzyme-linked immunosorbent assay (BmGPAC) based on a major secreted immunodominant antigen of B. microti (BmGPI12/BmSA1), and we provide evidence that this assay is superior for detection of active B. microti infections, compared to available microscopy methods and serological assays. The assay has been evaluated using supernatants of B. microti-infected erythrocytes cultured in vitro, sera from B. microti-infected laboratory mice, and sera from wild mice and human patients. Our data suggest that the BmGPAC assay is a reliable assay for detection of active B. microti infections and is superior to real-time PCR and antibody assays for diagnosis of acute B. microti infections, screening of the blood supply, and epidemiological surveys of humans and animal reservoir hosts.