Comparison of the Babesia duncani (WA1) IgG detection rates among clinical sera submitted to a reference laboratory for WA1 IgG testing and blood donor specimens from diverse geographic areas of the United States.

All reported cases of WA1 babesiosis have occurred in the Pacific coast region of the United States, suggesting that WA1 is limited to this geographic area. However, we detected WA1 IgG in 27% of clinical sera sent to our laboratory for WA1 IgG testing from across the United States over a 2-year period, suggesting that exposure to WA1 or a closely related organism occurs outside Pacific coast states. We sought to determine if this high WA1 IgG detection rate among clinical specimens merely reflects WA1 seroprevalence outside the Pacific region. WA1 IgG, as well as Babesia microti IgG, was measured in 900 blood donor specimens from 9 states. Overall seroprevalence was 2.0% for WA1 and 0.4% for B. microti; regional seroprevalences ranged from 0 to 4% and 0 to 2%, respectively. Additional studies were performed to determine if WA1 IgG reactivity was attributable to polyclonal B-cell activation associated with acute Epstein-Barr virus (EBV) infection; 40 WA1 IgG-positive clinical sera and the 18 WA1 IgG-positive blood donor specimens were all negative for EBV capsid antigen (EBVCA) IgM (a marker of acute EBV infection), and 40 EBVCA IgM-positive sera were all negative for WA1 IgG. These findings indicate that the high WA1 IgG detection rate among clinical specimens does not simply reflect the national WA1 seroprevalence among blood donors or nonspecific reactivity due to acute EBV infection. Rather, the findings suggest that infection with WA1 or a related organism is more common than indicated by the literature and is not limited to Pacific coast states.