Detection of Babesia bigemina-infected carriers by polymerase chain reaction amplification

A SpeI-AvaI fragment (0.3 kbp) from pBbi16 (a pBR322 derivative containing a 6.3-kbp Babesia bigemina DNA insert) was subcloned into the pBluescript phagemid vector and was sequenced by the dideoxy-mediated chain termination method. Two sets of primers were designed for the polymerase chain reaction (PCR) assay. Primer set IA-IB was used to amplify a 278-bp DNA fragment, and primer set IAN-IBN was used to prepare a probe directed to a site within the PCR-amplified target DNA. Digoxigenin-dUTP was incorporated into the probe during the amplification reaction. PCR amplification of target DNA obtained from in vitro-cultured B. bigemina and nucleic acid hybridization of amplified product with the nonradioactive DNA probe showed that a 278-bp fragment could be detected when as little as 100 fg of parasite genomic DNA was used in the assay. A fragment of similar size was amplified from genomic DNAs from several B. bigemina isolates but not from DNAs from Babesia bovis, Anaplasma marginale, or six species of bacteria or bovine leukocytes. Similarly, the PCR product could be detected in DNA samples purified from 200 microliters of blood with a parasitemia of as low as 1 in 10(8) cells and which contained an estimated 30 B. bigemina-infected erythrocytes. By a direct PCR method, B. bigemina DNA was amplified from 20 microliters of packed erythrocytes with a calculated parasitemia of 1 in 10(9) cells. With the analytical sensitivity level of the PCR-DNA probe assay, six cattle with inapparent, 11-month chronic B. bigemina infection were found to be positive. No PCR product was observed in bovine blood samples collected from a splenectomized, A. marginale-infected bovine, a 4-year chronic B. bovis-infected animal, or 20 uninfected cattle from Missouri which were subjected to amplification. The PCR-DNA probe assay was shown to be sensitive in detecting latently infected cattle. The specificity and high analytical sensitivity of the test provide valuable tools for performing large-scale epidemiological studies.