Detection of Babesia bovis carrier cattle by using polymerase chain reaction amplification of parasite DNA.

Carrier cattle infected with Babesia bovis are difficult to detect because of the low numbers of parasites that occur in peripheral blood. However, diagnosis of low-level infections with the parasite is important for evaluating the efficacies of vaccines and in transmission and epidemiological studies. We used the polymerase chain reaction (PCR) to amplify a portion of the apocytochrome b gene from the parasite and tested the ability of this method to detect carrier cattle. The target sequence is associated with a 7.4-kb DNA element in undigested B. bovis genomic DNA (as shown previously), and the amplified product was detected by Southern and dot blot hybridization. The assay was specific for B. bovis, since no amplification was detected with Babesia bigemina, Trypanosoma brucei, Anaplasma marginale, or leukocyte DNA. The target sequence was amplified in DNA from B. bovis Mexico, Texas, and Australia S and L strains, demonstrating the applicability of the method to strains from different geographic regions. The sensitivity of the method ranged from 1 to 10 infected erythrocytes extracted from 0.5 ml of blood. This sensitivity was about 1,000 times greater than that from the use of unamplified parasite DNA. By the PCR method, six B. bovis carrier cattle were detected 86% of the time (range, 66 to 100%) when they were tested 11 times, while with microscopic examination of thick blood smears, the same carrier cattle were detected only 36% of the time (range, 17 to 66%). The method provides a useful diagnostic tool for detecting B. bovis carrier cattle, and the sensitivity is significantly improved over that of current methods. The results also suggest that characteristics of the apocytchrome b gene may make this a valuable target DNA for PCR-based detection of other hemoparasites.