Detection of the agent of heartwater, Cowdria ruminantium, in Amblyomma ticks by PCR: Validation and application of the assay to field ticks

We have previously reported that the pCS20 PCR detection assay for Cowdria ruminantium, the causative agent of heartwater disease of ruminants, is mor e sensitive than xenodiagnosis and the pCS20 DNA probe for the detection of infection in the vector Amblyomma ticks. Here, we further assessed the rel iability of the PCR assay and applied it to field ticks. The assay detected DNA of 37 isolates of C. ruminantium originating from sites throughout the distribution of heartwater and had a specificity of 98% when infected tick s were processed concurrently with uninfected ticks. The assay did not dete ct DNA of Ehrlichia chaffeensis, which is closely related to C, ruminantium . PCR sensitivity varied with tick infection intensity and was high (97 to 88%) with ticks bearing 10(7) to 10(4) organisms but dropped to 61 and 28%, respectively, with ticks bearing 10(3) and 10(2) organisms. The assay also detected C, ruminantium in collections of Amblyomma hebraeum and Amblyomma variegatum field ticks from 17 heartwater-endemic sites in four southern A frican countries. Attempts at tick transmission of infection to small rumin ants failed with four of these collections. The pCS20 PCR assay is presentl y the most characterized and reliable test for C. ruminantium in ticks and thus is highly useful for field and laboratory epidemiological investigatio ns of heartwater.