A 16S RIBOSOMAL-RNA-BASED PCR ASSAY FOR DETECTION AND IDENTIFICATION OF GRANULOCYTIC EHRLICHIA SPECIES IN DOGS, HORSES, AND CATTLE

A PCR-based assay was developed for detecting DNA of granulocytic ehrl ichiae in blood samples front dogs, horses, and cattle, Primers were d esigned from 16S rRNA sequence information to specifically amplify DNA from a newly identified Swedish Ehrlichia species, The 16S rRNA nucle otide sequence of this Swedish species differs in only two and three p ositions from the sequences of Ehrlichia phagocytophila and Ehrlichia equi, respectively, which were also amplified by this PCR system, For evaluation, PCR results were compared with microscopic examination of stained blood smears for the detection of granulocytes containing ehrl ichiae (morulae). Thirty-four of 36 microscopically positive samples w ere also positive by PCR, and 6 microscopically negative samples were negative by PCR as well, Six samples, in which morulae-like structures had been seen, were negative by PCR, also at a lower annealing temper ature and when a reamplification of the first PCR products was perform ed, The identities of the PCR products from some canine and equine iso lates were verified by direct DNA sequencing and were found to be iden tical with the Ehrlichia sequence found in these animal species that h ad been obtained earlier, The sequences of a segment of approximately 600 nucleotides from two bovine isolates were identical to that of E, phagocytophila, whereas the sequence of another bovine isolate differe d in two positions from that of E. phagocytophila and in three positio ns from the sequences of the canine and equine isolates, Serum samples were analyzed by indirect fluorescent-antibody testing. Seventy-three percent of the animals which were positive by microscopy and PCR also had positive antibody titers, However, it was not possible to rely on a single serological result for diagnosis of present infection. It wa s, therefore, concluded that PCR was the most reliable method, useful in the clinical laboratory for specific and early diagnosis of granulo cytic ehrlichiosis in animals.