DIVERSITY OF 16S RIBOSOMAL-RNA GENES OF NEW EHRLICHIA STRAINS ISOLATED FROM HORSES WITH CLINICAL SIGNS OF POTOMAC HORSE FEVER

Ehrlichia risticii is the causative agent of Potomac horse fever, Vari ations among the major antigens of different local E. risticii strains have been detected previously. To further assess genetic variability in this species or species complex, the sequences of the 16S rRNA gene s of several isolates obtained from sick horses diagnosed as having Po tomac horse fever were determined. The sequences of six isolates obtai ned from Ohio and three isolates obtained from Kentucky were amplified by PCR, Three groups of sequences were identified. The sequences of f ive of the Ohio isolates were identical to the sequence of the type st rain of E. risticii, the Illinois strain. The sequence of one Ohio iso late, isolate 081, was unique; this sequence differed in 10 nucleotide s from the sequence of the type strain (level of similarity, 99.3%). T he sequences of the three Kentucky isolates were identical to each oth er, but differed by five bases from the sequence of the type strain (l evel of similarity, 99.6%), The levels of sequence similarity of isola te 081, the Kentucky isolates, and the type strain to the next most cl osely related Ehrlichia sp., Ehrlichia sennetsu, were 99.3, 99.2, and 99.2%, respectively, On the basis of the distinct antigenic profiles a nd the levels of 16S rRNA sequence divergence, isolate 081 is as diver gent from the type strain of E. risticii as E. sennetsu is. Therefore, we suggest that strain 081 and the Kentucky isolates may represent tw o new distinct Ehrlichia species.