New Ehrlichia species closely related to Ehrlichia chaffeensis isolated from Ixodes ovatus ticks in Japan

Seven Ehrlichia strains (six HF strains and one Anan strain) that were obta ined from laboratory mice by intraperitoneally inoculating homogenates of a dult Ixodes ol ovatus collected in Japan mere characterized. 16S rRNA seque nces of all six HF strains were identical, and the sequences were 99.7, 98. 2, and 97.7% identical to those of Anan strain, Ehrlichia chaffeensis (huma n monocytic ehrlichiosis agent), and E. muris, respectively. Partial GroEL amino acid sequencing also revealed that the six HF strains had identical s equences, which were 99.0, 98.5, and 97.3% identical to those of E. chaffee nsis, the Anan strain, and E. canis, respectively. All HF strains were leth al to mice at higher dosages and intraperitoneal inoculation, whereas the A nan or E. muris strain induced only mild clinical signs. Light and electron microscopy of moribund mice inoculated with one of the HF strains revealed severe liver necrosis and the presence of numerous ehrlichial inclusions ( morulae) in various organs. The study revealed that members off. canis geno group are naturally present in Ixodes ticks. HF strains that can cause seve re illness in immunocompetent laboratory mice would be valuable in studying the pathogenesis and the roles of both cellular and humoral immune respons es in ehrlichiosis caused by E, canis genogroup.