Genetic variants of Ehrlichia canis detected in two dogs in Japan

Ehrlichia canis, the causative agent of canine monocytic ehrlichiosis, infects monocytes and macrophages in dogs and is transmitted mainly by hard ticks Rhipicephalus sanguineus sensu lato. Although E. canis has been reported worldwide, autochthonous E. canis infection in Japan was not confirmed before 2020. In this study, molecular characterization of E. canis detected in two dogs in Tokyo in 2020 and 2023 was conducted. DNA extracted from spleen and blood samples was subjected to polymerase chain reaction (PCR) targeting the 16S ribosomal RNA (rRNA), groEL (heat shock protein), p28 (major outer membrane protein), and trp36 (tandem repeat protein 36) genes, followed by sequencing and phylogenetic analyses. Both cases shared identical 16S rRNA and groEL sequences with many E. canis strains, whereas sequence diversity was evident for p28 and trp36 genes. Interestingly, E. canis in one of the cases harbored a novel 26-amino-acid tandem repeat sequence in trp36, representing a previously unreported trp36 tandem-repeat variant Japan. Phylogenetic analyses placed the E. canis sequences from the two cases into different clusters, suggesting the presence of genetically distinct variants in Japan. This study provides evidence of genetic diversity among E. canis detected in dogs in Japan and suggests the possibility of local transmission or previously unrecognized introduction of this pathogen.