Genetic identification of rickettsiae isolated from ticks in Japan

Abstract

Following the description in Japan of Japanese spotted fever, caused by Rickettsia japonica, a search for the vector of this disease led to the isolation of several rickettsiae from various tick species. Sixty-three rickettsial isolates were obtained from six different tick species, and six type strains were described by PCR and monoclonal antibody testing. We identified these six strains by amplification and sequencing of the genes encoding 16S rRNA and citrate synthase. We confirmed that the isolates from Dermacentor taiwanensis and Haemaphysalis flava ticks were R. japonica isolates. In Ixodes ovatus, Ixodes persulcatus, and Ixodes monospinosus, we identified a Rickettsia identical or closely related to Rickettsia helvetica, a species that is pathogenic for humans and that to date has only been found in Europe. Finally, we identified a new genotype of unknown pathogenicity, genotype AT, that was isolated from Amblyomma testudinarium ticks and that is closely related to a Slovakian genotype obtained from Ixodes ricinus ticks.

FIG. 1.

Geographical distributions in Japan of the ticks from which the rickettsial isolates studied were isolated. (a) I. persulcatus; (b) I. monospinosus; (c) I. ovatus; (d) H. flava; (e) D. taiwanensis; (f) A. testudinarium. Green dots indicate the distributions of the ticks, and the red dots indicate the area where the ticks harboring the rickettsiae were collected.

FIG. 2.

Citrate synthase gene-based phylogenetic tree showing the positions among the rickettsiae of the six Japanese strains described in this study. The tree was constructed by the parsimony method. Bootstrap values are indicated at the nodes.