A novel Ehrlichia genotype detected in naturally infected cattle in North America

Abstract

During a research investigation to determine if cattle from British Columbia (BC), Canada were infected with Anaplasma marginale or other related rickettsial blood parasites, a novel Ehrlichia genotype was revealed. Blood from seven BC source cattle was bioassayed by intravenous inoculation into naïve splenectomised calves. Additional splenectomised calves were used as uninoculated negative control or A. marginale-inoculated positive control. Newly designed sets of primers specific for the msp5 gene of A. marginale or for the 16S rRNA gene were used to test blood samples collected from all source cattle and from all recipient calves prior to inoculation and up to 72 days post-inoculation. Results of the PCR assays as well as microscopic examination of stained blood smears failed to demonstrate A. marginale in any of the animals except for the positive control. The 16S rRNA PCR primers amplified DNA from samples from all BC source cattle, five of six of the corresponding recipient calves, and the A. marginale infected control animal. DNA sequence data indicated the presence of A. marginale only in the positive control calf. Blast analysis in GenBank showed that sequences of all other 16S rRNA PCR products clearly fit within the Ehrlichia genus in the Anaplasmataceae family which also includes members of the genus Anaplasma. Phylogenetic analyses using the 16S rRNA gene sequences strongly support the putative Ehrlichia organism as a distinct genotype with sequences of various strains of Ehrlichia canis as the closest clade. Ehrlichia ruminantium is the only other species of the genus known to naturally infect cattle, apart from the present Ehrlichia isolate. However, within the genus, E. ruminantium is phylogenetically the furthest removed species from the novel genotype. The finding of this novel Ehrlichia represents the first known natural ehrlichial infection in cattle in North America. Nevertheless, it is unclear whether cattle are an incidental or primary host, particularly since deer are recognized as reservoir hosts for other species of Ehrlichia. Although other Ehrlichia spp. are known to be pathogenic for animals and zoonotic, these features are presently unknown for this novel genotype.