Isolation and Molecular Analysis of a Novel Species That Causes Potomac Horse Fever.

Potomac horse fever (PHF), a severe and frequently fatal febrile diarrheal disease, has been known to be caused only by , an endosymbiont of digenean trematodes. Here, we report the cell culture isolation of a new species found in two locations in eastern Ontario, Canada, in 2016 and 2017 (in addition to 10 variable strains of ) from PCR-negative horses with clinical signs of PHF. Gene sequences of 16S rRNA and the major surface antigen P51 of this new species were distinct from those of all previously characterized strains and species, except for those from an uncharacterized species culture isolate from a horse with PHF in northern Ohio in 1991. The new species nonetheless had the characteristic intramolecular repeats within strain-specific antigen 3 (Ssa3), which were found in all sequenced Ssa3s of strains. Experimental inoculation of two naive ponies with the new species produced severe and subclinical PHF, respectively, and the bacteria were reisolated from both of them, fulfilling Koch's postulates. Serological assay titers against the new species were higher than those against Whole-genome sequence analysis of the new species revealed unique features of this bacterium compared with We propose to classify this new bacterium as sp. nov. This finding will improve the laboratory diagnosis of and vaccine for PHF, environmental risk assessment of PHF, and understanding of PHF pathogenesis and biology in general. Despite the detection of species DNA sequences in various trematode species and their hosts, only three species have been cell culture isolated and whole-genome sequenced and are known to infect mammals and/or cause disease. The molecular mechanisms that enable the obligatory intracellular bacterium to colonize trematodes and to horizontally transmit from trematodes to mammals, as well as the virulence factors associated with specific mammalian hosts, are unknown. Potomac horse fever (PHF) is a severe and acute systemic infectious disease of horses, with clinical signs that include diarrhea. is the only known bacterial species that causes PHF. Ingestion of insects harboring -infected trematodes by horses leads to PHF. Our discovery of a new species that causes PHF and whole-genome sequence analysis of this bacterium will improve laboratory diagnosis and vaccine development for PHF and will contribute to our understanding of ecology, pathogenesis, and biology.