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

Abstract

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 newspecies found in two locations in eastern Ontario, Canada, in 2016 and 2017 (in addition to 10 variable strains of) fromPCR-negative horses with clinical signs of PHF. Gene sequences of 16S rRNA and the major surface antigen P51 of this newspecies were distinct from those of all previously characterizedstrains andspecies, except for those from an uncharacterizedspecies culture isolate from a horse with PHF in northern Ohio in 1991. The newspecies nonetheless had the characteristic intramolecular repeats within strain-specific antigen 3 (Ssa3), which were found in all sequenced Ssa3s ofstrains. Experimental inoculation of two naive ponies with the newspecies produced severe and subclinical PHF, respectively, and the bacteria were reisolated from both of them, fulfilling Koch's postulates. Serological assay titers against the newspecies were higher than those againstWhole-genome sequence analysis of the newspecies revealed unique features of this bacterium compared withWe propose to classify this new bacterium assp. nov. This finding will improve the laboratory diagnosis of and vaccine for PHF, environmental risk assessment of PHF, and understanding of PHF pathogenesis andbiology in general.Despite the detection ofspecies DNA sequences in various trematode species and their hosts, only threespecies 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 bacteriumto 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 newspecies 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 ofecology, pathogenesis, and biology.