There is more than one specie of Neorickettsia causing Potomac Horse Fever

Improved methods for the genetic analysis of organisms is unveiling much of why, what we thought were the same diseases, behave differently in different cases: it may not be the same organism. Already it was understood that there were different subspecies of the organism that causes Potomac Horse Fever and may be responsible for the different severities experienced in different cases. But here we learn there are several species of Neorickettsia causing PHF symptoms. Worst is that PCR, the best diagnostic technique at this time for PHF, does not identify this Neorickettsia organism as PHF. A negative PCR test may cause some to suspend the best treatment for this infection the tetracycline class drugs. On the positive side this finding partially explains the poor response to vaccination to PHF seen in horses. This should help us make better vaccines for the future. The take home message if it looks like PHF, if it smells like PHF, and if it feels PHF, treat it like PHF even if PCR says it is not. See the article for more on this as we have just updated this article.
DrO

2020 Feb 25;11(1).
Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever.
Teymournejad O1, Lin M1, Bekebrede H1, Kamr A2, Toribio RE2, Arroyo LG3, Baird JD3, Rikihisa Y4.

Author information:
1. Laboratory of Molecular, Cellular, and Environmental Rickettsiology, Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA.
2. Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA.
3. Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada.
4. Laboratory of Molecular, Cellular, and Environmental Rickettsiology, Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA Rikihisa.1@osu.edu.

Abstract
Potomac horse fever (PHF), a severe and frequently fatal febrile diarrheal disease, has been known to be caused only by Neorickettsia risticii, an endosymbiont of digenean trematodes. Here, we report the cell culture isolation of a new Neorickettsia species found in two locations in eastern Ontario, Canada, in 2016 and 2017 (in addition to 10 variable strains of N. risticii) from N. risticii PCR-negative horses with clinical signs of PHF. Gene sequences of 16S rRNA and the major surface antigen P51 of this new Neorickettsia species were distinct from those of all previously characterized N. risticii strains and Neorickettsia species, except for those from an uncharacterized Neorickettsia species culture isolate from a horse with PHF in northern Ohio in 1991. The new Neorickettsia species nonetheless had the characteristic intramolecular repeats within strain-specific antigen 3 (Ssa3), which were found in all sequenced Ssa3s of N. risticii strains. Experimental inoculation of two naive ponies with the new Neorickettsia 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 Neorickettsia species were higher than those against N. risticii Whole-genome sequence analysis of the new Neorickettsia species revealed unique features of this bacterium compared with N. risticii We propose to classify this new bacterium as Neorickettsia finleia 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 Neorickettsia biology in general.

IMPORTANCE
Despite the detection of Neorickettsia species DNA sequences in various trematode species and their hosts, only three Neorickettsia 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 Neorickettsia 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. Neorickettsia risticii is the only known bacterial species that causes PHF. Ingestion of insects harboring N. risticii-infected trematodes by horses leads to PHF. Our discovery of a new Neorickettsia 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 Neorickettsia ecology, pathogenesis, and biology.