Publications

Detection of Neorickettsia risticii in antemortem fecal and postmortem fetal samples, with genomic insights from complete genome sequencing of a strain recovered from an aborted equine fetus

Uprety T, Durazo J, Paul L, Metiner K, Ruby R, Loynachan A, Janes J, Kenndy L, Cassone L, Molly E, Quick M, Morgan J, Beyhan S, Erol E

PMID: 40913922

Abstract

Neorickettsia risticii (N. risticii) is an obligatory intracellular bacterium that causes Potomac horse fever (PHF), a disease clinically characterized by diarrhea, pyrexia, and laminitis in horses. Although sporadic reports of N. risticii infection have been linked to abortion in mares, a detailed retrospective study, including genomic analysis of the pathogen from an aborted fetus, has not been published. This study examined 546 fecal samples from clinically ill horses (January 1, 2017-December 31, 2024) and 833 colon samples from aborted equine fetuses (September 20, 2018-December 31, 2024). Using real-time PCR, N. risticii was detected in 11.5 % of fecal samples and 1.08 % of fetal colon samples. Positive antemortem cases were detected between May and September, while fetal cases occurred from September to December. The fetuses were 5-8 months of gestational age, all showing microscopic evidence of colitis. A shotgun metagenomic approach was applied directly to an archived fetal colon sample using the MiSeq platform, yielding a complete genome of N. risticii (strain KY18-EqFetus) with 99.72 % nucleotide identity to the N. risticii strain Illinois reference genome. The genome was 879,923 bp with a GC content of 41.3 %, and 2024 variants (including indels and nucleotide polymorphisms) were identified. Notably, three genes coding for small hypothetical proteins present in the reference strain were absent in KY18-EqFetus. This study provides a comprehensive investigation of N. risticii in both antemortem and postmortem equine samples and reports the first complete genome assembly of the pathogen directly from an aborted equine.