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.
Authors: Tirth Uprety, Jacob Durazo, Litty Paul, Kemal Metiner, Rebecca Ruby, Alan Loynachan, J. Janes, Laura Kenndy, Lynne Cassone, Evely Molly, Maria Quick, Jocelynn Morgan, S. Beyhan, E. Erol
Journal: Veterinary microbiology
Summary
# Editorial Summary: *Neorickettsia risticii* Detection in Horses and Genomic Characterisation of an Aborted Fetus Strain Potomac horse fever, caused by the obligate intracellular bacterium *Neorickettsia risticii*, is primarily known for inducing diarrhoea, fever, and laminitis; however, links to abortion in pregnant mares remain poorly characterised in the peer-reviewed literature. Over an eight-year period, researchers screened 546 faecal samples from acutely ill horses and 833 colon samples from aborted fetuses using real-time PCR, detecting *N. risticii* in 11.5% and 1.08% of samples respectively, with clinical cases clustered between May–September and fetal cases from September–December, exclusively affecting fetuses aged 5–8 months of gestation. Complete genome sequencing of the recovered strain (KY18-EqFetus) from an archived fetal colon sample revealed 99.72% nucleotide identity to the reference Illinois strain, though notably absent were three genes encoding small hypothetical proteins, alongside 2,024 documented variants including insertions and deletions. For practitioners managing breeding mares, these findings establish *N. risticii* as a credible abortifacient during mid-to-late gestation and suggest that seasonal surveillance of diarrhoeic cases may provide an epidemiological indicator of abortion risk within a population. The genomic data furnish a foundation for future research into pathogenic mechanisms specific to fetal infection and potential strain-specific virulence factors.
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Practical Takeaways
- •N. risticii should be considered as a differential diagnosis in mares presenting with abortion, particularly in fetuses showing colitis; seasonal monitoring (May-December) may help identify at-risk pregnancies
- •Fecal PCR testing can detect N. risticii in clinically affected horses during spring-summer months, enabling earlier diagnosis and potential management of pregnant mares exposed to infected animals
- •Genomic characterization shows this pathogen has strain variation; understanding these differences may eventually help predict virulence and abortion risk in future cases
Key Findings
- •N. risticii detected in 11.5% of fecal samples from clinically ill horses and 1.08% of fetal colon samples from aborted fetuses
- •Clinical cases occurred May-September while fetal cases occurred September-December, suggesting seasonal transmission patterns
- •Complete genome of N. risticii strain KY18-EqFetus sequenced directly from aborted fetus with 99.72% nucleotide identity to reference strain but missing three genes coding for hypothetical proteins
- •All aborted fetuses were 5-8 months gestation with microscopic colitis as consistent pathological finding