Large Animal Internal Medicine Resident New Bolton Center West Grove, Pennsylvania, United States
Abstract:
Background: Potomac horse fever (PHF) is a common cause of infectious colitis in North American horses. Neorickettsia findlayensis, a newly recognized cause of PHF, is not detected by some diagnostic laboratory PHF PCR assays. This diagnostic gap suggests the prevalence and geographic distribution of N. findlayensis may be underestimated.
Hypothesis/
Objectives: To expand the described geographic distribution of N. findlayensis in the United States.
Animals: PHF-positive horse samples were identified by retrospective analysis of 3,945 PHF PCR sample submissions to the New York State Animal Health Diagnostic Center (AHDC) from 3,160 horses between 2023-2025.
Methods: Retrospective diagnostic study. Submission data and PHF PCR results were reviewed. Non-equine submissions and inconclusive results were excluded. The AHDC PHF assay detected both N. findlayensis and N. risticii but did not differentiate between them. Therefore, all banked PHF-positive samples were subsequently tested with a multiplex PCR to differentiate species.
Results: A total of 279 PHF-positive samples (7.1% of tests), representing 218 positive cases, were identified. Of these cases, 68 (31%) were N. findlayensis-positive, 151 (69%) were N. risticii-positive, and one was co-infected. Neorickettsia findlayensis was detected in 15 of 28 states that submitted PHF-positive samples: Arizona, Colorado, Connecticut, Idaho, Massachusetts, Michigan, Minnesota, North Carolina, North Dakota, New York, Pennsylvania, Texas, Vermont, Wisconsin, and Wyoming. Classic PHF clinical signs of fever, diarrhea, and laminitis were reported with infections of both Neorickettsia species.
Conclusions and Clinical Importance: N. findlayensis is more geographically widespread than previously recognized. Clinicians should verify whether their preferred laboratory can detect this organism.
