Potomac Horse Fever (PHF)

Monday, August 29, 2022

NY: Neorickettisa risticii, the causative agent of Potomac Horse Fever (PHF), is a seasonal pathogen that causes fever and life-threatening colitis leading to laminitis in equids. PHF cases occur during the summer and early fall months in the United States when equids become exposed by consuming infected aquatic insects, including mayflies and caddisflies. The Cornell Animal Health Diagnostic Center (AHDC) noted a rise in PHF positive submissions from NY starting in July this year, with 17% (21/125) of PHF PCR requests testing positive this summer, as of August 22nd. This percentage of positive NY cases represents a rise in comparison to past years (Fig 1), with 14% (27/190), 9% (19/221) and 6% (12/267) testing positive in 2019, 2020 and 2021 respectively. PHF cases from 2019 to present have originated from counties throughout NY state, with a higher number of cases diagnosed in Albany, Dutchess, Saratoga, and Tompkins counties (Fig 2).

To diagnose PHF in equids, veterinarians are encouraged to submit EDTA whole blood or feces in a leak-proof sterile container for PHF PCR prior to initiating treatment with oxytetracycline. The PHF PCR at the AHDC detects both Neorickettsia risticii and the novel species Neorickettsia finleia recently described by Teymournejad et al in the northern United States and Canada,1 but this PCR does not differentiate between the two species. When PHF PCR is performed after treatment with oxytetracycline has been initiated, it may not detect neorickettsial DNA and produce a false negative result. In this circumstance, PHF IFA can be used to detect antibodies. Commercially available PHF vaccines may also induce detectable serum antibodies, so previous vaccine history must be considered when interpreting titers. For more information about PHF diagnostics in equids, please contact the Veterinary Support Services team at the AHDC (607) 253-3900 or email vetsupport@cornell.edu.

Potomac Horse Fever submissions
Figure 1. PHF positive submissions originating from NY state, received by the Cornell Animal Health Diagnostic Centers from 2019 to August 2022.

Potomac Horse Fever Map NYS
Figure 2. Distribution of PHF positive cases in NY state submitted to the Cornell Animal Health Diagnostic Center from 2019 to August 2022.

1. Teymournejad O, Lin M, Bekebrede H, Kamr A, Toribio RE, Arroyo LG, Baird JD, Rikihisa Y. Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever. mBio. 2020 Feb 25;11(1):e03429-19. doi: 10.1128/mBio.03429-19. Erratum in: mBio. 2020 Apr 21;11(2): PMID: 32098825; PMCID: PMC7042704.