(NY) Potomac Horse Fever (PHF; Neorickettsia risticii) was diagnosed in a sixteen-year-old Thoroughbred mare via PCR performed on an EDTA whole blood sample. The mare presented with a fever of 102.6 F, lethargy, and severe diarrhea. PCR is the optimal test to diagnose an acute infection of PHF. Serum can also be submitted for PHF Indirect Fluorescent Antibody (IFA) testing. Because the incubation period is 10-18 days, acutely ill animals often have an elevated PHF IFA titer. Vaccinated animals may have a titer in a similar range. Paired acute and convalescent titers will help sort out response to infection versus a vaccine or previous exposure. N. risticii is an intracellular bacterium found in snails and aquatic insects. Horses accidentally ingest the emerged adult aquatic insects and become infected. Other ancillary tests were negative in this mare, and she was responding to IV oxytetracycline and clinically normal as of 6/21/18. This mare has received the PHF vaccine annually. It's a good reminder to keep PHF on your differential list regardless of vaccine status. Per the American Association of Equine Practitioners (AAEP) vaccination guidelines: "Vaccination against this disease has been questioned because field evidence of benefit is lacking. Proposed explanations for this include lack of seroconversion and multiple field strains whereas only one strain is present in available vaccines."