Systematic review broadens the map for Potomac horse fever agent

Potomac horse fever’s map may be wider than many clinicians think, according to a new systematic review of the global distribution of Neorickettsia risticii, the bacterium long linked to equine neorickettsiosis. The review, published in Veterinary Research Communications, pulls together scattered reports from multiple continents and adds to a broader shift already underway in the literature: Potomac horse fever is no longer viewed as a narrowly regional problem confined to the eastern U.S. Companion work in Veterinary Microbiology also frames equine neorickettsiosis as a global disease ecology issue, noting confirmed endemic clinical cases in parts of the U.S., Canada, Uruguay, and Brazil, and emphasizing the complex trematode-snail-aquatic insect life cycle that shapes where exposure occurs. (sciencedirect.com)

Why it matters: For veterinary professionals, the review reinforces two practical points: geography alone is an unreliable rule-out, and Potomac horse fever surveillance still has blind spots. Current guidance from the AAEP says PHF is caused by Neorickettsia risticii and N. findlayensis, occurs seasonally but variably, and can present with fever, diarrhea, laminitis, and abortion. The Merck Veterinary Manual also notes that the true geographic range remains uncertain, in part because older serology-based reports likely overstated exposure, while definitive confirmation depends on PCR or culture from clinical cases. That means equine practitioners may need to keep PHF on the differential beyond classic endemic areas, especially in horses with acute febrile enterocolitis during insect-heavy months or near aquatic habitats. (aaep.org)

What to watch: Expect more attention to molecular surveillance, species-level differentiation between N. risticii and N. findlayensis, and region-specific risk mapping as researchers try to define where clinically relevant transmission is actually occurring. (pubmed.ncbi.nlm.nih.gov)