New reviews widen the surveillance lens on Potomac horse fever

A pair of recent reviews is pushing Potomac horse fever back into the disease-surveillance conversation, not because the syndrome is new, but because its map may be. In Veterinary Research Communications, ThankGod Emmanuel Onyiche and Tan Li Peng systematically reviewed the global distribution of Neorickettsia risticii, while a Veterinary Microbiology review by Luis G. Arroyo, Alexandre S. Borges, and John D. Baird framed equine neorickettsiosis as a broader ecological problem shaped by trematodes, snails, aquatic insects, and regional habitats. Read together, the papers suggest the pathogen’s recognized range is wider, and more complicated, than many clinicians were taught. (pmc.ncbi.nlm.nih.gov)

That matters because Potomac horse fever has long carried a geographically narrow identity. AAEP says the disease was first described near the Potomac River in the eastern U.S., but has since been identified in multiple locations in the U.S. and Canada. More recent guidance now lists both N. risticii and N. findlayensis as causes of Potomac horse fever, reflecting how the taxonomy and pathogen picture have evolved. (aaep.org)

The newer literature also adds to the international context. The Veterinary Microbiology review describes clinical endemicity in multiple regions of the U.S. and Canada, along with parts of South America, including Uruguay and Brazil. Primary studies support that broader view: a Scientific Reports paper documented molecular detection of N. risticii in horses from Rio de Janeiro, and a 2026 PubMed-indexed report identified the organism in horses from southern Rio Grande do Sul, Brazil. A broader enterocolitis review likewise notes clinical disease along the Brazil-Uruguay border and molecular detection in Brazil. (pmc.ncbi.nlm.nih.gov)

At the same time, the reviews land in a field where distribution data have been messy. Merck Veterinary Manual cautions that many historical reports in the U.S. and Canada relied on indirect fluorescent antibody testing, and that false-positive titers mean the true geographic range remains uncertain. Merck says isolation or PCR-confirmed clinical cases have been reported only from a more limited set of U.S. states. That diagnostic caveat is one of the most useful contributions of the new reviews: they don’t just expand the map, they highlight how much of the map depends on the method used to draw it. (merckvetmanual.com)

Expert and industry guidance is broadly aligned on the clinical implications. AAEP’s 2025 Potomac horse fever guideline describes the disease as non-contagious and typically seasonal, with cases usually occurring in summer and fall, though timing can vary with weather. Clinical signs can include high fever, diarrhea, lethargy, laminitis, colic, edema, and abortion after transplacental transmission. AAEP and Merck both emphasize that ecology matters because Neorickettsia organisms are linked to digenean trematodes and aquatic hosts, helping explain why risk can appear focal, seasonal, and hard to predict from political boundaries alone. (aaep.org)

Why it matters: For equine veterinarians, the practical message is to think less in terms of “is this a classic PHF state?” and more in terms of syndromic fit, season, and local water-associated ecology. In horses with acute fever, colitis, laminitis, or abortion risk, especially during warmer months, a low index of suspicion based on geography alone may miss cases. The literature also supports leaning on PCR and other direct detection methods rather than overinterpreting serology, particularly when discussing disease risk with pet parents, planning herd prevention, or evaluating whether a practice’s vaccination recommendations still match local exposure patterns. AAEP notes the current vaccine is an aid in prevention, while Merck points to strain heterogeneity as one reason protection can be imperfect. (merckvetmanual.com)

There’s also a broader surveillance lesson here. Potomac horse fever is not simply a horse-to-horse infectious disease problem; it’s a habitat-linked pathogen system. That makes case detection dependent on veterinary awareness, diagnostic access, and ecological investigation. As more studies use PCR, sequencing, and whole-genome analysis, the field is likely to get a clearer picture of where N. risticii and related Neorickettsia species are circulating, and whether apparent expansion reflects true emergence, better testing, or both. A 2025 PubMed-indexed study on fetal and fecal detection, plus earlier work identifying N. findlayensis as another PHF agent, suggest that species differentiation and genomic surveillance will become more relevant to practice and epidemiology. (pubmed.ncbi.nlm.nih.gov)

What to watch: The next phase will likely center on better molecular surveillance in undercharacterized regions, clearer separation of exposure from clinical disease, and updated regional guidance on diagnosis, vaccination, and prevention as the epidemiology of equine neorickettsiosis comes into sharper focus. (merckvetmanual.com)