Reviews suggest a broader global footprint for Potomac horse fever

A pair of recent reviews is reframing how veterinarians should think about Potomac horse fever. The new systematic review in Veterinary Research Communications synthesizes the global literature on Neorickettsia risticii, while a companion review in Veterinary Microbiology takes a broader look at equine neorickettsiosis and the organism’s natural habitat. Together, they suggest the disease’s ecology and distribution are wider than its historical branding implies, with evidence spanning North America and parts of South America rather than a narrow Potomac River footprint. (sciencedirect.com)

That broader framing has been building for years. PHF was first recognized in the eastern U.S., but AAEP now describes it as a non-contagious equine disease caused by Neorickettsia risticii and N. findlayensis, and notes that cases have become associated with endemic areas beyond the original Mid-Atlantic cluster. Earlier work also documented a distinct Canadian ecotype of N. risticii, and Ontario investigators later reported confirmed PHF cases involving both N. risticii and N. findlayensis. In other words, the field has been moving away from a one-pathogen, one-region view for some time; these new reviews consolidate that shift. (aaep.org)

The ecological story is central to that change. Rather than spreading horse-to-horse, Neorickettsia organisms are maintained within digenean trematodes that cycle through multiple hosts. AAEP guidance and prior literature link PHF risk to freshwater environments because infected trematodes involve aquatic snails and insects, and horses are thought to become infected after ingesting infected aquatic insects or trematode stages. That biology helps explain the disease’s seasonal pattern, usually in summer and fall, and why recognized case distribution may track environmental conditions and local host ecology more than state or national borders. (aaep.org)

The newer literature also adds practical diagnostic detail. A 2025 study published in Veterinary Microbiology reported N. risticii detection in 11.5% of fecal samples from clinically ill horses submitted between 2017 and 2024, and in 1.08% of fetal colon samples from equine abortions submitted between 2018 and 2024. Another recent paper described real-time PCR methods that differentiate N. findlayensis from N. risticii in PHF cases. Those findings matter because PHF doesn’t always read like a textbook case, and species-level differentiation may become more relevant as surveillance expands and as clinicians encounter disease outside long-recognized endemic pockets. (pubmed.ncbi.nlm.nih.gov)

Direct outside commentary on the new reviews was limited in publicly accessible sources, but the broader industry message is consistent. AAEP’s disease guidance emphasizes variable clinical presentation, including high fever, diarrhea, laminitis, colic, edema, and abortion, and notes that all ages and breeds in endemic areas are susceptible. AAEP’s vaccination guidance also cautions that vaccination may not be fully protective, citing possible strain variation among field isolates. That fits with the reviews’ broader implication that geographic spread and organism diversity may be underappreciated in practice. (aaep.org)

Why it matters: For veterinary professionals, especially equine practitioners and diagnosticians, these reviews are less about a brand-new pathogen than about updating the mental map. If PHF risk is tied to aquatic ecology and multiple Neorickettsia species, then case recognition, client education, and prevention discussions may need to be more locally tailored than historically assumed. Horses with fever, enterocolitis, laminitis, or abortion histories in freshwater-associated environments may warrant PHF consideration even outside the disease’s traditional geographic shorthand. The reviews also support continued investment in PCR-based diagnostics and in surveillance that connects clinical cases with environmental reservoirs. (aaep.org)

There’s also a communication challenge for practices. “Potomac” can still imply a narrow regional disease to pet parents and even some horse professionals, but the evidence base now points to a broader North American and South American footprint, with ongoing work clarifying how often N. risticii versus N. findlayensis is involved in different regions. That may affect how veterinarians discuss seasonal risk, insect exposure, water-adjacent pasture management, and the limits of vaccination in endemic settings. (aaep.org)

What to watch: The next phase will likely focus on finer-scale geographic mapping, better understanding of trematode and aquatic insect hosts, and wider adoption of assays that distinguish N. risticii from N. findlayensis. If that work advances, the field could move from broad awareness of PHF risk to more precise regional surveillance and prevention strategies. (pubmed.ncbi.nlm.nih.gov)