Systematic review broadens the map for Potomac horse fever risk

A new systematic review is putting firmer boundaries around a disease that has often been discussed as if it were mainly a Mid-Atlantic or Great Lakes problem. In Veterinary Research Communications, researchers synthesized the published record on the global distribution of Neorickettsia risticii, the main bacterium associated with Potomac horse fever, while a companion 2026 review in Veterinary Microbiology makes the case that equine neorickettsiosis is better understood as a globally relevant ecological disease complex involving multiple hosts, trematodes, aquatic insects, and at least two equine pathogenic Neorickettsia species. (deepdyve.com)

That framing matters because PHF has steadily outgrown its original label. AAEP notes the disease was first recognized near the Potomac River, but current guidance describes it as a non-contagious infectious disease seen in endemic areas more broadly, typically in summer and fall, though cases can occur in other seasons depending on weather conditions. The newer Veterinary Microbiology review says clinical disease is considered endemic in multiple regions of the United States and Canada, as well as parts of Brazil and Uruguay. (aaep.org)

The ecology behind that spread remains central. According to the 2026 review, Neorickettsia species are endosymbionts of digenean trematodes with complex life cycles involving snails as first intermediate hosts, a range of second intermediate hosts, and vertebrate definitive hosts. Horses are aberrant hosts, typically infected after ingesting aquatic insects carrying infected trematodes. That helps explain why disease patterns often cluster around freshwater environments and why simple farm-to-farm contagion models don't fit PHF well. Merck's current guidance similarly ties vaccination decisions and risk assessment to endemic areas near freshwater streams, rivers, ponds, and irrigated pasture. (sciencedirect.com)

The pathogen story is also getting more nuanced. AAEP's current disease guideline lists both N. risticii and N. findlayensis as PHF agents. A 2022 PubMed-indexed diagnostic study described real-time PCR methods that can differentiate N. findlayensis from N. risticii, and an Ontario case series found both species among cultured isolates from PHF cases diagnosed between 2015 and 2019. In other words, surveillance based only on the historical N. risticii label may now miss meaningful epidemiologic detail. (aaep.org)

Recent and older field literature also supports the idea that the organism's habitat extends beyond where equine cases are routinely recognized. Merck notes confirmed detection from clinical PHF cases in a range of U.S. states, including California, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, Ohio, Oregon, Pennsylvania, Texas, and Virginia. Beyond horses, PubMed-indexed work has identified Neorickettsia closely related to N. risticii in wildlife, including coatis in Brazil, underscoring the broader ecological reservoir picture highlighted by the new reviews. (merckvetmanual.com)

There wasn't much direct outside commentary on the new systematic review yet, but current industry guidance aligns with its practical implications. AAEP's acute infectious diarrhea field guideline includes N. risticii and N. findlayensis among adult infectious differentials, and its PHF disease guideline emphasizes the variability of presentation, from high fever and anorexia to diarrhea, laminitis, and abortion. The 2026 review adds that molecular nucleic acid detection is now the preferred diagnostic approach because of turnaround time and analytical sensitivity, and it recommends treating suspected cases promptly in endemic areas, even before confirmatory results are back. (aaep.org)

Why it matters: For veterinary professionals, the biggest takeaway is that PHF surveillance may need to become more ecological and more local at the same time. A horse with acute febrile colitis in a "nonclassic" region may still warrant PHF testing if there's exposure to aquatic insects or freshwater habitats. The newer literature also suggests practices should think beyond a single-agent model: species-level diagnostics, awareness of wildlife and trematode-linked ecology, and realistic conversations with pet parents about vaccine limitations all matter. AAEP notes vaccination may not fully protect against infection, and older guidance points to strain variation as one likely reason. (aaep.org)

What to watch: The next step is likely not a single regulatory action, but a gradual shift in diagnostics and risk communication: more PCR-based differentiation of Neorickettsia species, more region-specific surveillance tied to aquatic habitats, and more work on vaccines that can address strain diversity and deliver broader protection. (pubmed.ncbi.nlm.nih.gov)