Systematic review widens the map for Potomac horse fever risk

A new systematic review on the global distribution of Neorickettsia risticii adds weight to a message equine practitioners have been hearing more often in recent years: Potomac horse fever is better understood as an ecologically complex, geographically broader disease than its name implies. The review, published in Veterinary Research Communications, arrives as a companion 2026 review in Veterinary Microbiology describes equine neorickettsiosis as endemic in multiple regions of the United States and Canada, with recognized disease also reported in Uruguay and Brazil. (sciencedirect.com)

That shift matters because Potomac horse fever has long been associated in practice with selected North American river systems and summer cases, but the literature now points to a wider and more heterogeneous distribution. In Ontario, investigators reviewing confirmed cases from 2015 to 2019 concluded that the disease was endemic and widely distributed in southern Ontario, while also documenting rising case counts over that period. The same study highlighted how historical descriptions of a similar equine disease in Ontario may date back nearly a century, suggesting the pathogen ecology has been established in some regions for far longer than modern surveillance records alone would indicate. (pmc.ncbi.nlm.nih.gov)

The biological explanation for that patchy but persistent distribution is the organism’s unusual life cycle. According to the 2026 Veterinary Microbiology review, Neorickettsia spp. are obligate endosymbionts of digenean trematodes, which move through snails, aquatic intermediate hosts, and vertebrate definitive hosts. Horses are considered aberrant hosts and become infected after ingesting parasitized aquatic insects. That ecology helps explain why disease risk can be highly local, seasonal, and difficult to predict using horse-level data alone. A 2020 Scientific Reports study from Rio de Janeiro, for example, detected N. risticii in 2.67% of sampled horses and argued that the finding was important because equine enterocolitis etiologies had been underinvestigated in Brazil. (sciencedirect.com)

The diagnostic picture is evolving, too. AAEP’s current disease guidance says Potomac horse fever is caused by N. risticii and N. findlayensis, and lists fever, diarrhea, anorexia, lethargy, laminitis, colic, edema, and abortion among the variable clinical signs. The 2026 review similarly says molecular detection of nucleic acids is now the preferred diagnostic method because of its speed and analytical sensitivity, and recommends antibiotic treatment early in suspected cases, especially in endemic areas. Ontario data add a practical point for clinicians: fecal PCR showed strong inter-laboratory agreement and, in some comparisons, identified more positives than blood PCR, reinforcing the value of submitting both blood and feces when Potomac horse fever is on the differential list. (aaep.org)

There’s also a species-diversity angle that could reshape how veterinarians think about “Potomac horse fever” as a single-agent disease. Research published in recent years has shown that N. findlayensis can also cause clinical Potomac horse fever-like disease, and the Ontario investigators reported that two cultured isolates from horses in eastern Ontario were this novel species rather than N. risticii. That helps explain at least some apparently negative N. risticii PCR results in clinically compatible cases and supports broader molecular approaches that can distinguish among Neorickettsia species. (pmc.ncbi.nlm.nih.gov)

Why it matters: For veterinary professionals, the systematic review is less about a single new outbreak signal and more about recalibrating surveillance and clinical suspicion. In endemic or adjacent regions, Potomac horse fever may deserve consideration even when geography doesn’t fit older textbook patterns. The literature also suggests that environmental exposure, aquatic insect ecology, and strain diversity all complicate prevention, while current vaccines remain an imperfect tool against a pathogen complex that may vary by region. For equine practices, that means sharper client education for pet parents, quicker empiric treatment decisions in compatible cases, and more attention to sample selection and test interpretation. (sciencedirect.com)

What to watch: The next phase will likely focus on filling surveillance gaps outside traditional endemic zones, refining PCR assays that separate N. risticii from N. findlayensis, and linking strain-level findings to geography, clinical severity, and vaccine performance. A 2025 Veterinary Microbiology study on fecal and fetal samples, including genome sequencing from an aborted equine fetus, suggests that genomics may become a more important part of that effort. (pubmed.ncbi.nlm.nih.gov)