Systematic review broadens the surveillance picture for Potomac horse fever

A new systematic review is trying to answer a basic but clinically important question: where, exactly, is Neorickettsia risticii found? The review, published in Veterinary Research Communications, assembles global reports on the agent behind Potomac horse fever, while a newly published review in Veterinary Microbiology argues that equine neorickettsiosis should be understood as a broader ecological and geographic problem than the disease’s name implies. Together, they suggest the profession may need to think less in terms of a narrow historic hotspot and more in terms of an expanding, better-documented surveillance footprint. (sciencedirect.com)

That matters because Potomac horse fever has long carried a regional identity. It was first described in horses near the Potomac River, but current guidance from AAEP says horses in endemic areas across the US and beyond remain at risk, with cases usually occurring in summer and fall, though weather can shift the timing. AAEP also now explicitly names both N. risticii and N. findlayensis as causative agents, reflecting how the science around this syndrome has evolved beyond the older one-pathogen framing. (aaep.org)

The ecology is one reason surveillance has been so fragmented. Merck describes N. risticii as an obligate intracellular bacterium maintained through a trematode-associated life cycle involving freshwater snails and aquatic insects. DNA has been detected in multiple aquatic insect groups, and experimental transmission through infected caddisflies has reproduced disease. That complex life cycle helps explain why local environmental conditions, water sources, insect emergence, and grazing management can shape risk in ways that don’t always line up neatly with old textbook maps. (merckvetmanual.com)

Recent literature is also filling in some of the geographic and diagnostic gaps. The Veterinary Microbiology review says clinical equine neorickettsiosis is considered endemic in multiple regions of the US and Canada, as well as parts of Brazil and Uruguay. A 2026 paper from southern Brazil reported the first genetic sequences of N. risticii from that region and confirmed circulation in symptomatic horses there. Another 2025 study from Kentucky detected N. risticii in 11.5% of fecal samples from clinically ill horses and in 1.08% of fetal colon samples from equine abortions submitted over multiple years, while also reporting a complete genome from an aborted equine fetus. (sciencedirect.com)

Expert guidance remains fairly consistent on the clinical front, even as the map broadens. AAEP lists high fever, anorexia, lethargy, variable diarrhea, colic, laminitis, edema, and abortion among the possible signs, and notes that fever can precede colitis by 7 to 14 days. Merck adds that laminitis develops in an estimated 20% to 30% of affected horses and that overall case fatality is about 5% to 30%, reinforcing why early recognition still matters more than the disease’s non-contagious status might suggest. Treatment with oxytetracycline is generally effective when started early, and Merck says response is often seen within 12 hours. (aaep.org)

Why it matters: For veterinarians, the bigger story is not just that Potomac horse fever exists in more places, but that diagnosis and prevention remain imperfect. Merck says definitive diagnosis should rely on PCR or isolation of the organism, and recommends testing both blood and feces because detection may not overlap between sample types. Serology can generate false positives, and field vaccination has been only marginally protective, in part because of strain heterogeneity. In practice, that means clinicians may need a lower threshold to include Potomac horse fever on differentials for seasonal febrile colitis, laminitis-associated enterocolitis, or abortion workups in regions that haven’t historically been viewed as classic endemic zones. Conversations with pet parents may also need to shift from “this is a local disease” to “this is a water- and insect-linked risk that depends on ecology, season, and travel history.” (merckvetmanual.com)

What to watch: The next phase will likely center on better mapping of trematode, snail, and aquatic insect hosts, plus wider use of improved molecular assays and genomic sequencing. That could sharpen regional risk estimates, clarify whether apparent spread reflects true expansion or better detection, and influence how equine practices approach seasonal surveillance, vaccination timing, and abortion diagnostics. (pmc.ncbi.nlm.nih.gov)