Systematic review widens the lens on Potomac horse fever

A new systematic review in Veterinary Research Communications is revisiting a familiar equine disease with a wider lens: where, exactly, is Neorickettsia risticii found? The paper assembles fragmented reports on the bacterium that causes Potomac horse fever, or equine neorickettsiosis, and lands at a useful moment for clinicians, as newer literature is already pushing the field to think beyond a narrowly North American story. A companion 2026 review in Veterinary Microbiology describes equine neorickettsiosis as a global ecological problem tied to trematodes, snails, aquatic insects, and regional environmental conditions, rather than a disease defined only by one river corridor or one country. (sciencedirect.com)

That broader framing matters because the disease’s history has shaped how veterinarians talk about risk. Potomac horse fever was first described in 1979 near the Potomac River, and AAEP vaccination guidance still reflects that origin story, while also acknowledging that cases have since been identified in other parts of the U.S. and Canada. More recent AAEP disease guidance now lists both N. risticii and N. findlayensis as causative organisms of PHF, underscoring that the clinical syndrome is more complex than older summaries suggested. (aaep.org)

The ecology is a big reason surveillance has lagged behind clinical suspicion. Merck Veterinary Manual notes that N. risticii is maintained through a trematode-associated life cycle involving freshwater snails and aquatic insects, including caddisflies, mayflies, damselflies, dragonflies, and stoneflies. Horses are thought to become infected mainly by ingesting infected aquatic insects, and cases tend to cluster in spring through early fall, especially around waterways. At the same time, Merck cautions that older indirect fluorescent antibody testing has a high false-positive rate, meaning exposure maps built on serology may not reflect the true distribution of clinical disease. (merckvetmanual.com)

That diagnostic uncertainty is one of the most useful implications of the new review. Merck states that PCR- or culture-confirmed clinical detection has been reported only from California, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, Ohio, Oregon, Pennsylvania, Texas, and Virginia, even though broader exposure has been suggested in more regions. Meanwhile, published molecular work has identified N. risticii in horses from Rio de Janeiro, Brazil, and recent Canadian reports have documented PHF in foals in southwestern Ontario, with current literature also recognizing N. findlayensis in the disease complex. In other words, the literature supports a wider footprint, but the quality and type of evidence still vary substantially by region. (merckvetmanual.com)

Industry guidance and expert-facing references also reinforce that prevention remains imperfect. AAEP says vaccination may not be fully protective and points to multiple field strains as one likely reason. Merck similarly notes that commercially available inactivated vaccines have been only marginally protective in the field and links vaccine failure to antigenic and genomic heterogeneity among more than 14 N. risticii strains isolated from natural cases. That concern is longstanding: earlier work tied poor vaccine performance to both weak antibody responses and strain heterogeneity, and newer genomic work continues to show meaningful variation among isolates. (aaep.org)

Why it matters: For equine veterinarians, this review is less about declaring PHF “everywhere” than about sharpening risk assessment. The practical message is to think ecologically and diagnostically at the same time. In a horse with acute fever, enterocolitis, ileus, laminitis, or abortion risk during insect season, PHF should stay on the differential when local water, snail, trematode, and aquatic insect exposure make transmission plausible, even if the practice is outside the disease’s classic historical footprint. PCR on both blood and feces remains especially important because organism detection may not coincide in the two sample types, and serology alone can mislead. (merckvetmanual.com)

The other clinical implication is communication with pet parents and farm managers. Because clinically ill horses are not considered contagious, the conversation is less about horse-to-horse spread and more about environmental exposure, rapid recognition, and early treatment. Merck reports that early oxytetracycline treatment can produce improvement within 12 hours, but laminitis remains a major complication, occurring in 20% to 30% of affected horses, and overall case fatality is still reported at 5% to 30%. Those numbers help explain why a better map of Neorickettsia distribution matters even if many surveillance questions remain unresolved. (merckvetmanual.com)

What to watch: The next phase will likely center on molecular epidemiology: more PCR-based surveillance, more sequencing of field strains, clearer separation of N. risticii from N. findlayensis, and better region-specific guidance on when vaccination and insect-exposure mitigation are most likely to help. Recent work on fetal and fecal samples, including a complete genome assembled from an aborted equine fetus, suggests that the evidence base is moving in that direction. (pubmed.ncbi.nlm.nih.gov)