Potomac horse fever review maps a wider global footprint

Potomac horse fever may need a broader map. A new systematic review in Veterinary Research Communications synthesizes the scattered literature on the global distribution of Neorickettsia risticii, while a second 2026 review in Veterinary Microbiology places equine neorickettsiosis in a wider ecological and geographic frame, including disease caused by both N. risticii and N. findlayensis. The central takeaway is that this is no longer best understood as a narrowly regional problem defined by its original Mid-Atlantic name. (sciencedirect.com)

That shift has been building for years. AAEP says Potomac horse fever is a non-contagious infectious disease that affects horses in endemic areas and usually appears in summer and fall, though cases may occur in any season depending on weather conditions. The organization’s current guidance also reflects an important taxonomy and causation update: the syndrome is now associated with both Neorickettsia risticii and Neorickettsia findlayensis. Merck’s review similarly notes that confirmed detection from clinical cases has been reported from multiple U.S. states and Canada, while cautioning that older serologic evidence likely overstated the true range because indirect fluorescent antibody testing can generate false positives. (aaep.org)

The newer reviews add global context to that picture. The Veterinary Microbiology paper says clinical cases are considered endemic in multiple regions across the United States and Canada, as well as in parts of South America, including Uruguay and Brazil. That aligns with primary research from Brazil, where investigators reported molecular detection of N. risticii in horses from Rio de Janeiro, and with a more recent PubMed-indexed report describing detection and phylogenetic analysis in horses from southern Rio Grande do Sul. Taken together, those findings suggest the organism’s recognized footprint is expanding not necessarily because the pathogen is new, but because molecular tools are identifying it in places where surveillance had been sparse. That last point is an inference based on the geographic spread of PCR-based reports and the reviews’ emphasis on fragmented literature. (sciencedirect.com)

Ecology remains central to understanding risk. AAEP and Merck both emphasize that transmission is linked to trematodes and aquatic insects, including caddisflies, mayflies, damselflies, and dragonflies, with exposure thought to occur through oral ingestion of infected trematodes or parasitized insects rather than direct horse-to-horse spread. AAEP also highlights practical risk factors, such as proximity to freshwater streams or rivers, irrigated pasture, and nighttime barn lighting that attracts insects. This matters because the “global distribution” question is really an ecology question: where the relevant snail, trematode, insect, and environmental conditions overlap, veterinarians may need to think about equine neorickettsiosis even outside traditional teaching maps. (aaep.org)

On the clinical side, the disease remains consequential. AAEP lists fever, diarrhea, anorexia, lethargy, laminitis, colic, edema, and abortion among the variable signs, while Merck reports an overall case fatality rate of roughly 5% to 30% and notes that laminitis can be severe. Merck also says a definitive diagnosis should rely on organism detection by culture or PCR, with testing of both blood and feces recommended because shedding in the two sample types may not coincide. That diagnostic nuance is especially relevant as clinicians confront a broader geographic literature: if awareness expands faster than testing strategy, cases may still be missed or misclassified. (aaep.org)

Expert and industry guidance points to another practical issue: prevention remains incomplete. AAEP’s vaccination guidance says the current commercial product is a killed, adjuvanted vaccine labeled as an aid in prevention, and it explicitly notes that vaccination may not be fully protective. Merck goes further, linking field vaccine failure in part to antigenic and genomic heterogeneity among isolates. For veterinarians, that means the new reviews are less a call to overpromise vaccination than a reminder to pair risk-based immunization with environmental management, seasonal vigilance, and fast diagnostics when compatible cases appear. (aaep.org)

Why it matters: For veterinary professionals, these reviews help reframe Potomac horse fever from a named syndrome with a familiar U.S. history into a surveillance problem with wider geographic and ecological boundaries. Practices serving horses near freshwater environments, irrigated pasture, or insect-heavy facilities may need to revisit client education, diagnostic algorithms for acute febrile colitis, and assumptions about where the disease “belongs.” The inclusion of N. findlayensis in current AAEP guidance also signals that clinicians should think in terms of equine neorickettsiosis more broadly, not just classic N. risticii infection. (aaep.org)

What to watch: The next step is likely more molecular surveillance and phylogenetic work, particularly in under-sampled regions, to clarify where these organisms are established, how much strain diversity is clinically relevant, and whether current vaccines and testing approaches are keeping pace with what the epidemiology now appears to show. (sciencedirect.com)