Systematic review broadens the map for Potomac horse fever

A newly published systematic review is putting Potomac horse fever back on the surveillance radar by asking a basic question with practical consequences: where, exactly, is Neorickettsia risticii found? The March 18, 2026, review in Veterinary Research Communications compiles evidence on the global distribution of the bacterium, while a companion-style 2026 review in Veterinary Microbiology argues that equine neorickettsiosis should be understood through a broader international and ecological lens, not only as a regional equine disease. (deepdyve.com)

That framing reflects how the field has shifted over time. Potomac horse fever was first recognized in the U.S. Mid-Atlantic and has long been treated as a seasonal disease associated with freshwater environments and ingestion of infected aquatic insects. But the transmission story has steadily expanded as researchers clarified the bacterium’s relationship with digenean trematodes and their multi-host life cycles involving snails, aquatic insects, and definitive hosts. Experimental and field work over the years helped establish that horses can be infected after exposure to infected aquatic insects or trematode stages, supporting the environmental nature of the disease rather than horse-to-horse spread. (aaep.org)

More recent discoveries have complicated the picture further. In 2020, investigators reported a novel Neorickettsia species, later recognized as N. findlayensis, that can also cause Potomac horse fever-like disease in horses in eastern Ontario and the northern U.S. AAEP now explicitly lists both N. risticii and N. findlayensis in its disease guidance. That matters because older literature and some field assumptions still focus narrowly on N. risticii, while newer diagnostic and epidemiologic work suggests clinicians may be dealing with more than one closely related pathogen under the same clinical umbrella. (pubmed.ncbi.nlm.nih.gov)

The newer reviews appear to synthesize evidence that recognized disease and pathogen detection extend beyond the best-known U.S. endemic areas. The Veterinary Microbiology review says clinical cases are considered endemic in multiple regions of the U.S. and Canada, as well as parts of South America, including Uruguay and Brazil. Supporting that broader view, a recent PubMed-indexed report described detection and phylogenetic characterization of N. risticii in horses from southern Rio Grande do Sul, Brazil. Earlier work from Brazil also documented the organism in horses from Rio de Janeiro. Taken together, those reports suggest the literature is moving from isolated regional observations toward a more coherent map of international circulation. (sciencedirect.com)

Direct expert reaction to the new systematic review was limited in publicly accessible sources, but established veterinary guidance reinforces the clinical relevance. AAEP describes Potomac horse fever as a non-contagious infectious disease, usually seen in summer and fall, with signs ranging from fever and anorexia to diarrhea, laminitis, edema, and abortion. Cornell’s Animal Health Diagnostic Center has previously warned that positive submissions can rise sharply during active seasons and noted that PCR testing is most useful before oxytetracycline is started, since treatment can reduce detectable neorickettsial DNA and lead to false negatives. Cornell also notes that its PCR detects both N. risticii and N. findlayensis but does not differentiate between them. (aaep.org)

Why it matters: For veterinary professionals, the surveillance message is bigger than a literature update. If the pathogen’s environmental range is wider than traditional teaching suggested, then equine practitioners may need a lower threshold to consider Potomac horse fever or equine neorickettsiosis in compatible summer and fall cases, even outside classic hot spots. The reviews also underscore a familiar challenge in equine infectious disease: ecology, diagnostics, and nomenclature are evolving at the same time. A broader distribution of Neorickettsia spp., plus the existence of N. findlayensis, could affect how veterinarians interpret PCR results, discuss vaccine expectations, and advise pet parents managing horses near rivers, streams, irrigation systems, or heavy aquatic insect exposure. (sciencedirect.com)

There’s also a disease-surveillance angle. A systematic review can’t prove expansion on its own; in part, it may reveal where researchers have looked rather than where the bacterium truly is. Still, the practical implication is the same: under-recognition is possible where testing is sparse, species differentiation is limited, or cases are attributed to nonspecific enterocolitis or laminitis syndromes. That makes this review useful not because it settles the map, but because it highlights how incomplete the map may still be. This is an inference drawn from the synthesis of the reviews, current guidance, and diagnostic limitations. (deepdyve.com)

What to watch: The next developments to watch are whether reference labs expand species-specific testing, whether future surveillance studies better define environmental and host reservoirs outside North America, and whether equine guidelines begin to reflect a more explicitly global risk map for Potomac horse fever and related neorickettsial disease. (sciencedirect.com)