Reviews suggest a broader global footprint for Potomac horse fever
Potomac horse fever’s map is getting bigger. A new systematic review in Veterinary Research Communications pulls together the scattered global literature on Neorickettsia risticii, the main cause of equine neorickettsiosis, and argues that the pathogen’s distribution is broader, and more complex, than many clinicians may assume from the disease’s Mid-Atlantic name alone. A companion review in Veterinary Microbiology reinforces that point, describing equine neorickettsiosis as endemic in multiple regions of the U.S. and Canada, with reported clinical disease and pathogen detection also extending into parts of South America, including Uruguay and Brazil. The reviews also place N. risticii in a wider ecological context: the bacterium persists within digenean trematodes and is linked to aquatic snails and insects, which helps explain why recognized risk is tied to freshwater habitats and seasonality. (aaep.org)
Why it matters: For equine practitioners, the main takeaway is that Potomac horse fever surveillance can’t stay anchored to older geographic assumptions. AAEP’s current guidance notes that PHF is caused by Neorickettsia risticii and N. findlayensis, affects horses in endemic areas, and typically appears in summer and fall, although timing can vary with weather. That matters diagnostically because fever, diarrhea, laminitis, colic, edema, and abortion can present in different combinations, and newer literature shows the etiologic picture is broader than a single organism in a single region. Ontario case data published in recent years identified both N. risticii and N. findlayensis, while a 2025 study detected N. risticii in fecal samples from clinically ill horses and in fetal colon samples from equine abortions, underscoring the value of molecular testing and the need to keep PHF on the differential beyond classic presentations. (aaep.org)
What to watch: Expect more emphasis on region-specific risk mapping, molecular diagnostics that distinguish Neorickettsia species, and surveillance work that links equine cases to local aquatic host ecology. (pubmed.ncbi.nlm.nih.gov)