Systematic review broadens the map for Potomac horse fever

Potomac horse fever may be more globally distributed than many clinicians assume, according to a new systematic review of Neorickettsia risticii, the bacterium long linked to equine neorickettsiosis. The review, published March 18, 2026, in Veterinary Research Communications, pulls together scattered reports from multiple continents and adds to a growing body of literature suggesting the ecology of this pathogen is broader, and more complex, than the traditional North America-centered picture. A related 2026 review in Veterinary Microbiology also frames equine neorickettsiosis as a global disease system, noting recognized clinical disease in the U.S., Canada, Uruguay, and Brazil, and emphasizing the role of trematodes, snails, aquatic insects, and other hosts in transmission. (deepdyve.com)

Why it matters: For equine veterinarians, the takeaway isn’t just geography, it’s case recognition. AAEP’s current Potomac horse fever guidance says the disease is caused by both N. risticii and N. findlayensis, is non-contagious, and typically appears in summer and fall, although seasonality can vary with weather. Clinical signs can include high fever, diarrhea, lethargy, colic, laminitis, and abortion. Cornell has also highlighted the diagnostic challenge: PCR sensitivity can drop after oxytetracycline treatment starts, and available PCR assays may detect both N. risticii and N. findlayensis without distinguishing between them. In practice, that means veterinarians may need to think beyond classic endemic maps, especially when compatible cases appear near freshwater habitats or in regions with limited surveillance. (aaep.org)

What to watch: Expect more attention on environmental surveillance, species-level diagnostics, and whether updated risk maps change vaccination and case-workup discussions in endemic and emerging regions. (aaep.org)