Review maps wider global footprint for Potomac horse fever agent

Potomac horse fever may be more globally relevant than its name suggests, according to a new systematic review of Neorickettsia risticii, one of the bacteria that causes equine neorickettsiosis. The review, published March 18, 2026, in Veterinary Research Communications, pulls together scattered reports from multiple continents and adds to a broader 2026 review in Veterinary Microbiology arguing that equine neorickettsiosis should be viewed through a wider ecological and geographic lens, not just as a regional US disease. That second review says clinical disease is considered endemic in multiple regions of the United States and Canada, as well as parts of Brazil and Uruguay, and notes that disease is caused by N. risticii and the related species N. findlayensis. (deepdyve.com)

Why it matters: For veterinarians, the key takeaway is surveillance and suspicion. AAEP’s current Potomac horse fever guidance describes the disease as non-contagious and linked to ingestion of trematodes in freshwater habitats or in aquatic insects such as mayflies and caddisflies, while also noting that one killed, adjuvanted vaccine is available but has shown variable results, likely because of strain differences. A wider documented distribution of Neorickettsia spp. could affect how clinicians think about seasonal risk, differential diagnoses for fever, colitis, and laminitis, and vaccine conversations with pet parents moving horses into or through endemic areas. Recent surveillance summaries show PHF is still being reported in the field, including a US case in Washington in the first quarter of 2025 and a Pennsylvania case reported in May 2025. (aaep.org)

What to watch: Expect follow-up work on environmental reservoirs, geographic risk mapping, strain diversity, and whether current prevention guidance and vaccination strategies need to be refined as the disease’s range is better defined. (sciencedirect.com)