New reviews widen the surveillance lens on Potomac horse fever

Potomac horse fever’s map may be wider than many clinicians assume. A new systematic review in Veterinary Research Communications pulls together the scattered global literature on Neorickettsia risticii, the main cause of Potomac horse fever, while a recent companion review in Veterinary Microbiology argues for a broader, ecology-based view of equine neorickettsiosis. Together, the papers suggest the organism’s documented footprint extends beyond the classic U.S. endemic narrative, with evidence from Canada and parts of South America, including Brazil and Uruguay, and they underscore how uneven surveillance and older serology-based reports have likely blurred the true range of disease. Current AAEP guidance also reflects an evolving picture, noting that Potomac horse fever can be caused by both N. risticii and Neorickettsia findlayensis. (aaep.org)

Why it matters: For veterinary professionals, the big takeaway isn’t just geography, it’s diagnostics and risk communication. Merck notes that older indirect fluorescent antibody data likely overstated exposure in some regions because of false-positive titers, and recommends definitive diagnosis by PCR or organism isolation from blood or feces. That matters as clinicians assess summer and fall febrile colitis cases, abortion risk, and laminitis in horses that may not fit the traditional “Potomac River” mental map. The reviews also reinforce that transmission is tied to trematodes and aquatic intermediate hosts, which makes local ecology, not just state lines, central to case suspicion and prevention planning. (merckvetmanual.com)

What to watch: Expect more attention to molecular surveillance, species-level differentiation between Neorickettsia pathogens, and region-specific prevention advice as researchers refine where clinically relevant exposure is actually occurring. (aaep.org)