New reviews sharpen the global picture of Potomac horse fever

Potomac horse fever’s map may be broader, and murkier, than many clinicians assume. A new systematic review in Veterinary Research Communications pulls together scattered evidence on the global distribution of Neorickettsia risticii, the classic cause of Potomac horse fever, while a companion review in Veterinary Microbiology places equine neorickettsiosis in a wider ecological context that now includes both N. risticii and N. findlayensis. Together, the papers argue that confirmed circulation and disease extend beyond the Potomac region’s historical framing, with reports from multiple parts of the U.S. and Canada and evidence from South America, while also underscoring how uneven diagnostics and older serology-based reports still blur the true map. (sciencedirect.com)

Why it matters: For equine veterinarians, the practical takeaway is that geography alone is a weak rule-out. AAEP now defines Potomac horse fever as a disease caused by both N. risticii and N. findlayensis, and Merck notes that older indirect fluorescent antibody surveys likely overstated range because of false positives, making PCR on blood and feces the more useful diagnostic anchor in suspected cases. That matters in horses with acute fever, depression, diarrhea, or laminitis risk during seasonal windows, especially as strain diversity and species diversity may also help explain why field vaccine performance can be inconsistent. (aaep.org)

What to watch: Expect follow-on work to focus less on broad seroprevalence maps and more on PCR-confirmed cases, vector ecology, and whether regional strain differences should reshape surveillance and vaccination strategy. (merckvetmanual.com)