New reviews sharpen the global picture of Potomac horse fever

A pair of recent reviews is pushing Potomac horse fever surveillance toward a more global, ecology-first view. The new systematic review in Veterinary Research Communications synthesizes the fragmented literature on Neorickettsia risticii, while a second review in Veterinary Microbiology reframes equine neorickettsiosis as a disease system tied to trematodes, aquatic insects, and regional ecology, not just a historically named syndrome from the U.S. Mid-Atlantic. (sciencedirect.com)

That shift has been building for years. Potomac horse fever was first recognized along the Potomac River in the early 1980s, but subsequent work documented disease or pathogen detection far beyond that original footprint. Merck now notes PCR- or culture-based confirmation from multiple U.S. states, and AAEP’s current disease guidance says Potomac horse fever is caused by both N. risticii and N. findlayensis, reflecting how the case definition has expanded as molecular tools improved. (pmc.ncbi.nlm.nih.gov)

The new reviews matter because the literature on N. risticii has long been patchy and methodologically mixed. Older distribution claims often relied on indirect fluorescent antibody testing, but Merck cautions that those surveys had a high false-positive rate, meaning exposure maps may not equal true pathogen distribution. By contrast, molecular studies have added firmer evidence from places such as Brazil. One peer-reviewed report from Rio de Janeiro identified N. risticii DNA in naturally infected horses and found a South American genotype distinct from two North American groups, supporting the idea that geographic clustering is real and epidemiologically relevant. (merckvetmanual.com)

The ecological picture is also wider than many clinicians were taught. Potomac horse fever is non-contagious, and transmission is linked to digenean trematodes and the inadvertent ingestion of infected aquatic insects. That helps explain its seasonal pattern and why cases can appear in seemingly unrelated regions that share the right water-associated intermediate hosts. The Veterinary Microbiology review’s framing of the disease as a habitat- and life-cycle-driven problem, rather than a narrowly regional one, aligns with AAEP and Merck guidance on seasonality, endemicity, and environmental exposure. (sciencedirect.com)

There wasn’t much direct quote-based expert reaction available in open sources, but current guidance from major equine authorities points in the same direction. AAEP’s October 1, 2025 disease guideline explicitly recognizes both causative species, and its vaccination guidance has long warned that field protection may be limited by the presence of multiple strains while commercial products are based on a single strain. Merck similarly emphasizes that definitive diagnosis should rely on PCR or culture, not serology alone. Taken together, that’s an implicit industry message: surveillance and prevention need to catch up with the organism’s diversity. (aaep.org)

Why it matters: For veterinary professionals, these reviews reinforce three practical points. First, PHF should stay on the differential outside its classic historical geography, particularly in horses with fever followed by enterocolitis signs during summer and fall. Second, diagnostics matter: paired serology may support a case, but PCR on both blood and feces is more actionable in real time because organism shedding in those sample types may not coincide. Third, prevention remains imperfect. Available vaccines are still used as an aid in prevention, but strain heterogeneity and the addition of N. findlayensis to the disease picture complicate expectations around field efficacy. (merckvetmanual.com)

The broader surveillance implication is that “where PHF exists” is still partly a diagnostics question. Regions with active molecular testing are more likely to look endemic than regions relying on historical serosurveys or limited enterocolitis workups. That suggests at least some apparent expansion may reflect better detection rather than wholly new emergence, though the genotype data from South America also supports genuine geographic structuring of the pathogen. This is an inference from the available evidence, rather than a direct claim from any single source. (merckvetmanual.com)

What to watch: The next phase will likely center on PCR-based surveillance, species-specific testing for N. risticii versus N. findlayensis, and more work on trematode and aquatic insect ecology to clarify where clinical risk is truly changing versus where diagnostics are simply improving. (pmc.ncbi.nlm.nih.gov)