Systematic review widens the map for Potomac horse fever

A newly published systematic review is giving Potomac horse fever a wider epidemiologic frame. The paper, published March 18, 2026, in Veterinary Research Communications, synthesizes global reports on Neorickettsia risticii, the classic causative agent of Potomac horse fever, also called equine neorickettsiosis. Read alongside a May 2026 review in Veterinary Microbiology, the message is that this is no longer best understood as a disease confined to a few familiar North American hot spots. (deepdyve.com)

That shift matters because the disease’s history has long shaped clinical thinking. Potomac horse fever was first recognized in horses near the Potomac River in the late 1970s, and for years was strongly associated with specific river-adjacent regions in the eastern United States. More recent guidance from AAEP notes that cases are seasonal, typically appearing in summer and fall, and that the disease has since been identified in multiple geographic areas in the United States and Canada. (aaep.org)

The newer review literature adds important nuance. The Veterinary Microbiology article states that equine neorickettsiosis is caused by the closely related intracellular bacteria N. risticii and N. findlayensis, and describes clinical endemicity across multiple regions of the United States and Canada, plus parts of South America, including Brazil and Uruguay. It also reinforces the ecological explanation for that spread: Neorickettsia species persist as endosymbionts of digenean trematodes, with horses becoming aberrant hosts after ingesting infected aquatic insects. That helps explain why risk is tied less to horse movement alone and more to local freshwater ecosystems, insect exposure, and seasonality. (sciencedirect.com)

The clinical picture remains familiar, but the reviews suggest the surveillance lens should widen. AAEP lists fever, diarrhea, anorexia, lethargy, colic, edema, laminitis, and abortion among recognized signs, while Merck notes that definitive diagnosis should rely on isolation or identification of the organism, with PCR favored for speed and sensitivity. The Veterinary Microbiology review similarly says molecular nucleic-acid detection is now the preferred diagnostic method and argues that treatment should begin promptly in suspected cases in endemic areas, even before confirmation, because early therapy improves prognosis. (aaep.org)

Industry and expert commentary has long reflected the same concern: clinicians can miss the disease if they anchor too tightly to the name. AAEP’s current guidance explicitly includes both N. risticii and N. findlayensis under the Potomac horse fever umbrella. Merck’s vaccination guidance recommends annual spring boosters in endemic areas, particularly for horses near freshwater streams, rivers, ponds, and heavily irrigated pastures, but also notes that vaccination is imperfect. Earlier AAEP guidance similarly says available vaccines may not be fully protective, with strain variation among proposed explanations. (aaep.org)

Why it matters: For equine veterinarians, this is less about a brand-new pathogen than about recalibrating risk assessment. If the literature is right, the challenge is not simply where Potomac horse fever has historically been diagnosed, but where the trematode-host-aquatic insect ecology can support transmission. That has implications for case recognition, client education, and prevention messaging to pet parents managing horses near waterways, irrigated pasture, or heavy insect pressure. It also supports using PCR early and keeping equine neorickettsiosis on the list for acute febrile enterocolitis, laminitis-associated systemic illness, and abortion workups during vector season. This is an inference drawn from the ecology and diagnostic guidance in the reviews and practice references. (sciencedirect.com)

Another practical issue is prevention. The literature continues to point to limited long-term prevention tools and incomplete vaccine performance. The Veterinary Microbiology review identifies broadly protective vaccination against diverse strains as an ongoing research priority, and AAEP guidance has already warned that currently available vaccination may lessen severity without fully preventing infection. For practices in endemic or emerging-risk regions, that means prevention still depends heavily on awareness, seasonal vigilance, and rapid treatment rather than vaccination alone. (sciencedirect.com)

What to watch: The next step is whether these reviews translate into better geographic surveillance, more precise mapping of trematode and aquatic insect reservoirs, and updated vaccine or diagnostic strategies that reflect both species diversity and a wider apparent range. (deepdyve.com)