With warmer weather on the horizon, veterinarians and horse owners across Canada grow increasingly concerned about Potomac Horse Fever (PHF). 

Once rare, the disease is appearing more often in regions north of the United States. Potomac is now considered endemic in Ontario, Quebec, Nova Scotia, Manitoba, Alberta and British Columbia.

What Is Potomac Horse Fever?

Potomac Horse Fever (PHF), also called equine neorickettsiosis or equine monocytic ehrlichiosis, is a serious intestinal disease affecting horses during warmer months. First recognized in 1979 near the Potomac River in Maryland, PHF has spread across North and South America and now is on the rise in parts of Canada. The disease is caused by Neorickettsia bacteria found inside parasitic flatworms that infect aquatic insects such as mayflies, caddisflies and dragonflies. Horses become infected when they ingest these insects while grazing or drinking from contaminated water.

Why Are Cases Increasing?

Dr. Luis Arroyo from the Ontario Veterinary College says, “Climate change and shifting insect populations may be contributing to the spread of Potomac Horse Fever into new regions, including parts of Ontario where it wasn’t previously seen. Increased awareness and better diagnostic tools are also helping identify more cases.”

Symptoms: What to Watch For

Common signs include high fever, lethargy, loss of appetite, diarrhea/colitis, colic-like symptoms, laminitis and abortion in pregnant mares. PHF can lead to laminitis, and studies show that about 25 to 40 per cent of affected horses develop laminitis. PHF associated laminitis frequently involves multiple limbs and can be severe, so early treatment is especially important. 

Arroyo shared an encouraging statistic from a recently published review paper (2026): “For horses who suffer from PHF‑associated colitis, there is a 73 per cent survival rate when treatment is started promptly.”

Diagnosis and Treatment

Diagnosis typically uses blood and fecal testing including PCR. There are different strains and species of PHF>. Neorickettsia findlayensis is a newer species, now confirmed in Ontario, Quebec, Alberta and B.C. Species-specific PCR tests can tell whether a horse has N. risticii or N. findlayensis.

Treatment usually includes oxytetracycline and supportive care such as IV fluids.

Prevention: Protecting Your Horses

While no preventative method is perfect, horse owners can take several steps to reduce risk. There are many different strains and two known species of the Potomac Horse Fever bacteria. Experts advise that horse owners should talk to their veterinarian about vaccination. While it doesn’t prevent all cases, it may help lessen the severity of illness and is still a useful part of an overall risk‑reduction plan.

Horse owners should consult their veterinarian about vaccination and inquire about specific disease risks in their area and areas they plan on travelling to with their horse. AAEP guidelines explain the seasonality of Potomac Horse Fever and recommend administering vaccines a month ahead of expected cases. For primary vaccinations, two doses are given three to four weeks apart. Horse owners should discuss the frequency of revaccination with their veterinarian. PHF has a strong seasonal pattern (late spring through fall, peaking in summer).

Reduce insect exposure by limiting standing water, turning barn lights off at night and cleaning water sources. Infection can occur from contaminated water containing stages of the parasite/trematode that carry the bacteria.

Maintain pasture and stable hygiene to limit contact with aquatic insects. Horses within eight kilometres of waterways have substantially higher risk because of insect emergence.

Additional Resources

Sign up for Equine Guelph E-news to receive timely health alerts, including updates re-posted from the Equine Disease Communication Center (EDCC). These alerts help horse owners stay informed about emerging disease threats. 

In February 2025, Equine Guelph partnered with EDCC to relaunch the Biosecurity Risk Calculator, a practical resource that helps horse owners assess and reduce a wide range of disease risks on their farms.

Watch the Potomac Horse Fever Webinar with Drs. Ashley Whitehead (UCVM) and Luis Arroyo (OVC), hosted by Canadian Animal Health Surveillance System. This webinar from May 2021 provides an update on Potomac Horse Fever (PHF) in Canada, with information on pathogenesis, disease strains, epidemiology and a review on how recent advances in laboratory diagnostics, such as genotyping, could impact clinical practice.

In addition, the following papers are available for review on the topic:

Arroyo LG, Moore A, Bedford S, Gomez DE, Teymournejad O, Xiong Q, Budachetri K, Bekebrede H, Rikihisa Y, Baird JD. Potomac horse fever in Ontario: Clinical, geographic, and diagnostic aspects. Can Vet J. 2021 Jun;62(6):622-628. PMID: 34219771; PMCID: PMC8118184.

Luis G. Arroyo, Alexandre S. Borges, John D. Baird, Brian D. Perry, Yasuko Rikihisa, Stephen E. Greiman, Equine neorickettsiosis: A global perspective of the natural habitat of the bacteria and clinical disease, Veterinary Microbiology, Volume 316, 2026,110963, ISSN 0378-1135, https://doi.org/10.1016/j.vetmic.2026.110963.

(Equine Guelph; horse pictured does not have Potomac Horse Fever)