07/29/2025
🚨Potomac Horse Fever
(PHF) is a potentially serious, sometimes fatal disease in horses caused by the bacterium Neorickettsia risticii. It’s most common in late spring through early fall, often peaking in late summer. It is typically associated with horses grazing near rivers, marshes or having those nearby.
🌡️Symptoms can appear suddenly and vary in severity:
Fever (anything over 101.5 is a fever)
Lethargy
Loss of appetite
Diarrhea
Colic-like signs
Laminitis (common and dangerous complication)
Dehydration
Cause
Neorickettsia risticii is transmitted through aquatic insects (like mayflies, caddisflies, and damselflies) that become infected by feeding on snails carrying the bacterium. Horses accidentally ingest the insects while grazing or drinking.
Key Facts
Seasonality: Most common from late spring to early fall
Transmission: Oral ingestion (not spread horse-to-horse)
Some horses show mild signs, but others may rapidly deteriorate.
Possible Treatment
Antibiotics: Usually given IV
Supportive care: IV fluids, anti-inflammatories, electrolytes, probiotics
**Early treatment = much better outcomes**
Prevention
Vaccination is available, but not fully protective. We do find it reduces severity if they do come down with Potomac. 📞call to schedule boosters. If you are in a Potomac heavy area we recommend a booster every 3 months during summer months.
Environmental control:
Turn off barn lights at night to avoid attracting mayflies
Stable horses during peak insect activity
Potomac Horse Fever is not contagious between horses. It can be life-threatening especially if laminitis develops.
❗If you see fever and/or colic-like symptoms, call us immediately.❗