08/04/2026
Potomac Horse Fever (PHF)
What is Potomac Horse Fever?
Potomac horse fever (PHF) is an infectious disease that causes colitis (inflammation of the large intestine) in horses. It is most commonly seen in endemic regions, including Oregon. The primary causative agent is Neorickettsia risticii, although additional Neorickettsia species have recently been identified.
The bacteria are carried within trematodes (flukes) that parasitize aquatic snails and insects. Horses become infected after ingesting Neorickettsia spp. infected trematodes within aquatic insects or within free-living trematodes while grazing or drinking. Once ingested, the bacteria invade cells of the gastrointestinal tract and other tissues, resulting in systemic illness.
When is PHF Most Common?
Because transmission depends on aquatic insects, PHF is most commonly diagnosed during the summer and early autumn. Horses with access to freshwater environments, such as ponds, streams, rivers, or irrigated pastures, are at increased risk of infection.
Clinical Signs
Common signs of PHF include:
• Fever
• Depression, lethargy, and decreased appetite (anorexia)
• Diarrhea, ranging from soft manure to profuse "pipe-stream" diarrhea
• Colic
• Abortion in pregnant mares
Complications
PHF can progress to systemic inflammatory response syndrome (SIRS) due to severe intestinal inflammation, bacterial translocation, and endotoxemia. Horses with SIRS are at risk for life-threatening complications, including laminitis.
Signs of SIRS may include:
• Increased heart rate (tachycardia)
• Increased respiratory rate (tachypnea)
• Congested or darkened mucous membranes with a prolonged capillary refill time
• Laminitis
If your horse develops any of these signs, contact your veterinarian immediately.
Diagnosis
PHF is typically diagnosed using polymerase chain reaction (PCR) testing of whole blood and f***l samples. Diagnostic testing should be interpreted alongside the horse's clinical signs and history.
Treatment
Prompt treatment improves the likelihood of recovery and typically includes:
• Tetracycline antibiotics (such as oxytetracycline)
• Intravenous fluid therapy
• Anti-inflammatory medications
• Supportive care, including treatment for endotoxemia and prevention of laminitis
With early recognition and intensive treatment, survival rates at referral hospitals are approximately 70%.
Prevention
While PHF cannot be completely prevented, the risk of infection can be reduced by limiting exposure to aquatic insects and their intermediate hosts.
Recommendations include:
• Turn off barn lights at dusk to reduce attraction of aquatic insects.
• Prevent horses from drinking from natural freshwater sources when possible.
• Remove dead insects from feed and water sources.
• Vaccinate horses with an inactivated Neorickettsia risticii vaccine. Although vaccination does not provide complete protection, it may reduce the severity of disease.
Learn More
For current disease information and active case reports, visit the Equine Disease Communication Center:
https://www.equinediseasecc.org/potomac-horse-fever
