Potomac Horse Fever

What is Potomac Horse Fever? 

Potomac Horse Fever (PHF) is a non-contagious infection caused by the rickettsial organism Neorickettsia risticii (previously known as Ehrlichia risticii). The disease can affect any age, breed or sex of horse. PHF cases usually occur in summer and fall  but may occur in any season depending on weather conditions.

Clinical Signs: Highly variable, including: 

  • High fever to 107° F (41.6o C) reported. Fevers may occur 7–14 days before signs of colitis develop 
  • Diarrhea: variable, ranging from absent to severe 
  • Depression 
  • Anorexia 
  • Lethargy 
  • Laminitis (with or without diarrhea) may progress from mild to severe 
  • Mild to moderate colic 
  • Decreased gastrointestinal sounds 
  • Edema of limbs and ventral body, prepuce of males 
  • Abortion (by transplacental transmission)

Note: Concurrent infections with Salmonella have been documented.

Diagnosis: Diagnosis is made by a veterinarian using PCR to detect DNA from the rickettsial organism in the feces, blood, serum or aborted fetal tissue. Detection of increased serum titer can be used to make a diagnosis. Diarrhea caused by PHF can be identical to diarrhea from other intestinal infections.

Treatment: Tetracycline antimicrobials are used under the supervision of a veterinarian. Fever and diarrhea usually resolves within 72 hours of treatment. In severe cases intravenous fluids, antibiotics and non-steroidal anti-inflammatory drugs may be indicated to hydrate the horse and treat shock.

Prevention: A vaccine is available for PHF. Although the vaccine is not 100% effective, it can decrease the severity of the disease.


August 11, 2023
Chaudière-Appalaches, QC
Information Only
July 24, 2023
West Virginia, United States
Confirmed Case(s) - No Quarantine
July 12, 2023
Hamilton County, OH
Confirmed Case(s) - No Quarantine