Potomac horse fever (PHF) is a disease affecting horses and is best known for its gastrointestinal signs such as enteritis and diarrhea. This diseases is caused by E. risticii & Neorickettsia risticii, organisms that live inside several different cells of the equine body. This disease is also referred to as equine monocytic ehrlichiosis, or equine ehrlichial colitis.Shop Allivet for all your Horse Potomac Fever Vaccines and SAVE now. Find the lowest prices on Horse Potomac Fever Vaccines now.