Equine Potomac Horse Fever Study Guide

Overview and Clinical Importance

Potomac Horse Fever (PHF), also known as Equine Neorickettsiosis or Equine Monocytic Ehrlichiosis, is an acute, potentially fatal enterocolitis caused by the obligate intracellular gram-negative bacterium Neorickettsia risticii (formerly Ehrlichia risticii). First identified in 1979 near the Potomac River in Maryland, this disease has since been recognized throughout North America and is now endemic in more than 40 U.S. states and parts of Canada, Brazil, Uruguay, and Europe.

PHF is a high-yield topic on the NAVLE because it represents a unique transmission cycle involving trematodes and aquatic insects, requires differentiation from other causes of acute colitis (Salmonella, Clostridium), and has specific treatment protocols with oxytetracycline. The disease is characterized by fever, profuse watery diarrhea, and a high risk of laminitis (20-30% of cases), making rapid diagnosis and treatment critical for survival.

Etiology and Epidemiology

Causative Agent

Neorickettsia risticii is an obligate intracellular, gram-negative coccobacillus belonging to the family Anaplasmataceae (order Rickettsiales). Key characteristics include: