Potomac Horse Fever
Basic Information 
Epidemiology
Geography and Seasonality
• Originally reported in the Potomac River Valley.
• Cases have now been described in most states, though they occur most frequently in the northeast, mid-Atlantic, and western parts of the country.
• Housing near (within ~5 miles of) a river in endemic regions is correlated with an increased incidence of Potomac horse fever (PHF).
Clinical Presentation
History, Chief Complaint
• Inappetence, colic, fever and diarrhea as for other causes of colitis (see “Colitis/Diarrhea, Acute” in this section).
• Occasionally, a transient fever is noted 7 to 14 days before signs of colitis develop shortly after infection, although this is often missed by the owner.
• Some horses present for acute laminitis in the absence of diarrhea.
Etiology and Pathophysiology
• N. risticii is an intracellular gram-negative coccus.
• The etiopathogenesis of PHF has not been fully elucidated, but current evidence suggests that the vector for N. risticii transmission is a trematode (helminth), with freshwater operculate snails or aquatic insects such as caddis flies likely intermediate hosts.
• The reservoir for N. risticii in the environment is also unknown, although many small mammals and birds may carry the infected trematode.
• The exact mode of transmission to horses is unknown, but infection is presumed to occur via ingestion of infected snails or insects or ingestion of water containing infected trematodes released in secretions from freshwater snails.
• After it has been ingested, N. risticii infects monocytes, thus evading the host’s immune response.
• The organism also has a predilection for large colon epithelial cells, resulting in loss of microvilli and active reduction of electrolyte and fluid transport. This results in diarrhea and electrolyte, fluid, and protein loss as for other causes of colitis (see “Colitis/Diarrhea, Acute” in this section).
• Laminitis occurs in 30% to 40% of horses with PHF, although specific reasons for this increased incidence compared with other causes of equine colitis are not known.
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