Identification of an enzootic diarrhea (''Shasta River crud'') in northern California as Potomac horse fever

A disease entity known as the ''Shasta River crud'' (SRC) has been recognized for at least 25 years in areas of Siskiyou and Shasta counties, in far northern California. It is characterized by anorexia, lethargy, diarrhea, variable fever, and in some cases laminitis, and is tetracycline responsive. The incidence is seasonal, with cases occurring in the spring and summer months near the Shasta and Klamath rivers. Limited serologic testing has provided some suggestion that the illness may be linked to Ehrlichia risticii, the agent of Potomac horse fever. Hen we report the identification of E. risticii genomic DNA in blood buffy-coat cells from horses with SRC, and so provide the first definitive evidence that this enzootic diarrhea is in fact Potomac horse fever.