Equine monocytic Ehrlichiosis (Potomac horse fever) in horses in Uruguay and southern Brazil

A disease named locally as churrio or churrido equino (i.e., equine scours) has occurred for at least 100 years in Uruguay and southern Brazil in farm s along both shores of the Merin lake. This report describes cases of churr ido equino and provides serologic, pathologic, and DNA-based evidence indic ating that the disease is in fact equine monocytic ehrlichiosis (Potomac ho rse fever). Results of an epidemiological investigation conducted on an end emic farm are also presented. Clinical signs in 12 horses were fever, depre ssion, diarrhea, dehydration, and sometimes colic and distal hind limb edem a. Postmortem findings of 3 horses were of acute enterocolitis. Inclusion b odies containing ehrlichial organisms were found in the cytoplasm of macrop hages of the large colon of 1 horse. Eleven of the 12 horses were serologic ally positive to Ehrlichia risticii (indirect fluorescent antibody assay) a nd, of 3 paired samples, 2 showed seroconversion. Ehrlichia risticii DNA wa s identified by a nested polymerase chain reaction in peripheral blood of a n affected horse. A healthy horse inoculated with peripheral blood from an affected horse developed the disease and antibodies to E. risticii. The dis ease had a peak incidence in March (summer) and was statistically associate d with a marshy ecosystem near the Merin lake, where large numbers of Pomac ea spp. (Ampullariidae) snails were found. Incidence density was almost 8 t imes higher in normative horses than in native horses. It was concluded tha t the previous diarrheic disease of horses known in Uruguay and southern Br azil as churrido equino is equine monocytic ehrlichiosis.