Potomac horse fever.

E. risticii, the cause of classic Potomac horse fever, is now known to produce two disease syndromes: EEC and EEA. The pathogen appears to commonly infect horses based on seroepidemiologic studies; however, the method of transmission remains unknown. The most common clinical disease is EEC, commonly called Potomac horse fever, which presents a wide spectrum of clinical signs. Diagnosis is currently dependent on serology, which frequently does not lead to a definitive diagnosis and at best results in a retrospective diagnosis. A new diagnostic approach, polymerase chain reaction, may offer a rapid and accurate diagnostic test but is yet to be proven. Antimicrobials found to be highly effective in treating EEC include intravenous oxytetracycline and the oral combination of erythromycin and rifampin. Without an understanding of the mode of transmission, prevention of EEC will remain impossible. Although two commercial bacterins are available, vaccination does not result in complete protection. EEA is a recently recognized clinical syndrome caused by E. risticii. After recovery from EEC, pregnant mares may abort. Based on histologic examination, the aborted fetus appears to be a victim of an ehrlichial colitis as well as placentitis. The effect of antimicrobials and vaccination on the occurrence of EEA has yet to be investigated.