Other Causes of Tick-Borne Ehrlichioses, Including Ehrlichia ewingii

At least three species of tick-borne bacteria in the genera Ehrlichia (Ehrlichia chaffeensis and Ehrlichia ewingii) and Anaplasma (Anaplasma phagocytophilum) cause moderately severe to life-threatening infections in humans, and they are collectively termed human ehrlichioses. In addition, Ehrlichia canis was recently isolated from a patient without clinical signs of illness; the role of this pathogen as a cause of human disease remains incompletely defined. In this chapter, an understanding of the epidemiology and ecology of E. ewingii and characterization of the disease it causes are relatively nascent. Areas for future investigation in the natural history of E. ewingii include field investigations that define roles of tick species other than A. americanum and vertebrate hosts other than dogs and deer. Understanding the frequency and mechanism of asymptomatic canine infection may provide insights to the pathogenesis of E. ewingii infections in natural and non-natural hosts. Prospective active surveillance is needed to better identify (i) the numbers of es of disease in humans caused by E. ewingii, (ii) the geographical distribution of disease in humans, and (iii) the spectrum of clinical disease in various human cohorts. More robust laboratory techniques need to be developed, including methods to isolate E. ewingii in cell culture and serological techniques to distinguish this infection from human monocytic ehrlichiosis. It is likely that other tick-borne ehrlichiae will be recognized eventually as additional agents of human ehrlichioses around the world. The key to identifying novel agents lies in the perspicacity of, and cooperation among, clinicians, veterinarians, epidemiologists, and laboratorians.