Studies on Ehrlichia chaffeensis and Borrelia lonestari, tick-borne agents transmitted by Amblyomma americanum
Varela, Andrea Soares
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Over the past decade, the lone star tick (LST), Amblyomma americanum, has become recognized as an important vector of several known or suspected zoonotic agents in the southern United States. Among these are Ehrlichia chaffeensis, the causative agent of human monocytotropic ehrlichiosis, E. ewingii, and agent of human and canine ehrlichiosis, and Borrelia lonestari, the putative agent of “southern tickassociated rash illness” (STARI). White-tailed deer (WTD) are a primary host for all motile LST stages and serve as the principal reservoir host for E. chaffeensis. However, studies that reveal the course of E. chaffeensis infection in reservoir hosts are scarce. Reports of E. ewingii and B. lonestari in LST from the southern U.S. exist but additional reports would add to an understanding of the distribution of these bacteria. In addition, B. lonestari has not yet been isolated in culture, making the development of specific diagnostic assays to differentiate STARI from Lyme disease problematical. This dissertation presents studies which investigated these three tick-borne agents. Findings in Chapter 3 demonstrated that the course of primary infection with E. chaffeensis via different routes of needle inoculation did not differ. Chapter 4 showed that WTD with a primary infection of E. chaffeensis were not protected from secondary infection with a genetic variant. In Chapter 5, inoculation of domestic goats with E. chaffeensis did not cause active infection, suggesting that goats are an unsuitable model for E. chaffeensis infection. In Chapter 6, a three-year survey of LST from northeastern Georgia found prevalences of E chaffeensis, E. ewingii, and B. lonestari at 2.0% (8/398), 4.8% (19/398), and 1.0% (4/398), respectively, substantiating the presence of these organisms and risk of human exposure in northeastern Georgia. The detection of B. lonestari in LST from northeastern Georgia was critical for obtaining the first culture isolate of B. lonestari. The isolation and molecular, immunologic, and ultramicroscopic characterization used to identify the spirochetes are described in Chapter 6. These studies contribute to an understanding of recently recognized bacteria transmitted by LST.