Etiology and Transmission The causative agent is Rickettsia rickettsii, a minute, gram-negative, organism which may be found in ticks, the tissues of infected eggs, and the lesions of patients who have contracted the disease. The Rocky Mountain wood tick (Dermacentor andersoni), the American dog tick (Dermacentor variabilis), and the Lone Star tick (Amblyomma americanum) are the vectors in the United States that are responsible for