The Pacific Coast tick, Dermacentor occidentalis Marx (henceforth D. occidentalis) is the most widely distributed tick in California and is found in chaparral and shrubland areas from northern Baja California to California and Oregon (Furman & Loomis, 1984). D. occidentalis is a three-host, hard-shell tick that feeds on a variety of vertebrates, such as rodents, rabbits, cattle, deer, horses and humans. Surveys of this tick have shown its ability to vector human pathogens such as Francisella tularensis (tularemia), Coxiella burnetii (Q fever), Anaplasma phagocytophilum (human granulocytic anaplasmosis), Ehrlichia chaffeensis (human monocytic ehrlichiosis), Rickettsia rickettsii (Rocky Mountain spotted fever, RMSF) and Rickettsia philipii 364D (hereafter R. philipii) as well as the non-pathogenic spotted fever group Rickettsia, R. rhipicephali (Parker, Brooks & Marsh, 1929; Cox, 1940; Lane et al., 1981; Holden et al., 2003; Wikswo et al., 2008; Shapiro et al., 2010). Rickettsia philipii, was originally described as an unclassified Rickettsia found by Bell in D. occidentalis from California (Philip et al., 1978). It is closely related to Rickettsia rickettsii but can be serologically and genetically distinguished (Philip, Lane & Casper, 1981; Karpathy, Dasch & Eremeeva, 2007). Although discovered in 1966, and long suspected of being able to cause disease, it was only recently confirmed to be associated with eschars and lymphadenopathy in people at the site of a tick bite (Lane et al., 1981; Shapiro et al., 2010; Johnston et al., 2013).