23 The Family Rickettsiaceae

The Rickettsiaceae family is composed of two genera, Rickettsia and Orientia, which are obligate intracellular bacteria that belong to the order Rickettsiales. The species of these genera are divided into two groups based on antigenic, molecular, and ecological patterns: (1) the typhus group, composed of the species Rickettsia prowazekii, Rickettsia typhi, and Orientia tsutsugamushi, which are transmitted by lice, fleas, and mites, respectively; and (2) the spotted fever group (SFG), which is composed of more than 23 valid species. The transmission of great majority of species in SFG is associated with ticks, with the exception of Rickettsia felis and Rickettsia akari, which are associated with fleas and mites, respectively. Other Rickettsia species, such as Rickettsia bellii and Rickettsia canadensis, are not included in either of the two groups. In SFG, at least 12 species of Rickettsia cause infections in humans (Rickettsia rickettsii, Rickettsia conorii, Rickettsia africae, Rickettsia parkeri, Rickettsia australis, Rickettsia honei, Rickettsia sibirica, Rickettsia japonica, Rickettsia massiliae, Rickettsia aeschlimannii, R. akari, and R. felis). However, species of nonpathogenic rickettsiae or of unknown pathogenicity might have a key role in the natural history of the pathogenic species; ticks infected by a kind of rickettsia that is nonpathogenic to humans (e.g.: Rickettsia montana, Rickettsia peacockii) may become unable to maintain (via transovarial transmission) infection by other pathogenic species (e.g., R. rickettsii). This fact is of great practical importance because there are populations of ticks infected with nonpathogenic rickettsiae for which the infection rate is often higher compared to pathogenic rickettsiae.