The definition of the Rickettsiaceae family has been based mainly on nonspecific phenotypic characters. Originally, small gram-negative bacteria, associated (or not) with arthropods and necessitating (or not) eukaryotic cells for growth, were considered Rickettsiaceae. During the past 20 years, gene sequencing and genetic phylogeny have deeply challenged this classification. The controversy has centered on how much difference between strains should constitute a subspecies.^1–5 Among the agreed-upon changes, Orientia was created from an independent branch of its phylum. The Ehrlichia group has been reclassified⁶ into four genera, with Ehrlichia and Anaplasma being associated with ticks, Neorickettsia with helminths, and Wolbachia with both arthropods and helminths. This chapter is limited to the Rickettsiales. All are intracellular alphaproteobacteria associated with eukaryotic hosts (arthropods or helminths). Based on antigenic and genetic data, pathogenic rickettsiae are traditionally divided into three groups—the spotted fever group, the typhus group, and the scrub typhus group (Table 187-1). The spotted fever group accounts for most tick-borne rickettsioses. The typhus group comprises two human pathogens transmitted by insects. Epidemic typhus is caused by Rickettsia prowazekii and is transmitted by the body louse. Murine typhus is caused by Rickettsia typhi and is transmitted by rat and cat fleas. The scrub typhus group comprises Orientia tsutsugamushi only and is transmitted by “chiggers.”

New genetic tools and the use of cell culture assays have allowed the description of many new rickettsioses and ehrlichioses during the past 30 years (Table 187-2).⁷ Three ehrlichioses and 12 rickettsioses have been described since 1980. Three major conditions determined the description and separation of these species. Some were discovered after clinical description in countries where spotted fever had been unknown (Japan and Rickettsia japonica, Flinder’s Island and Rickettsia honei, and Russia and Astrakhan fever). Some were recognized by bacterial identification based on culture and polymerase chain reaction (PCR) in places where the new pathogen was confounded with another known rickettsial pathogen (Rickettsia africae with Rickettsia conorii, Rickettsia heilongjianghensis with Rickettsia sibirica, R. sibirica mongolitimonae and Rickettsia aeschlimannii with R. conorii, Rickettsia felis with Rickettsia typhi, and Anaplasma phagocytophilum and Ehrlichia ewingii with Ehrlichia chaffeensis). Some were identified through association by physicians and microbiologists when an atypical unknown disease (E. chaffeensis, Rickettsia slovaca, Rickettsia raoultii, and Rickettsia helvetica) was being explored.⁷

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