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Rickettsial diseases include a wide range of infections caused by alphaproteobacteria in the Order Rickettsiales. The Order Rickettsiales consists of two families, with the family Rickettsiaceae containing the genera Rickettsia and Orientia. The other family, Anaplasmataceae, contains the genera Ehrlichia, Anaplasma, Neorickettsia, and Wolbachia. All of these organisms are obligate intracellular bacteria, with the Rickettsiaceae growing in host cell cytoplasm, and Anaplasmataceae growing within host cell vacuolar structures.

There are at least 27 agents within the Order Rickettsiales that can infect humans (Table 144-1). Within the Rickettsiaceae, the typhus group (TG) rickettsia include R. typhi (endemic typhus), R. prowazekii (epidemic typhus), and O. tsutsugamushi (scrub typhus). The spotted fever group (SFG) includes a variety of rickettsia species including R. rickettsii (Rocky Mountain spotted fever, RMSF), R. conorii (Mediterranean spotted fever, MSF), and R. africae.1 In the family Anaplasmataceae, the Wolbachia are not known to infect humans and will not be discussed here. Multiple species within the Ehrlichia genus can cause human ehrlichiosis, with E. chaffeensis being the most prominent. The genus Anaplasma contains pathogens of veterinary importance, but A. phagocytophilum is the most common etiologic agent of human anaplasmosis. Neorickettsia includes the pathogenic species N. sennetsu that causes sennetsu fever.


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