The order Rickettsiales includes a diverse group of microorganisms. The criteria used to define genera (Rickettsia and Coxiella) within the order include morphology, association with various arthropods (primarily ticks, fleas, mites or lice), obligate intracellular parasitism, and serologic relatedness. Rickettsia, Coxiella and species within those genera have adapted to existence within the various arthropods and also frequently infect humans and other vertebrates, usually as accidental hosts. The table below summarizes the various rickettsioses, their respective etiologic agents, and their salient epidemiologic features.
Rickettsia rickettsii, the etiologic agent of Rocky Mountain Spotted Fever (RMSF), is transmitted by a tick bite. Although prevalence varies, RMSF is endemic throughout the Western hemisphere. In the U.S., cases are most frequently reported in the South Atlantic and South Central states. Predominant manifestations of RMSF include fever, headache, myalgia, and rash often extending to palms. The triad of fever, headache, and rash is reported in 50% of the cases.
Rickettsia conorii, the etiologic agent of Mediterranean Spotted Fever (MSF), also referred to as Boutonneuse fever, is endemic in the Mediterranean regions of Europe and Africa as well as India. It is transmitted by ticks, which also use dogs and rodents as hosts. Predominant manifestations of MSF include fever, headache, myalgia,rash, and rash on palms. The triad of fever, headache, and rash is reported in 50% of the cases. In adults, fever ranges from 103 to 105°F. Maculopapular rash and petechiae first appear on the extremities then progress to the trunk. The incubation period, from actual infection to acute onset of symptoms, ranges from 5 to 7 days. Complications are rare and the mortality rate is low, even in untreated cases. Symptoms may last 10 to 20 days.
Rickettsia typhi, the etiologic agent of murine or endemic typhus, is distributed worldwide. It is transmitted primarily, but not solely, by rat fleas (Xenopsylla cheopis), which bite humans. Predominant manifestations of murine typhus include the sudden onset of fever, headache, and malaise and maculopapular rash. The triad of fever, severe headache, and rash is reported in 50% of the cases. Fever in adults ranges from 103 to 104°F. Most patients also experience pronounced muscular weakness. The incubation period, from actual infection to acute onset of symptoms, ranges from 8 to 16 days with an average of 11 days. Complications are rare and the mortality rate is lower than in epidemic typhus. Symptoms may last 9 to 14 days.