Clinical Information

Species of Rickettsia are small (0.3-0.5 mcm x 1-2 mcm) obligately intracellular bacteria (Proteobacteria). They have a gram-negative cell wall structure. Rickettsiae are found in arthropod hosts for at least part of their life cycle.

Rickettsial infections in the United States are caused by 2 major groups within the genus Rickettsia: spotted fever group and typhus fever group. The spotted fever group includes Rickettsia rickettsii (Rocky Mountain spotted fever), Rickettsia akari, Rickettsia conorii (Boutonneuse fever), Rickettsia australis (Queensland tick typhus), and Rickettsia sibirica (North Asian tick typhus). The typhus fever group includes Rickettsia typhi (murine typhus; endemic typhus) and Rickettsia prowazekii (epidemic typhus).

R rickettsiae is the most common rickettsial species encountered in the United States and is transmitted through a tick vector (Dermacentor species or, less commonly, Rhipicephalus sanguineus). Following a 2- to 14-day incubation period, patients most commonly present with fever, chills, and myalgia. A maculopapular rash typically appears 2 to 5 days after fever onset, though approximately 10% of patients will not develop a rash. Antibodies to the spotted fever group agents are detectable within 7 to 10 days after illness onset. Demonstration of either seroconversion or a 4-fold change in IgG-specific antibody titers in acute and convalescent serum samples is consistent with acute or ongoing disease.