Rickettsia

Rickettsia is a genus of small, obligate intracellular, gram-negative coccobacilli. Because they depend on the host cell for nutrients such as CoA and NAD⁺, rickettsiae must live inside host cells — they cannot be grown on standard culture media and stain poorly with Gram stain (Giemsa stain works better). Nearly all rickettsial infections are transmitted by arthropod vectors: ticks, lice, mites, or fleas.

The two classic diseases sit in two groups. In the spotted fever group, Rocky Mountain spotted fever (RMSF) is caused by Rickettsia rickettsii and transmitted by Dermacentor ticks; its hallmark triad is headache, fever, and a rash that begins on the wrists and ankles and spreads inward to the trunk, often involving the palms and soles. In the typhus group, epidemic typhus (R. prowazekii, spread by the human body louse) produces a rash that starts centrally on the trunk and spreads outward, sparing palms and soles; endemic typhus (R. typhi) is flea-borne. Rickettsiae infect vascular endothelial cells, causing the widespread small-vessel vasculitis behind the rash, headache, and potential organ damage.

Treatment for rickettsial disease is doxycycline, started empirically on clinical suspicion — RMSF can be rapidly fatal if therapy waits for serologic confirmation.

USMLE Step 1 tests Rickettsia as classic vignette material: expect questions on the rash pattern distinguishing RMSF from typhus, the obligate intracellular lifestyle, vector associations, and doxycycline as the drug of choice.