Rickettsiae are Gram-negative obligate intracellular parasites. Types of rickettsiae cause a variety of diseases that are typically characterised by a triad of fever, headache and rash. A notable exception is Q fever (cause by Coxiella burnetti) which causes pneumonia but no rash. The Weil-Felix reaction is positive except in Q fever. Rickettsial diseases are all treated with tetracyclines.

Rash starts on the peripheries (wrist, ankles) before spreading centrally. It is initially maculopapular before becoming vasculitic

Endemic to east coast of US | | Q fever | Coxiella burnetti | No vector | No rash but causes pneumonia | | Endemic typhus | Rickettsia typhi | Flea | Rash starts centrally then spreads to the peripheries | | Epidemic typhus | Rickettsia prowazekii | Human body louse | | | Ehrlichliosis | Ehrlichia | Tick | |

A dry fracture of a Vero cell exposing the contents of a vacuole where Coxiella burnetti are busy growing. Note the intracellular nature of the organism. Credit: NIAID

RMSF

Rickettsia rickettsii

Endemic typhus

Rat fleas

Rickettsia typhi

Epidemic typhus

Human body lice

Rickettsia prowazekii

starts centrally and spreads outwards

Weil-Felix reaction

Rickettsial infections have antibodies against Rickettsia

Those anti-rickettsial antibodies cross-react and agglutinate --> positive Weil-Felix reaction

Some serotypes of (non-motile) Proteus are also positive

Coxiella = negative Weil-Felix reaction