Rickettsiae
Rickettsiae are Gram-negative obligate intracellular parasites. Types of rickettsiae cause a variety of diseases that are typically characterised by 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.
| Disease | Cause | Vector | Notes |
|---|---|---|---|
| Rocky Mountain spotted fever | Rickettsia ricketsii | Tick | Headache and fever are common 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 |
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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