Rickettsiae

Snapshot

A 24-year-old man presents to his primary care physician for a rash. He had been hiking in Tennessee during a retreat at his new job last week. He reports that he pulled a tick off of himself, though he did not think much of it at the time. Since then, he has had fevers and headaches daily. A few days after the onset of fever, he developed a rash that started on his wrists and ankles and now has spread to his chest. On physical exam, there are small blanching erythematous macules on his chest and petechiae on his extremities. His palms and soles are spared. He is started on empiric antibiotics. (Rocky Mountain spotted fever)

Introduction

Classification
- Rickettsia
  - spotted fever group (tick-borne)
    - Rickettsia rickettsii (Rocky Mountain spotted fever)
      - most common
    - Rickettsia conorii (Mediterranean spotted fever)
    - Rickettsia akari (Rickettsialpox)
    - Rickettsia africae (African tick bite fever)
  - typhus group
    - Rickettsia prowazekii (louse-borne epidemic typhus)
    - Rickettsia typhi (flea-borne murine typhus)
- Orientia
  - scrub typhus group
    - Orientia tsutsugumushi (mite-borne)
      - formerly Rickettsia tsutsugamushi
- obligate intracellular gram-negative bacteria
- transmission via blood-feeding arthropod vectors
  - ticks, lice, and flea
Epidemiology
- incidence depends on geographic distribution of the vector
- North America (R. rickettsii and R. akari)
- Europe (R. conorii)
- Asia (R. conorii and O. tsutsugamushi)
- Africa (R. africae and R. conorii)
- South America (R. prowazekii)
- worldwide (R. typhi)
Pathogenesis
- the bacteria is transmitted into the human body via arthropod saliva when bitten
- vascular endothelial cells are targeted by the bacteria
- replication can cause local hemorrhage
Prognosis
- most resolve with treatment
- epidemic typhus (R. prowazekii) may recur (Brill-Zinsser disease)
Rocky Mountain spotted fever (R. rickettsii) has highest mortality

Presentation