Rickettsia and Their Properties

Study Notes

Rickettsia are non-motile, gram-negative, coccobacilli.
They are obligate intracellular bacteria, meaning they must live inside host cells.
Rickettsia cannot be grown in artificial media.
They are transmitted by arthropod vectors, such as ticks, mites, and fleas.
Rickettsia infect vascular endothelial cells.

Rickettsia belong to the order Rickettsiales, and are further classified into two families: Rickettsiaceae and Anaplasmataceae.
Genera within these families include Rickettsia, Orientia, Ehrlichia, Wolbachia, Anaplasma, and Neorickettsia.
Coxiella and Bartonella were previously in the Rickettsiaceae family, but are no longer considered to be obligate intracellular bacteria and are not transmitted by arthropods.

Rickettsiae cause a group of diseases known as typhus or spotted fever.
Typhus group:
- Spotted fever:
  - R. akari (Rickettsialpox)
  - R. rickettsii (Rocky Mountain Spotted Fever)
  - R. conorii (Mediterranean spotted fever)
  - R. africae (African tick-bite fever)
- Typhus:
  - R. prowazekii (Epidemic typhus)
  - R. typhi (Endemic typhus)
Antigenic structures of Rickettsia:
- OMP antigens: Species-specific and highly immunogenic.
- LPS antigens: Group-specific and used in the Weil-Felix reaction.

Reservoirs:
- Ticks and mites
- Lice and fleas
Transmission:
- Through the bite of an arthropod
- Rubbing or scratching an abraded surface contaminated with rickettsia
Spread in the body:
- Through the lymphatics, where they multiply in lymph nodes.
- They then enter the bloodstream and eventually reach endothelial cells.
Endothelial cell infection:
- Rickettsia adhere to endothelial cells using OmpA and OmpB proteins.
- They are phagocytosed by endothelial cells and multiply by binary fission within the cell.
- Cell lysis occurs, except in spotted fever.

Endemic Typhus (Flea-borne)
- Vector: Rat flea (R. typhi)
- Reservoir: Rodents
- Mode of transmission: Similar to epidemic typhus
- Clinical manifestation: Similar to epidemic typhus, but less severe.
Epidemic Typhus
- Vector: Human body louse (P. humanus capitis)
- Transmission: Rubbing or scratching of abraded skin, ingestion of feces contaminated with rickettsia, rarely inhalation.
- Clinical manifestation: Acute febrile disease with headache, muscle and joint pain, rash (beginning on the upper trunk and spreading to the entire body), myalgia.
- Complications: Intestinal pneumonitis, CNS involvement (mental confusion, coma).
- Risk factors: High louse population, poor hygiene.
- Brill-Zinsser disease: Reactivation of epidemic typhus due to waning immunity.
Rocky Mountain Spotted Fever
- Vector: Ticks
- Clinical manifestation: Fever, headache, rash (on extremities and trunk), myalgia, anorexia.
- Complications:
  - Vascular damage (permeability, edema, hemorrhage, DIC).
  - Interstitial pneumonitis.
  - CNS involvement.
Indian Tick Typhus
- Vector: Tick (Rhipicephalus sanguineus)
- Clinical manifestation: Similar to RMSF.
- An eschar (a necrotic lesion) is present at the site of the tick bite.

Serology:
- Antibody detection:
  - Non-specific (Weil-Felix test): Less sensitive and specific, but inexpensive.
  - Specific (IFA/ELISA): More sensitive and specific.
  - The Weil-Felix test relies on the presence of LPS antigen of rickettsia in Proteus species.