Rickettsia and Their Properties

General Properties

• 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.

Classification

• 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.

Rickettsia VS Virus

• Rickettsia possess a gram-negative cell wall.
• They contain both DNA and RNA.
• Rickettsia multiply through binary fission.
• They can be killed by antimicrobial agents.
• Rickettsia are visible under a light microscope.
• They cannot pass through bacterial filters.
• These characteristics distinguish them from viruses.

Rickettsia Infection

• 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.

Pathogenesis

• 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.

Diseases Caused by Rickettsia

• 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.

Laboratory Diagnosis

• 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.