34. The causative agent of louse-borne typhus (Rickettsia prowazeki), Mediterranean fever (Rickettsia conorii), Q fever (Coxiella burnetii), human ehrlichiosis (Ehrlichia) and cat-scratching disease (Bartonella henselae). General characteristics – classification, morphology, biology, pathogenesis and immunity. Microbiological diagnosis, diseases, therapy.

I. General Characteristics

• Obligate intracellular Gram-negative rods.
• Poor Gram stain visibility; better seen with Giemsa or Gimenez stain.
• Non-motile, non-spore-forming, pleomorphic.
• Aerobic, requires host ATP → "energy parasite".
• Infects endothelial cells, leading to vasculitis and increased vascular permeability.
• Arthropod-borne (ticks, lice, fleas, mites) – key to transmission.
• Divided into groups:
• Spotted fever group: e.g., R. rickettsii, R. conorii.
• Typhus group: e.g., R. prowazekii, R. typhi.

II. Morphology & Life Cycle

• Requires cell culture, embryonated eggs, or tissue for growth.
• Uses actin polymerization to move within and between host cells.

III. Virulence Factors

• Phospholipase D and membrane peroxidation: help escape phagosome.
• Endothelial cell infection → vascular damage → rash, edema, organ ischemia.
• Induce host cytokines → inflammatory response.

IV. Epidemiology

• Reservoirs: Wild rodents, humans (for epidemic typhus), small mammals.
• Vectors:

• R. rickettsii – ticks (Dermacentor)
• R. prowazekii – body louse
• R. typhi – fleas
• Worldwide distribution; endemic areas depend on vector presence.

VI. Diagnosis

• Clinical suspicion (fever + rash + vector exposure).
• Serology (Weil-Felix test) – obsolete, low specificity.
• Immunofluorescence antibody (IFA) – gold standard.
• PCR – detects rickettsial DNA in blood or tissue biopsy.
• Skin biopsy: shows vasculitis with intracellular bacteria.