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Rickettsiae / Coxiella

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Topic updated on 03/14/17 2:39pm

Taxonomy
  • Bacteria
    • Rickettsiaceae
      • Rickettsia (genus)
        • R. rickettsii
        • R. typhi
        • R. prowazekii
    • Coxiella
      • Coxiella burnetii
Biology
  • Pathogenesis
    • transmission
      • vector is disease specific
      • transmission by an arthropod vector
        • except Coxiella
          • transmitted by aerosol
          • causes pneumonia
    • reservoir
      • rodents and dogs
    • molecular biology
      • obligate intracellular
        • cannot make enough ATP
        • need CoA and NAD+
      • invasion of endothelial cells → vasculitis
Diseases
  • Rickettsial diseases usually exhibit the classic triad of
    • headache
    • fever
    • rash
  • Rocky Mountain spotted fever (tick)
    • Rickettsia rickettsii
    • symptoms
      • petechial rash on palms and soles 
        • STARTS distally, MIGRATES proximally
          • to wrists, ankles, then trunk
        • caused by small hemorrhages
      • headache
      • fever
      • can progress to
        • hepatosplenomegaly
        • thrombocytopenia
        • DIC
    • endemic to East Coast
      • don't let the name fool you
    • recall that a palm and sole rash is seen in "CARS"
      • Coxsackievirus A infection (hand, foot, and mouth disease)
      • Rocky Mountain spotted fever
      • Secondary syphilis
  • Endemic typhus (fleas)
    • R. typhi
    • exists constantly throughout a population
  • Epidemic typhus (human body louse)
    • R. prowazekii
    • rapidly spreads through large swath of population
  • Q fever (inhaled aerosols)
    • Coxiella burnetii
    • causes mild pneumonia
    • "Q fever is Queer"
      • no rash
      • no vector
      • negative Weil-Felix
      • endospore form
        • can survive outside host for a long time
        • resists heat and drying
      • does not have Rickettsia as its genus name
  • Presentation depends on organism
    • Rickettsial rash starts on hands and feet
    • Typhus rash starts centrally and spreads outward
      • without involving palms or soles
    • "Rickettsia on the wRists, Typhus on the Trunk"
Laboratory
  • Characteristics
    • Gram-negative
      • however, too small to Gram stain
    • aerobic
  • Diagnosis
    • Weil-Felix test
      • no longer used but may be tested on USMLE
      • patient serum is mixed with Proteus antigens
        • antirickettsial antibodies cross-react and agglutinate
      • Weil-Felix is negative in Coxiella infection
    • serological IFA test is most widely used
Treatment
  • Tetracycline
    • start treatment without laboratory confirmation


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