Rickettsia rickettsii

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Rickettsia rickettsii / (Redirected from R. rickettsii)

Background

  • Causes Rocky Mountain spotted fever (RMSF)

Microbiology

  • Obligate intracellular bacterium
  • Structurally Gram-negative, but difficult to stain
    • Needs Gimenez method or acridine orange stain
  • Contain lipopolysaccharide (LPS) as well as OmpA and OmpB autotransporters

Life Cycle

  • Transmitted by ticks
  • Has a deleterious effect on ticks
  • Transmitted trans-stadially (stage to stage) and trans-ovarially in ticks, as well as horizontally through vertebrate hosts
    • Trans-ovarial transmission uncommon for D. variabilis
  • Only the adult ticks feed on humans, during prolonged feeding of 1 to 2 weeks
    • Infection requires a minimum of 8 hours of tick attachment, though can theoretically be as quick as 10 minutes
    • Can also be infected during tick removal, when it is crushed between the fingers
  • As few as a single bacillus can cause disease

Pathophysiology

  • From cutaneous inoculation, bacilli spread via lymphatics and small blood vessels to the larger blood vessels
  • There, the OmpA, OmpB, Sca1, and Sca2 proteins induce phagocytosis by vascular endothelial cells
  • From there, they replicate and spread to adjacent cells

Epidemiology

  • Wide global distribution
    • Most cases in the US are in the south Atlantic and south-central regions
      • Highly endemic in North Carolina
    • Also occurs in Argentina, Brazil, Colombia, Panama, Costa Rica, and Mexico
  • Most cases occur in late spring and summer
  • Higher with occupational tick exposures
  • Other spotted fever species include R. conorii (Europe, Africa, and South Asia), R. sibirica (eastern Russia and Asia), R. africae (sub-Saharan African and West Indies), R. parkeri (North and South America), and R. slovaca (Europe), as well as R. felis (worldwide)

Clinical Manifestations

Rash

  • Begins as small (1–5 mm in diameter), blanching, pink macules on the ankles, wrists, or forearms that subsequently spread to the arms, legs, and trunk
  • Classic spotted or generalized petechial rash, including involvement of the palms and soles, usually appears by day 5 or 6
  • Rash may not occur in black patients and elderly patients

Comparison to Other Spotted Fevers

  • Probably the most severe spotted fever
    • Mortality likey above 20%
    • Others typically less severe disease course
  • Others often have eschar at tick bite site

Differential Diagnosis

Diagnosis

  • Microscopy and Culture
    • Can be isolated from blood, though it's not commonly done
  • Serology
    • Enables retrospective diagnosis, since antibodies are only detectable in convalescent phase
    • Indirect immunofluorescence and enzymes immunoassay
      • Titre of 1:64 is diagnostic, but does not distinguish between spotted fevers
      • Takes 7 to 10 days for IgM
      • Usually wait 2 to 3 weeks after onset for convalescent
  • Molecular Testing
    • PCR possible but not sensitive

Management

  • Supportive management, with aggressive fluid resuscitation
  • Antimicrobials