Harvey's Garden

Rickettsia diseases

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Rocky Mountain spotted fever

  • Etiology/Pathophysiology
    • Causative agent: Rickettsia rickettsii
      • Obligate intracellular, gram-negative coccobacillus.
    • Vector: Dermacentor ticks (wood tick, dog tick).
    • Pathophysiology: Organism has a predilection for endothelial cells, leading to widespread vasculitis, increased vascular permeability, edema, and organ damage.
  • Clinical Features
    • Hx of tick bite is often absent (~40% of cases).
    • Geographic location: South-Central & Southeastern US (e.g., North Carolina, Oklahoma). Name is a misnomer.
    • Incubation period: 2-14 days.
    • Abrupt onset of constitutional Sx:
      • High fever
      • Severe headache (often retro-orbital)
      • Myalgias/arthralgias
    • Rash (appears 2-5 days after fever):
      • Initially macular, blanches.
      • Starts on wrists and ankles.
      • Spreads centripetally to the trunk.
      • Characteristically involves palms and soles.
      • Evolves into a non-blanching petechial/purpuric rash.
  • Diagnostics
    • Dx is primarily clinical. Do not delay treatment awaiting confirmation.
    • Labs (nonspecific but classic associations):
      • Thrombocytopenia (due to endothelial damage and platelet consumption)
      • Hyponatremia (due to ADH secretion)
      • ↑ LFTs
    • Confirmatory Tests:
      • Gold Standard: Indirect immunofluorescence assay (IFA) for IgM/IgG antibodies. Titers do not rise until 7-10 days into illness (retrospective).
      • Skin biopsy of rash with immunohistochemical (IHC) staining.
  • Treatment
    • First-line for ALL patients (including children <8 and pregnant women): Doxycycline.
      • The risk of mortality from untreated RMSF far outweighs the minimal risk of dental staining from a short course of doxycycline in children.
  • Complications
    • Result from widespread vasculitis.
    • Neurologic: Encephalitis, confusion, seizures.
    • Cardiopulmonary: Myocarditis, pulmonary edema, ARDS.
    • Vascular: Gangrene of digits/limbs requiring amputation.
    • High mortality rate (~20%) if untreated.

Typhus

  • General Features (Rickettsial Diseases)
    • Obligate intracellular, gram-negative coccobacilli.
    • Classic triad: Fever, headache, rash.
    • Pathology: Invade endothelial cells → vasculitis.
    • Dx: Primarily clinical, confirmed with serology (indirect immunofluorescence assay). Weil-Felix test is historical but may appear on exams.
    • Tx: Doxycycline for all types, including in children.

Epidemic Typhus

  • Etiology
    • Rickettsia prowazekii
  • Transmission
    • Vector: Human body louse (Pediculus humanus corporis)
    • Reservoir: Humans
    • Associated with poor hygiene, crowding (war, famine, refugee camps).
    • Rare US reservoir: Flying squirrels.
  • Clinical Features
    • Abrupt onset of high fever, severe headache, confusion, myalgias.
    • Rash appears ~5 days after fever: Maculopapular rash that starts on the trunk/axilla and spreads centrifugally to extremities, characteristically SPARING the face, palms, and soles.
  • Complications
    • Myocarditis, delirium, coma, gangrene.
    • Brill-Zinsser disease: Recrudescent form of epidemic typhus that can occur years after the primary infection.

Endemic (Murine) Typhus

  • Etiology
    • Rickettsia typhi
  • Transmission
    • Vector: Rat flea (Xenopsylla cheopis)
    • Reservoir: Rodents (rats)
  • Clinical Features
    • Milder than epidemic typhus.
    • Gradual onset of fever, headache, myalgias.
    • Rash is less common (<50% of pts) and less severe; typically maculopapular on the trunk.

Scrub Typhus

  • Etiology
    • Orientia tsutsugamushi (Note: Not a Rickettsia genus, but clinically similar).
  • Transmission
    • Vector: Chiggers (larval mites).
    • Reservoir: Rodents. Common in Asia, Australia, Pacific Islands.
  • Clinical Features
    • Fever, headache, myalgias.
    • Key Finding: Painless eschar (dark, crusted lesion) at the site of the chigger bite is pathognomonic.
    • Maculopapular rash may develop.
    • Lymphadenopathy is common.

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