Rickettsia rickettsii - Rocky Mountain Spotted Fever

 

Clinical Background

Rickettsia rickettsii is a tick-borne illness (zoonosis) and the etiologic agent of Rocky Mountain Spotted Fever (RMSF).

Epidemiology

  • Incidence – 3-5/1,000,000
  • Age – peak incidence 5-9 years
  • Transmission
    • Via Dermacentor (variabilis, andersoni), and Amblyomma, and Rhipicephalus sanguineus ticks in the U.S.
      • Geographical distribution is restricted to the western hemisphere
    • Humans are accidental hosts
    • 95% of the cases occur April through September

Organism

  • Gram-negative coccobacilli of the Rickettsiaceae family – obligate intracellular organisms
  • Characteristic feature of the Rickettsiae – life cycle requires multiplying in an arthropod
  • Invertebrate hosts are both reservoirs and vectors
  • Rickettsia are part of a family of organisms responsible for the following rickettsial diseases:
    • Spotted fever and typhus (vector: tick, louse, flea or gamasid mite)
    • Scrub typhus (vector: chigger)
    • Ehrlichiosis (vector: tick)
    • Neorickettsiosis
    • Q-fever

Risk Factors

  • Dog exposure to ticks
  • Residence in a wooded area
  • Residence in Central and Mid-Atlantic states
  • Male sex

Clinical Presentation

  • The incubation period between tick bite and onset of symptoms is 2 days-2 weeks
  • RMSF is difficult to differentiate from viral illness
  • Non-specific signs and symptoms
    • Classic triad – fever, headache and rash
    • Rash typically appears on the second or third day of illness
      • Rash begins as macules on the wrists, palms, ankles and soles of feet, which then results in petechiae form
      • Rash finally spreads to the trunk
    • Rash is hallmark of infection but usually follows systemic symptoms; its absence should not rule out a possible rickettsial etiology
  • Other symptoms include malaise, myalgias, vomiting and photophobia
    • Mild pulmonary involvement, manifested by cough and infiltrates, is found in about one-third of patients with RMSF
  • Neurologic (25% of patients)
    • Meningitis
    • Meningoencephalitis
    • Seizures
    • Confusion
    • Focal neurologic deficits
  • Cardiovascular
    • Myocarditis
    • Pericarditis
  • Mortality is dependent on cardiac and central nervous system involvement or delay in treatment

Treatment

  • Initiation of early antibiotic therapy is necessary to reduce mortality
    • Decision to treat should not be delayed until lab confirmation of organism