Rickettsia rickettsii - Rocky Mountain Spotted Fever

Rickettsia rickettsii - Rocky Mountain Spotted Fever

 

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

Epidemiology

  • Incidence – 3-5/1,000,000
  • Age – greatest incidence in children <10 years old
  • Transmission
    • Via Dermacentor and Amblyomma spp ticks in the U.S.
    • Humans are accidental hosts
    • 95% of the cases occur April through September

Organism

  • Gram-negative coccobacilli which are obligate intracellular organisms
  • A characteristic feature of the Rickettsiae is that they multiply in an arthropod as part of their life cycle
  • With spotted fever, the 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)
    • Neorickettsiasis
    • Q-Fever

Risk Factors

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

Clinical Presentation

  • The incubation period between tick bite and onset of symptoms is 2-6 days
  • Rocky Mountain Spotted Fever is difficult to differentiate from viral illness
  • Non specific signs and symptoms
    • Fever, headache and rash are the classic triad
  • Rash typically appears on the second or third day of illness
    • Rash begins as macules on the wrists, palms, ankles and soles of feet, then petechiae form
    • Rash finally spreads to the trunk
    • Rash is a hallmark of infection, but it usually follows systemic symptoms; its absence should not rule out a possible rickettsial etiology
  • Onset of disease is sudden in about half of the cases
  • 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 Rocky Mountain Spotted Fever
  • Serious central nervous system impairment seen in 25% of patients
  • Mortality is dependent on cardiac and central nervous system involvement or delay in treatment

Diagnosis

  • Diagnosis is made based upon clinical grounds, symptoms and/or serology and history of tick exposure
  • The best evidence for infection is a significant change on 2 appropriately timed specimens where both tests are done in the same laboratory at the same time
    • Appearance of an IgM antibody response normally occurs 7-14 days after the onset of disease

Differential Diagnosis

  • Lyme disease
  • Human monotropic ehrlichiosis
  • Human granulocytic anaplasmosis
  • Parvovirus B19
  • Epstein-Barr virus
  • Disseminated gonococcus
  • Leptospirosis
  • Secondary syphilis
  • Kawasaki disease
  • Mycoplasma pneumoniae

Treatment

  • Initiation of early antibiotic therapy is necessary to reduce mortality

See Also