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 - peak incidence in children 5-9 years old
• Transmission
  • Via Dermacentor and Amblyomma spp 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 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)
  • Neorickettsiosis
  • 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 impairments seen in 25% of patients
• 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

Diagnosis

• Indications for testing - flu like illness with or without rash in association with tick bite exposure
• Laboratory testing
• Diagnosis is made based upon clinical grounds, symptoms and/or serology and history of tick exposure
• Antibody testing
  • 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
    • Negative initial IgM does not exclude the disease

• Febrile Antibody testing
  • More specific than Weil Felix but still has cross-reactivity with Brucella and Salmonella
  • Must be used in conjunction with clinical presentation
• Weil-Felix - too nonspecific to be used in diagnosis
• Culture - not routinely available because of biosafety issues (research labs only)

Differential Diagnosis

• Dengue
• Disseminated gonococcus, meningococcus
• Epstein-Barr virus
• Human granulocytic anaplasmosis
• Human monotropic ehrlichiosis
• Kawasaki disease
• Leptospirosis
• Lyme disease
• Mycoplasma pneumoniae
• Parvovirus B19
• Rickettsia typhi
• Secondary syphilis
• Vasculitis