Rocky Mountain Spotted Fever

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Basics

Description

  • Life-threatening, small vessel vasculitis
  • Caused by infection with Rickettsia rickettsii, an obligate intracellular gram-negative coccobacillus, predominantly transmitted by three species of ticks in the United States
  • Member of spotted fever subgroup of rickettsial diseases
  • Seasonal endemic disease but may occur in other areas and throughout the year
  • Classic symptoms of fever, headache, and rash following tick exposure are often not present.

Epidemiology

  • Most common rickettsial disease in the United States
  • Seasonal: April to September accounts for 90% of cases.
  • Geographic
    • Restricted to countries of Western Hemisphere
    • Cases reported from all states except Alaska and Hawaii
    • Occurs most often in mid-Atlantic and south central regions: 1994 to 2003, >50% of cases in North Carolina, South Carolina, Tennessee, Oklahoma, Arkansas
    • Less often seen in Rocky Mountain states
    • Also occurs in southern Canada, Mexico, and Central and South America
  • Single isolated cases most common in United States; clusters are reported infrequently in United States (4.4% familial) but are more typical in certain endemic areas (e.g., Brazil)
  • Up to 2/3 of patients are <15 years old.

Incidence

  • Annual incidence: 7 cases per million people (2002 to 2007); the highest recorded level in >80 years of national surveillance
  • The recent increase in incidence and decrease in case fatality may be due to changes in reporting, diagnostic abilities, and possibly climate change.
  • Cyclic (every 30 to 40 years) fluctuations in incidence; 250 to 1,200 cases reported per year
  • More often reported in Native American, whites, males, and children; incidence highest in 5- to 9-year-olds
  • Fatal outcome reported in 23% of untreated and 5% of treated cases
  • Geographic variations in case fatality occur, likely due to different levels of pathogenicity, host factors, and delayed recognition in less endemic regions.
  • 15% reported deaths in children <10 years of age

Prevalence

4–22% of children show significant antibody titers in endemic areas, likely representative of subclinical disease.

Risk Factors

  • R. rickettsii–infected tick exposure
  • Environment or occupation with increased forest exposure in endemic region

General Prevention

  • Avoid tick-infested areas; limit skin exposure with long, light-colored clothing, tucked-in socks, or boots; inspect frequently.
  • Use tick repellants or impregnated clothing.
    • N,N-Diethyl-meta-toluamide (DEET) most effective
    • Essential oils that offer natural alternatives considered safe (soybean, lemon eucalyptus, citronella, and clove)
  • Remove ticks promptly.
    • Do not crush; may increase transmission
    • Avoid direct contact; remove with tweezers or gloved fingers close to skin.
    • Apply steady upward traction until tick’s grip is released.
    • Clean wound.
    • Matches, petroleum jelly, nail polish, and rubbing alcohol are not effective for removal.
  • Vaccine not available in the United States; may not prevent disease but does prevent deaths

Pathophysiology

  • Transmission usually occurs from tick bite (reservoir):
    • Usually >4 hours of attachment needed to transmit disease (often 24 hours)
    • Can occur by transfusion or aerosol route
  • Incubation period 2 to 14 days, average 7 days
  • R. rickettsii spreads through the lymphatic system, causing a small vessel vasculitis that affects all organs, especially skin and adrenals; increased vascular permeability and focal areas of endothelial proliferation
  • Causes hyponatremia, hypoalbuminemia, edema, and hypotension
  • Immunity is conferred following disease.

Etiology

Wood tick (Dermacentor andersoni) in Rocky Mountain States and southwest Canada; dog tick (Dermacentor variabilis) in east central region and areas of Pacific coast; Rhipicephalus sanguineus in Arizona and Northern Mexico; Amblyomma cajennense and Amblyomma aureolatum in Central and South America

Commonly Associated Conditions

  • Patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency account for a disproportionate number of deaths.
  • Serious biologic weapon threat due to virulence causing severe disease; difficulty establishing diagnosis; low levels of immunity; agent available in nature; high infectivity; and feasibility of propagation, stabilization, and dispersal; thus, development of a cross-protective vaccine against all Rickettsia is desirable for biodefense as well as for travel medicine.

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