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.

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.