Rocky Mountain Spotted Fever
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- 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.
- Most common rickettsial disease in the United States
- Seasonal: April to September accounts for 90% of cases.
- 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.
- 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
4–22% of children show significant antibody titers in endemic areas, likely representative of subclinical disease.
- R. rickettsii–infected tick exposure
- Environment or occupation with increased forest exposure in endemic region
- 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
- 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.
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.