DESCRIPTION

• Potentially 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 throughout the year
• Classic symptoms of fever, headache, and rash following tick exposure may not be present.

EPIDEMIOLOGY

• Most common rickettsial disease in the United States
• Seasonal disease with 90% of cases occurring between April and September
• Geographic
  • Contrary to its name, less often seen in the Rocky Mountain states
  • In the United States, highest incidence is in the mid-Atlantic and south-central regions
  • North Carolina, Tennessee, Arkansas, Oklahoma, and Missouri account for >60% of cases in the United States.
  • Cases have been reported in all states except Alaska and Hawaii.
  • Also occurs in southern Canada, Mexico, Central America, and South America
• Cases have been consistently increasing over the last 20 years. Increase in incidence and decrease in case fatality may be due to changes in reporting, diagnostic abilities, and possibly climate change.
• There is a higher reported incidence in males and Native Americans.
• Incidence is highest in adults >40 years old.
• Among pediatric patients, the highest incidence age range is 5 to 9 years.
  • Children represent 6% of infections but 22% of deaths.
  • Children <10 years old are 5 times more likely to have fatal infection than adults.

ETIOLOGY

R. rickettsii is transmitted by the following tick species in various areas:

• American dog tick (Dermacentor variabilis): Midwest, East, and Southern United States, as well as areas of the Pacific coast
• Rocky Mountain wood tick (Dermacentor andersoni): Rocky Mountain states and southwest Canada
• Brown dog tick (Rhipicephalus sanguineus): Arizona and northern Mexico
• Amblyomma cajennense and Amblyomma aureolatum: Central and South America

RISK FACTORS

• R. rickettsii–infected tick exposure
• Environment or occupation with increased forest exposure in endemic region
• Fatal outcomes are significantly higher with delayed treatment.
• Geographic variations in case fatality occur, which may be due to different levels of pathogenicity, host factors, or delayed recognition in less endemic regions.
• Higher fatality rates in children <4 years old and in patients with glucose-6-phosphate-dehydrogenase deficiency (G6PD), alcohol abuse, or delayed antibiotic treatment

GENERAL PREVENTION

• Avoid tick-infested areas; limit skin exposure with long, light-colored clothing, tucked-in socks, or boots; inspect skin and clothing frequently.
• Shower and complete skin check for ticks soon after returning indoors.
• Wash and dry clothes on hot setting to kill ticks soon after returning indoors.
• Examine pets who may bring ticks indoors after in woods or grassy areas.
• Tick repellants or impregnated clothing
  • N,N-Diethyl-meta-toluamide (DEET) applied directly to skin is one of the most effective tick repellants and is safe for children >2 months of age.
  • Permethrin impregnated clothing is also effective in preventing tick bites.
  • Natural alternatives (lemon eucalyptus, citronella, soybean, clove) are generally considered safe and may be better tolerated, but the effectiveness is likely lower.
• Remove ticks promptly.
  • Do not crush; may increase transmission.
  • Avoid direct contact; remove with tweezers or gloved fingers.
  • Grasp tick close to skin surface and apply steady upward traction until the tick’s grip is released.
  • Clean the wound.
  • Matches, petroleum jelly, nail polish, and rubbing alcohol are not effective for removal.
• There is no vaccine against R. rickettsia.
• Prophylactic antibiotics after tick bite are not recommended (treatment is only indicated if symptoms develop).
• There is no evidence of human-to-human transmission (other than via blood transfusion). Quarantine or isolation of infected individuals is not required.

PATHOPHYSIOLOGY

• Transmission occurs from tick bite (tick is both vector and reservoir of disease).
  • Minimum of 6 to 10 hours of attachment is needed to transmit disease.
  • Can rarely occur by blood transfusion, contact with crushed tick, or aerosol route
• Incubation period of 3 to 12 days, with average of 7 days
• R. rickettsii causes a small vessel vasculitis with resulting increased vascular permeability and focal areas of direct endothelial damage that impacts all organs.
• Can cause associated hyponatremia, edema, and hypotension
• Immunity is conferred following disease.

COMMONLY ASSOCIATED CONDITIONS

Patients with G6PD deficiency account for a disproportionate number of deaths.