• Rickettsia rickettsii, obligate intracellular Gram-negative bacterium, infected tick vector (within 6-10 hours of attachment); infects endothelial cells causing vascular injury, disseminates via blood leading to secondary end-organ damage; American dog tick (eastern and south central US), brown dog tick (southwestern US, Mexico), and Rocky Mountain wood tick (western mountain states); North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri account for >60% of cases; rural environments, April to September, occasionally fall and winter.

Symptoms develop 2 to 14 days after tick bite, may be abrupt or gradual in onset. Classic triad of fever, headache, and rash; initial symptoms are fever and headache; nausea, malaise, fatigue, myalgia, and arthralgia; sometimes acute abdominal pain.

• Skin: Day 3 to 5 ~70% to 90% of patients will have pink erythematous 1- to 4- mm macules progressing to red purpuric papules (palpable purpura), starts on the ankles/wrists, spreads toward trunk, and then palms and soles (centripetal spread). Rare necrosis and eschars on digits, scrotum, and earlobes; gangrene possible; children often have peripheral and periorbital edema, conjunctival injection; ~10% have “spotless” Rocky Mountain spotted fever; rash may not be obvious in darker skin types, which may delay in diagnosis.
  
• GI: Hepatosplenomegaly.
  
• GU: Acute renal failure.
  
• Neuro: Seizures, altered mental status.

Complications include sepsis and multi-organ failure.

Serology/indirect fluorescent antibody (sensitivity: 95%): Four-fold increase in IgG titer in acute and convalescent serum (5 days and 14-21 days after symptom onset); IgM is not diagnostic because of cross-reactivity with lipopolysaccharide of bacterial pathogens. If no acute serum, then IgG titers above 1:640 are diagnostic if obtained 2 weeks after symptom onset.

Skin biopsy before or within 12 hours of antibiotics (sensitivity: 70%-90%). Direct immunofluorescence testing or immunoperoxidase staining.

Polymerase chain reaction of blood. Low sensitivities.

Complete blood cell count shows a normal white blood cell at presentation and thrombocytopenia as disease progresses.

Possible prolonged prothrombin time/partial thromboplastin time, abnormal liver function tests, hyponatremia, azotemia, and acute renal failure.

Doxycycline (regardless of age) 100 mg po bid until patient has been afebrile for at least 3 days.

Chloramphenicol is second line; start immediately on clinical suspicion while testing is pending, particularly in areas where incidence is highest; supportive care depending on the sequelae.