Pathophysiology
1) tick-borne disease caused by Rickettsia rickettsii 2) tick vectors are Dermacentor variables (dog tick) and D. andersoni (wood tick)
Signs and Symptoms
Initial presentation – 1) fever 2) headache 3) myalgia 4) nausea/vomiting Rash – 5) develops 3-4 days after onset of symptoms 6) starts as pink flat macules on wrists and ankles and spreads to proximal arms, legs, and torso and then palms and soles 7) becomes hemorrhagic, petechial, and nonblanching Untreated – 8) delirium 9) seizures 10) coma 11) photophobia
Characteristic Test Findings
Laboratory – 1) thrombocytopenia 2) anemia 3) hypoalbuminemia 4) increased AST and ALT 5) hyponatremia 6) increased acute-phase reactants 7) increased titers on indirect immunofluoresecence assay 8) increased CSF cell count and protein
Histology/Gross Pathology
1) gram-negative cell wall 2) increased vascular permeability with edema, hypovolemia, and ischemia 3) Rickettsia organisms infect vascular endothelial cells
Associated Conditions
1) hypotension 2) pulmonary edema 3) encephalitis/cranial nerve palsies/confusion 4) bleeding 5) cardiac arrhythmias 6) conjunctivitis 7) fulminant course if patient also has G6PD deficiency
Inheritance/Epidemiology
1) occurs in all 50 states 2) high incidence in children but high mortality in elderly 3) 30% mortality in untreated cases
Treatment
doxycycline is drug of first choice
Tips for USMLE
1) must have high degree of suspicion to correctly diagnose 2) patients often do not remember tick bite 3) rash often misdiagnosed as measles or rubella 4) lesions if present spread centrpetally 5) 10-15% of cases do not have rash, so lack of rash does not rule out the disease
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