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clinical:
| • | abrupt onset fever, chills, headache, myalgias (triad = fever, myalgia, headache) one week after tick bite |
| • | then (3-4 days later) rash at wrist, ankles and forearms |
| • | bilateral palpable purpura on palms and soles is RMSF until proven otherwise |
| • | spreads centrally and develops petechiae and purpura |
epidemiology:
| • | endemic geographic area (Texas, Oklahoma, south atlantic states) |
| • | during summer (95% of cases April 1 to September 30) |
| • | organism = Rickettsia rickettsii |
| • | vector: Eastern US = Dermacantor variabilis (the American dog tick) |
| • | vector: Western US = Dermacantor andersoni |
| • | tick must be attached 6 or more hours to transmit disease |
| • | the organisms invade endothelial cells and cause vasculitis and the characteristic petechial rash |
differential diagnosis:
| • | parvovirus B19 (stocking and glove purpura); |
| • | atypical measles and echo 9 (bilateral symmetric macular purpura) |
| • | 1963-67 killed measles vaccine (vs. RMSF – only measles has pulmonary symptoms) |
| • | treat while waiting for measles complement titers because mortality rate 10-15% |
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diagnosis:
| • | Weil-Felix test: increase from acute to serum convalescent titers |
| • | more rapid diagnosis = skin biopsy for immunofluorescence to identify the causative organism in the walls of cutaneous blood vessels |
treatment:
| • | doxycycline 100mg PO BID X 7 days (unless pregnant or child <8 years old) |
| • | chloramphenicol 500mg PO QID X 7 days |
| • | 100mg BID is effective against each of the "big three" |
| • | Lyme disease (if < 8 years old, then amoxicillin) |
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