We usually treat RMSF with doxycycline for 7 days ( or at least 2 days after fevers resolved).
Rash may take a week or two to go away. One thing I cannot know for sure is if the rash you describe is from your infection- or a possible drug allergic reaction- It is not clear if you had the rash before you started the doxycycline- If this is not the case, and the rash only started when you took the medication, and it keeps worsening, I would suggest seeing your doctor again as he may stop or change antibiotics.
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