Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF) is a severe tick borne rickettsial infection caused by Rickettsia rickettsii, which infects vascular endothelium.

• Vectors: Dermacentor variabilis, D. andersoni, Rhipicephalus sanguineus (brown dog tick, Arizona, Mexico), and Amblyomma cajennense (South America).
• Peak in spring and summer; incubation is 3 to 12 days.
• Case fatality is 5 to 10% with treatment delay and 20 to 25% untreated; highest in children, elderly, and in the setting of delayed doxycycline beyond day 5.

• Fever, severe headache, myalgia, nausea, vomiting, and abdominal pain.
• Rash 2 to 5 days after onset: blanching macules on wrists and ankles spreading centripetally and to palms and soles, evolving into petechiae.
• Late complications: encephalopathy, seizures, pulmonary oedema (non-cardiogenic), acute kidney injury, DIC, gangrene of digits, and hearing loss.
• Hyponatraemia, thrombocytopenia, and raised transaminases.

• Empirical treatment on clinical suspicion; do not wait for confirmation.
• PCR of skin biopsy of rash (sensitivity about 70%) or whole blood, immunohistochemistry on biopsy, and paired serology (IFA IgG fourfold rise after 2 to 4 weeks).
• Blood: thrombocytopenia, hyponatraemia, elevated AST and ALT, CK, and creatinine.
• Exclude meningococcaemia, dengue, measles, typhoid, ehrlichiosis, TTP, Kawasaki disease, and secondary syphilis.

• Doxycycline 100 mg every 12 hours orally or IV (2.2 mg/kg twice daily for children under 45 kg) for at least 5 to 7 days, and at least 3 days after fever resolution (and clinical improvement).
• Doxycycline is the drug of choice in all ages, including pregnancy when the risk of untreated disease outweighs risks; chloramphenicol (50 to 75 mg/kg/day in 4 doses) is a poor alternative associated with higher mortality.
• Supportive care in ICU: fluids, correction of hyponatraemia, platelet and plasma transfusion for DIC, mechanical ventilation, and renal replacement.
• Prevent through tick checks, repellents (DEET, permethrin), and prompt removal.
• Report cases to public health authorities.