Rocky Mountain Spotted Fever (RMSF)
Rocky Mountain Spotted Fever is a reportable disease in Oklahoma. Rocky Mountain Spotted Fever (RMSF) is a disease caused by the bacteria Rickettsia rickettsii (R. rickettsii). In Oklahoma, the tick most closely associated with RMSF is the American dog tick (Dermacentor variabilis). The other known tick vectors of R. rickettsii include the Rocky Mountain wood tick (Dermacentor andersoni), brown dog tick (Rhipicephalus sanguineus), and the cayenne tick (Amblyomma cajennense). Humans nearly always become infected with the bacteria following the bite of an infected tick. Tick bites due to the American dog tick are common in Oklahoma, but fortunately only 1–3% of the tick population is infected with R. rickettsii and capable of transmitting RMSF.
At least 4–6 hours of tick attachment and feeding are usually required to pass on the infection to humans. The symptoms of RMSF usually begin 3–14 days following the infected tick bite. There is usually a sudden onset of fever, fatigue, headache, and muscle soreness. A fine, bumpy red rash often appears on the extremities about 3–5 days after the fever, which may quickly spread to the palms, soles, and trunk. However, not all persons with RMSF develop a rash. Other common symptoms of RMSF include nausea, vomiting, abdominal pain, and red eyes.
Diagnosis is based upon clinical signs and symptoms, but laboratory testing can also be used to help diagnose RMSF. Blood samples taken in the early stage of illness (acute phase) and follow up samples taken 2–4 weeks later (convalescent phase) are used to confirm cases of RMSF through serologic testing. However, laboratory testing can take days to weeks for results to become available, so treatment should never be delayed while awaiting laboratory confirmation.
The first line treatment for all cases of RMSF is antibiotics (doxycyline). Treatment should be started immediately whenever RMSF is suspected because it can be a very serious disease; overall, 3–5% of all persons with RMSF die, but death is uncommon with prompt diagnosis and treatment. Factors associated with worse outcomes include delayed antibiotic therapy and age over 40 years. Although RMSF can be severe, preventive antibiotic therapy in non-ill persons following a tick bite is not recommended.
