RMSF is only found in the western hemisphere. In the United States, the areas of greatest incidence are the southeastern and south central regions of the country. Nationwide, the average annual incidence increased during the last decade, from less than 2 cases per million persons in 2000 to over 6 cases per million in 2010, with large variability between years. It is likely that many more cases go unreported. In Florida, the reported incidence has increased markedly in recent years, possibly to increased disease awareness and reporting. 163 cases of RMSF were reported from 2002 through 2011. Of these, 77% were acquired in Florida and the rest were acquired while the person was traveling in another state or country. Of the infections acquired in Florida, the majority were reported from counties in the northern and central regions of the state. In Florida, cases of RMSF/SFR are reported year-round without distinct seasonality, though peak transmission typically occurs during the summer months.
Symptoms typically begin 2 to 14 days after a tick bite, and can initially include fever, headache, vomiting, muscle aches, and lack of appetite. Later signs include stomach pain, joint pain, and diarrhea. The characteristic red non-itchy rash may first appear 2-5 days after onset of fever on the wrists and ankles, then progress to the palms and soles, then the arms, legs, and trunk. However, a rash appears in 90% of RMSF cases; some people experience no rash at all.
RMSF can be difficult to diagnose because many of the symptoms are non-specific. It is important to tell the physician about any tick bites that have occurred. Cases detected early in the course of disease can be easily treated with antibiotics. If a physician thinks that a person has RMSF, it is recommended that treatment begin right away, without waiting for laboratory results. Cases detected later in the course of illness are more likely to be severe and require hospitalization. In the past, the disease would kill up to 30% of those infected. With current treatment, approximately 0.5% of the people who become ill with RMSF die from the disease. The elderly, American Indians (particularly men), and chronic alcohol abusers may be at increased risk for severe disease.
Studies have shown that the tick needs to be attached to a person for at least 6-20 hours to effectively transmit the infection, so it is important to regularly check for ticks and remove them right away.
In the southern United States, other rickettsial organisms have been identified that cross-react with tests for Rickettsia rickettsii. These include Rickettsia parkeri and Rickettsia ambyommi. It is speculated that some of the less severe cases of RMSF may be caused by one of these organisms.
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