Rocky Mountain Spotted FeverRocky Mountain spotted fever (RMSF) is caused by the bacterium Rickettsia rickettsii and in Florida is transmitted primarily by the American Dog Tick (Dermacentor variabilis). Ticks become infected either by feeding on rickettsemic small mammals that amplify the bacteria or transovarially (from infected female to progeny). The tick can then infect a human or animal during a subsequent feeding. Humans are considered accidental hosts and are not part of the natural transmission cycle. Dogs are also susceptible to infection.
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 was 2.2 cases per million people, 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. 154 cases of RMSF were reported from 1999 through 2008 (19 in 2008). Of these, 70% were acquired in Florida and the rest were acquired while the person was traveling in another state or country. Over half (71%) the cases were male, and most of the cases were also white (81%). Of the infections acquired in Florida, the majority were reported from counties in the northern and central regions of the state. The number of cases increased during the summer months, though cases are reported in Florida year-round. Symptoms and TreatmentSymptoms typically begin 5 to 10 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 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 only appears in 60 to 70% 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 87% of those infected. With current treatment, approximately 3% to 5% of the people who become ill with RMSF die from the disease. The elderly, African-Americans (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. More information is available at our R. parkeri page. Prevention of Tick-borne Diseases Other ResourcesCenters for Disease Control and Prevention (CDC) Rocky Mountain Spotted Fever Warner RD, Marsh WW. Zoonosis Update: Rocky Mountain Spotted Fever. JAVMA 2002. http://www.avma.org/reference/zoonosis/znrockymountain.asp Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever, Ehrlichioses, and Anaplasmosis - United States (PDF 1.1 MB)http://www.cdc.gov/mmwr/PDF/rr/rr5504.pdf Chen L, Sexton D. What's new in Rocky Mountain Spotted Fever? Infect Dis Clin N Am. 2008; 22: 415-432 Dantas-Torres F. Rocky Mountain Spotted Fever. Lancet Infect Dis. 2007; 7: 724-32.
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