Rocky Mountain Spotted Fever
Rocky 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.
Dermacentor variabilis
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 Treatment
Symptoms 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 Resources
Centers for Disease
Control and Prevention (CDC) Rocky Mountain Spotted Fever
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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