From 1999 to 2006 there was a mean of 1237 cases per year (range:
495–2236) of Rocky Mountain spotted fever (RMSF) reported
in the United States.7,8 Of these cases, approximately 90% occur
between April and September. Approximately two thirds of the cases
occur in children under age 15 years, with the highest age-specific
incidence occurring between ages 5 and 9 years.9 Although
the disease is rare in infants, RMSF has been described to occur
in more than one family member at the same time. The geographic
connotation of its name notwithstanding, RMSF is endemic to much
of the western hemisphere, including Mexico, Costa Rica, Argentina,
Panama, Colombia, and the continental United States.10-14 In
the United States, RMSF is most common in the south Atlantic states
(eg, North Carolina, South Carolina, Georgia, Virginia, and Maryland),
as well as Tennessee, Mississippi, Missouri, Arkansas, and Oklahoma.
Ticks that transmit the disease vary by region. In the western United
States, wood ticks (Dermacentor andersoni) are
primary carriers and vectors of infection, whereas in the eastern
United States, the dog tick (D. variabilis), and
in the south-central region, the Lone Star tick (Amblyomma
americanum), represent the most common arthropod hosts.1 Even
in areas where most human cases are reported, only approximately
1% to 3% of the tick population will carry the
causative agent, Rickettsia rickettsii, and very
few humans actually become infected despite suffering a tick bite.9 Ticks become
infected by feeding on the blood of infected animals, through fertilization,
or by transovarial passage. Once attached, the infected tick is
able to transmit the disease to humans during feeding. After attachment
to humans has occurred (6–24 hours), rickettsiae are released
from the salivary glands and multiply in the endothelial cells lining
the small blood vessels, resulting in cell damage. For all age groups,
reported risk factors include exposure to dogs, residence in a wooded
area, and male sex.