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  • Exposure to tick bite in an endemic area.

  • Influenza-like prodrome followed by fever, severe headache, and myalgias; occasionally, delirium and coma.

  • Red macular rash appears between days 2 and 6 of fever, first on the wrists and ankles and then spreading centrally; it may become petechial.

  • Mortality over 70% in untreated patients.

  • Serial serologic examinations by indirect fluorescent antibody confirm the diagnosis retrospectively.

General Considerations

Despite its name, most cases of Rocky Mountain spotted fever (RMSF) occur outside the Rocky Mountain area. More than half of the cases are from five states: North Carolina, Tennessee, Oklahoma, Missouri, and Arkansas (eFigure 32–6). RMSF is endemic in Central and South America with a small fatal familial cluster reported from Panama (Table 32–3). Native Americans are at high risk for infection. The causative agent, R rickettsii, is transmitted to humans by the bite of ticks, including the Rocky Mountain wood tick, Dermacentor andersoni, in the western United States, and the American dog tick, D variabilis, in the eastern United States. Several hours of contact between the tick and the human host are required for transmission. The brown dog tick, Rhipicephalus sanguineus, is a vector in eastern Arizona and responsible for many Native American cases. Epidemic RMSF, as described in Arizona and Mexico, is associated with massive local infestations of the brown dog tick in domestic dogs, which may explain why the incidence of RMSF in the three most highly affected communities in an Arizona epidemic from 2003 to 2012 was 150 times the US national average. Recent studies show that the seroprevalence for dogs on the Arizona Mexican border is lower than anticipated at about 5% seroprevalence for spotted fever group rickettsial antibodies.

eFigure 32–6.

Annual incidence (per million persons) for spotted fever rickettsiosis (SFR) in the United States, 2018. (

Other hard-bodied ticks transmit the organism in the southern United States and in Central and South America and are responsible for transmitting it among rodents, dogs, porcupines, and other animals. Human cases reemerged in Mexico in 2008 after decades of quiescence (since the 1940s). As of 2018, northern Mexico reported nearly 4000 RMSF cases.

There are 25 genotypes of R rickettsii in four different groups, and potential genomic-clinical correlations are underway. Several other rickettsial species cause mild, nonlethal infections in the United States, including R parkeri, R phillipi, and R massiliae. These are discussed in the “tick typhus” section.

In the United States, the estimated annual incidence of RMSF is as high as seven cases per million persons (primarily occurring from April through September), with a higher incidence among children and men. Better diagnostic capacity and improved surveillance are thought responsible for the changing epidemiology.

A Brazilian spotted fever with higher ...

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