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1. ROCKY MOUNTAIN SPOTTED FEVER

ESSENTIALS OF DIAGNOSIS

  • 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

Rocky Mountain spotted fever (RMSF) is caused by R rickettsii and is endemic in parts of the Americas (Table 32–3). In the United States, the numbers of RMSF cases have increased over the last two decades, peaking in 2017 at 6248. Despite its name, most cases of RMSF occur outside the Rocky Mountain area. More than half of US cases are from five states: North Carolina, Tennessee, Oklahoma, Missouri, and Arkansas (eFigure 32–6). Human cases reemerged in northern Mexico in 2008 after decades of quiescence (since the 1940s). As of 2018, northern Mexico reported nearly 4000 RMSF cases.

eFigure 32–6.

Annual incidence (per million persons) for spotted fever rickettsiosis (SFR) in the United States, 2018. (https://www.cdc.gov/rmsf/stats/index.html#anchor_1531851146113)

R rickettsii is transmitted to humans by the bite of ticks. Several hours of contact between the tick and the human host are required for transmission. Ticks that can transmit the infection include the Rocky Mountain wood tick, Dermacentor andersoni, in the western United States, and the American dog tick, D variabilis, in the eastern United States. 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. 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.

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.

A Brazilian spotted fever with higher mortality than RMSF is thought to be due to a virulent strain of R rickettsii. A host of spotted fever species have been identified from human patients over the last 20 years throughout the world including species from China (Rickettsia sp. XY99), Slovakia (R slovaca), Morocco (R aeschlimannii...

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