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Rickettsial infections are caused
by pleomorphic gram-negative organisms that contain both DNA and
RNA. They are obligate intracellular parasites, have typical bacterial
cell walls and cytoplasmic membranes, and divide by binary fission.
The taxonomy of rickettsiae continues to undergo an extensive reorganization.
The order Rickettsiales has changed and now includes only
two families, the Anaplasmataceae and the Rickettsiaceae.1,2 Three
groups of disease are still commonly classified as rickettsial diseases:
(1) the spotted fever and the typhus group from the family Rickettsiaceae,
(2) ehrlichioses and anaplasmoses from the family Anaplasmataceae,
and (3) scrub typhus (Orientia tsutsugamushi).3
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Rickettsial infections have many features in common, including
multiplication of the organism in an arthropod host; geographic
and seasonal occurrences that are related to the arthropod life
cycle, activity, and distribution; zoonotic illnesses with humans
as incidental hosts (except for louse-borne typhus); and fever,
rash (except some cases of ehrlichiosis and anaplasmosis), headache,
myalgias, and respiratory tract symptoms. These organisms produce
a vasculitis following replication within the endothelial lining and
smooth-muscle cells of blood vessels leading to generalized capillary
and small-vessel endothelial damage, increased vascular permeability, thrombus
formations, and tissue necrosis.4,5 This process
consumes platelets and results in the characteristic thrombocytopenia.
Many of the other initial symptoms and signs are referable to this
pathogenesis, which can affect any organ system. Although the thrombus
mediated vascular occlusion that occurs may play a role in severe rickettsial
infections, disseminated intravascular coagulation (DIC) occurs
rarely.5 Hyponatremia, which is another laboratory
hallmark of many rickettsial infections, is the result of initial
active secretion of salt into renal tubules. Subsequently, the syndrome
of inappropriate production of antidiuretic hormone (SIADH) can
further aggravate the hyponatremic state. Organisms from the typhus
(except scrub typhus) and spotted fever groups contain endotoxins,
and most will survive only briefly outside of a host (reservoir
or vector).
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The epidemiology, clinical manifestations and outcomes of each
rickettsial disease are discussed below. Because diagnostic methods and
treatment are similar for most rickettsial disorders, these are
discussed at the end of the chapter.
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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 ...