Chapter 294

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

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).

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.

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 ...

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