Rickettsiae

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           Diseases In the United States, there are two rickettsial diseases of significance: Rocky Mountain spotted fever, caused by Rickettsia rickettsii, and Q fever, caused by Coxiella bumetii.

Several other rickettsial diseases such as epidemic, endemic, and scrub typhus are important in developing countries.

The Weil-Felix test, which detects antiricketttial antibodies in a patient's serum by agglutination of the Proteus organisms, is based on this cross-reaction.

Transmission The most striking aspect of the life cycle of the rickettsiae is that they are maintained in nature in certain arthropods such as ticks, lice, fleas, and mites and, with one exception, are transmitted to humans by the bite of the arthropod.

Virtually all rickettsial diseases are zoonoses (ie, they have an animal reservoir), with the prominent exception of epidemic typhus, which occurs only in humans.

The incidence of the disease depends on the geographic distribution of the arthropod vector and on the risk of exposure, which is enhanced by such things as poor hygienic conditions and camping in wooded areas.

Pathogenesis The typical lesion caused by the riekettsiae is a vasculitis, particularly in the endothelial lining of the vessel wall where the organism is found.

The typical rash, which appears 2-6 days later, begins with macules that frequently progress to petechiae.

The diagnosis must be made on clinical grounds and therapy started promptly, because the laboratory diagnosis is delayed until a rise in antibody liter can be observed.

The name of the disease is misleading, because it occurs primarily along the East Coast of the United States, where the dog tick, Dermacentor variabilis, is located.

The name "Rocky Mountain spotted fever" is derived from the region in which the disease was first found.1 The tick is an important reservoir of R rickettsii as well as the vector; the organism is passed by the transovarian route from tick to tick, and a lifetime infection results.

Frequent examinalion of Ihe skin for ticks is important in preventing Rocky Mountain spotted fever; the tick must be atlached for several hours to transmit the disease.