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Rickettsiae are obligate intracellular bacteria; that is, they can grow only within cells. They are the agents of several important diseases, namely typhus, spotted fevers such as Rocky Mountain spotted fever, Q fever, ehrlichiosis, and anaplasmosis. Other less important rickettsial diseases such as endemic and scrub typhus occur primarily in developing countries.

Outbreaks of endemic typhus occur among the homeless in large cities in the United States. This disease is caused by Rickettsia typhi. The organism is transmitted by fleas with rats as the reservoir. Rickettsial pox, caused by Rickettsia akari, is a rare disease found in certain densely populated cities in the United States.

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 607.



Rickettsia rickettsii causes Rocky Mountain spotted fever, a life-threatening disease that occurs primarily in the Southeastern states, for example, North Carolina of the United States. Rickettsia prowazekii causes epidemic typhus, also a life-threatening disease that occurs mainly in crowded, unsanitary living conditions during wartime.

Important Properties

Rickettsiae are very short rods that are barely visible in the light microscope. Structurally, their cell wall resembles that of gram-negative rods, but they stain poorly with the standard Gram stain.

Rickettsiae are obligate intracellular parasites, because they are unable to produce sufficient energy to replicate extracellularly. Therefore, rickettsiae must be grown in cell culture, embryonated eggs, or experimental animals. Rickettsiae divide by binary fission within the host cell, in contrast to chlamydiae, which are also obligate intracellular parasites but replicate by a distinctive intracellular cycle (see Chapter 25).

Several rickettsiae, such as R. rickettsii, R. prowazekii, and Rickettsia tsutsugamushi (renamed Orientia tsutsugamushi), possess antigens that cross-react with antigens of the OX strains of Proteus vulgaris. The Weil-Felix test, which detects antirickettsial antibodies in a patient’s serum by agglutination of the Proteus organisms, is based on this cross-reaction.


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. The rickettsiae circulate widely in the bloodstream (bacteremia), infecting primarily the endothelium of the blood vessel walls.

The exception to arthropod transmission is Coxiella burnetii, the cause of Q fever, which is transmitted by aerosol and inhaled into the lungs (see later). Virtually all rickettsial diseases are zoonoses (i.e., they have an animal reservoir), with the prominent exception of epidemic typhus, which occurs only in humans. It occurs only in humans ...

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