any of a group of small bacteria that commonly multiply only inside living cells and that include a number of species causing disease in humans or animals. The term “rickettsia” is often used to refer particularly to microorganisms belonging to the tribe of bacteria known as Rickettsieae, typically regarded as including the genera Coxiella, Orientia, Rickettsia, and Rochalimaea. But the word is also sometimes applied to any bacteria belonging to the family Rickettsiaceae (which contains the Rickettsieae) or to the order Rickettsiales (which contains the Rickettsiaceae). The word comes from the name of the American pathologist Howard Taylor Ricketts (1871–1910), who in 1908 identified the microorganism that causes Rocky Mountain spotted fever; that bacterium is now called Rickettsia rickettsii. Ricketts died of typhus, another rickettsia-caused disease, while studying it in Mexico.

Rickettsias are generally rod-shaped or coccus-shaped (round) and are often carried by arthropods (such as fleas, lice, mites, and ticks), which may transmit them to humans or other mammals. Numerous insect-infesting rickettsia species cause no mammalian animal diseases.

Diseases caused by rickettsias vary in severity from mild to life threatening. They are commonly characterized by sudden onset and typically cause lethargy, high fever, headache, muscle aches, skin rash (in most cases), and damage to the lining of the blood vessels; damage to tissues of the central nervous system may follow. Rocky Mountain spotted fever is the most severe and commonly reported rickettsial disease in the U.S. But a number of other illnesses are also caused by rickettsias belonging to what is known as the spotted fever group. Among them are rickettsialpox, a mild disease caused by Rickettsia akari; Australian (or North Queensland) tick typhus, caused by R. australis; Boutonneuse fever, caused by Rickettsia conorii; Oriental spotted fever, caused by Rickettsia japonica; and Siberian tick typhus, caused by Rickettsia sibirica. Rickettsias in the so-called typhus group are responsible for the diseases epidemic typhus, caused by Rickettsia prowazeki, and murine, or “endemic,” typhus, caused by Rickettsia typhi. There also is a scrub typhus group, represented by the species Orientia tsutsugamushi (formerly Rickettsia tsutsugamushi), which causes scrub fever, also known under a variety of other names, such as tropical typhus, tsutsugamushi disease, and tsutsugamushi fever. Among other diseases of note sometimes referred to as rickettsial or rickettsial-like are trench fever, caused by Bartonella quintana (formerly Rochalimaea quintana); cat-scratch disease, caused by Bartonella henselae; and Q fever, caused by Coxiella burnetii, which can be transmitted by aerosols or food. Diagnosis of most of the rickettsial diseases is facilitated by the development in the blood of infected patients of specific antibodies that can be detected by serologic tests.

Prevention of rickettsial diseases involves eradication of the arthropod vectors or, where this is not feasible, their avoidance, through the use of protective clothing, chemical repellents, and the like. Human vaccines have been developed against a few rickettsial diseases, such as epidemic typhus, but are not commercially available in the U.S. Rickettsial diseases can generally be treated effectively with antibiotics such as tetracycline and chloramphenicol.

For further information on this topic, see the Bibliography, sections 448. Viruses, bacteria, 487. Medicine.