rickettsia

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Related to Rickettsia prowazeki: typhus fever, Rickettsia rickettsii

rickettsia

(rĭkĕt`sēə), any of a group of very small microorganisms, many disease-causing, that live in vertebrates and are transmitted by bloodsucking parasitic arthropods such as fleasflea,
common name for any of the small, wingless insects of the order Siphonaptera. The adults of both sexes eat only blood and are all external parasites of mammals and birds. Fleas have hard bodies flattened from side to side and piercing and sucking mouthparts.
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, lice (see louselouse,
common name for members of either of two distinct orders of wingless, parasitic, disease-carrying insects. Lice of both groups are small and flattened with short legs adapted for clinging to the host.
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), and ticks. Rickettsias are named after their discoverer, the American pathologist Harold Taylor Ricketts, who died of typhustyphus,
any of a group of infectious diseases caused by microorganisms classified between bacteria and viruses, known as rickettsias. Typhus diseases are characterized by high fever and an early onset of rash and headache.
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 in Mexico after confirming the infectious agent of that rickettsial disease. Rickettsias are gram-negative, coccoid-shaped or rod-shaped bacteria; unlike other bacteria, but like viruses, they require a living host (a living cell) to survive. Rickettsias from infected vertebrates, usually mammals, live and multiply in the gastrointestinal tract of an arthropod carrier but do not cause disease there; they are transmitted to another vertebrate, possibly one of another species, by the arthropod's mouthparts or feces.

Types of Rickettsial Diseases

Rickettsia prowazekii causes louse-borne typhus, carried from person to person by two species of lice. Flea, or murine, typhus, caused by R. mooseri, is transmitted from rodents to people by fleas. Trench fever, caused by R. quintana, was an epidemic disease in World War I; it is transmitted by the rat flea from rat to person or from person to person. Trench fever disease reservoirs (perpetuation of the disease in wild animal populations) exist in some parts of E Europe, Mexico, and N Africa. Various typhuslike rickettsial diseases, such as Rocky Mountain spotted feverRocky Mountain spotted fever,
infectious disease caused by a rickettsia. The germ is harbored by wild rodents and other animals and is carried by infected ticks that attach themselves to humans.
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 and African tick typhus, are transmitted by ticks from animal hosts to people. Mite-borne rickettsial infections include rickettsialpox, caused by Rickettsia akari and transmitted from house mice to people, and scrub typhus, or tsutsugamushi fever, caused by R. tsutsugamushi and found in Japan and SE Asia. Q fever, caused by Coxiella burnetii, a more hardy rickettsia viable outside the living host, is usually transmitted to humans by inhalation of contaminated airborne particles or from contaminated materials, often from infected livestock; it is an occupational hazard among dairy farm and slaughterhouse workers. A new rickettsia, Ehrlichia chaffeenis, which results in human ehrlichiosisehrlichiosis
, any of several diseases caused by rickettsia of the genus Ehrlichia. Ehrlichiosis is transmitted by ticks. Both human forms tend to develop about nine days after a tick bite.
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, was identified in 1986.

Symptoms and Treatment

The similar symptoms of rickettsial infections often make it difficult to distinguish one disease from another. In people the organisms grow in cells lining blood and lymph vessels; a rash, fever, and flulike symptoms are usually present. Q fever also causes lung damage. All rickettsial diseases respond to treatment with antibiotics such as doxycycline (a tetracyclinetetracycline
, any of a group of antibiotics produced by bacteria of the genus Streptomyces. They are effective against a wide range of Gram positive and Gram negative bacteria, interfering with protein synthesis in these microorganisms (see Gram's stain).
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) and chloramphenicolchloramphenicol
, antibiotic effective against a wide range of gram-negative and gram-positive bacteria (see Gram's stain). It was originally isolated from a species of Streptomyces bacteria.
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.

Rickettsia

 

a genus of small pathogenic bacteria that multiply only in host cells. It is named after the American pathologist H. T. Ricketts (1871–1910), who in 1909 discovered the causative agent of Rocky Mountain spotted fever. Rickettsia is a short rod (0.3 × 0.3 to 2 μm) with rounded ends occurring singly or in pairs. The rods are nonsporogenous, nonmotile, and gram-negative; they multiply by transverse division. The rickettsiae are no larger than some viruses but are classified as bacteria since they divide and have cell walls, cytoplasmic membranes, ribosomes, and nuclei. In addition, they synthesize protein, the nucleic acids DNA and RNA, ATP, and the enzymes of intermediate metabolism, chiefly those of the tricarboxylic acid cycle.

Being intracellular parasites, the rickettsiae make use of growth factors from cells of the host organism. The cytoplasmic membrane of the rickettsiae is highly permeable, a result of their adaptation to a parasitic mode of life. The rickettsiae do not grow on the ordinary nutrient media and require chick embryos or animal cell cultures. They are sensitive to unfavorable physical or chemical factors and to some antibiotics.

The rickettsiae include the causative agents of epidemic typhus (Rickettsia prowazekii), murine typhus (R. mooseri), Q fever (Coxiella burnetii), and other endemic or widespread rickettsioses.

A. A. IMSHENETSKII