rickettsia

—rickettsial, adj.

/ri ket"see euh/, n., pl. rickettsiae /-see ee'/, rickettsias /-see euhz/.

1. any member of the genus Rickettsia, comprising rod-shaped to coccoid microorganisms that resemble bacteria but can be as small as a large virus and reproduce only inside a living cell, parasitic in fleas, ticks, lice, and mites and transmitted by bite to vertebrate hosts, including humans, causing such severe diseases as typhus and Rocky Mountain spotted fever.

2. any rickettsia or rickettsialike microorganism of the orders Rickettsiales and Chlamydiales.

[1915-20; < NL, after Howard T. Ricketts (1871-1910), U.S. pathologist; see -IA]

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Any of the rod-shaped bacteria that make up the family Rickettsiaceae (named for Howard Ricketts).

They are rod-shaped or variably spherical, and most are gram-negative (see gram stain). Natural parasites of certain arthropods, they can cause serious diseases in humans and other animals, to which they are usually transmitted by a bite from an arthropod carrier. Because certain species can survive considerable drying, rickettsias can also be transmitted when arthropod feces are inhaled or enter the skin through abrasion. Typhus, trench fever, and Rocky Mountain spotted fever are rickettsial infections. The most effective treatment includes timely and prolonged administration of broad-spectrum antibiotics.

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▪ microorganism group

plural  Rickettsiae,  

      any member of three genera (Rickettsia, Coxiella, Rochalimaea) of bacteria in the family Rickettsiaceae. The rickettsiae are rod-shaped or variably spherical, nonfilterable bacteria, and most species are gram-negative. They are natural parasites of certain arthropods (notably lice, fleas, mites, and ticks) and can cause serious diseases—usually characterized by acute, self-limiting fevers—in humans and other animals.

      The rickettsiae range in size from roughly 0.3 to 0.5 micrometre (μm) by 0.8 to 2.0 μm (1 μm = 10^-6 metre). Virtually all rickettsiae can reproduce only within animal cells. Rickettsiae are usually transmitted to humans by a bite from an arthropod carrier. Because certain species can withstand considerable drying, transmission of rickettsia can also occur when arthropod feces are inhaled or enter the skin through abrasion. Most rickettsiae normally infect animals other than humans, who become involved as dead-end hosts only accidentally. Epidemic typhus and trench fever are exceptions, since humans are the only host of proven importance. The other rickettsial infections occur primarily in animals, which serve as reservoirs from which bloodsucking arthropods acquire the rickettsial bacteria and in turn transmit them to other animals and, occasionally, humans.

       Some disease-causing rickettsiaeThe largest rickettsial genus, Rickettsia, is generally subdivided into the typhus group, the spotted fever group, and the scrub typhus group. This genus alone is responsible for a number of highly virulent diseases including Rocky Mountain spotted fever, epidemic typhus, Brill-Zinsser disease, scrub typhus, and others, as shown in the Table (Some disease-causing rickettsiae).

      Protective measures against rickettsial disease agents include the control of arthropod carriers when necessary and immunization. Animals that recover from a rickettsiosis exhibit long-lasting immunity. Artificial immunity, as a preventive, is variably effective, typhus and the spotted fevers being among the easiest to immunize against. The most effective treatment of most rickettsioses includes the timely and prolonged administration of large amounts of broad-spectrum antibiotics (antibiotic) such as tetracycline or, if tetracycline cannot be used, chloramphenicol.

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