rickettsia

rickettsia, any of a group of very small microorganisms, many disease-causing, that live in vertebrates and are transmitted by bloodsucking parasitic arthropods such as fleas, lice (see louse), and ticks. Rickettsias are named after their discoverer, the American pathologist Harold Taylor Ricketts, who died of typhus 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 fever 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 ehrlichiosis, 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 tetracycline) and chloramphenicol.

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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Rickettsia is a genus of motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci (0.1 μm in diameter), rods (1-4 μm long) or thread-like (10 μm long). Obligate intracellular parasites, the Rickettsia depend on entry, growth, and replication within the cytoplasm of eukaryotic host cells (typically endothelial cells). Because of this, Rickettsia cannot live in artificial nutrient environments and are grown either in tissue or embryo cultures (typically, chicken embryos are used). In the past they were regarded as microorganisms positioned somewhere between viruses and true bacteria. The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.

Rickettsia species are carried as parasites by many ticks, fleas, and lice, and cause diseases such as typhus, rickettsialpox, Boutonneuse fever, African Tick Bite Fever, Rocky Mountain spotted fever, Australian Tick Typhus, Flinders Island Spotted Fever and Queensland Tick Typhus in human beings. They have also been associated with a range of plant diseases. Like viruses, they only grow inside living cells. The name rickettsia is often used for any member of the Rickettsiales. They are thought to be the closest living relatives to bacteria that were the origin of the mitochondria organelle that exists inside most eukaryotic cells.

The method of growing Rickettsia in chicken embryos was invented by Ernest William Goodpasture and colleagues at Vanderbilt University in the early 1930s.

Human pathogenesis

The Rickettsia human pathogens are generally grouped as follows:

Spotted fever group

* R. rickettsii (Western hemisphere)

Rocky Mountain spotted fever

* R. akari (USA, former Soviet Union)

Rickettsialpox

* R. conorii (Mediterranean countries, Africa, Southwest Asia, India)

Boutonneuse fever

* R. siberica (Siberia, Mongolia, northern China)

Siberian tick typhus

* R. australis (Australia)

Australian tick typhus

* R. japonica (Japan)

Oriental spotted fever

* R. africae (South Africa)

African Tick Bite Fever

Typhus group

* R. prowazekii (Worldwide)

Epidemic, recrudescent and sporadic typhus

* R. typhi (Worldwide)

Murine (endemic) typhus

Scrub typhus group

* The causative agent of scrub typhus formerly known as R. tsutsugamushi has been reclassified into the genus Orientia.

Plant pathogenesis

The following plant diseases have been associated with Rickettsia-like organisms.

* Beet latent Rosette RLO

* Citrus Greening bacterium

* Clover leaf RLO

* Grapevine infectious necrosis RLO

* Grapevine Pierce's RLO

* Grapevine yellos RLO

* Larch witche's broom disease

* Peach phoney RLO

Genomics

Certain segments of Rickettsial genomes resemble that of mitochondria. The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding genes. Unlike free-living bacteria, it contains no genes for anaerobic glycolysis or genes involved in the biosynthesis and regulation of amino acids and nucleosides. In this regard it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used. ATP production in Rickettsia is the same as that in mitochondria. In fact, of all the microbes known, the Rickettsia is probably the closest "relative" (in phylogenetic sense) to the mitochondria. Unlike the latter, the genome of R. prowazekii, however, contains a complete set of genes encoding for the tricarboxylic acid cycle and the respiratory chain complex. Still, the genomes of the Rickettsia as well as the mitochondria are frequently said to be "small, highly derived products of several types of reductive evolution".

The recent discovery of another parallel between Rickettsia and viruses may become a basis for fighting HIV infection. Human immune response to the scrub typhus pathogen, Orientia tsutsugamushi rickettsia, appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. A probable reason for this actively studied phenomenon is a certain degree of homology between the rickettsia and the virus - namely, common epitope(s) due to common genome fragment(s) in both pathogens. Surprisingly, the other infection reported to be likely to provide the same effect (decrease in viral load) is the virus-caused illness dengue fever.

Naming

The genus Rickettsia is named after Howard Taylor Ricketts (1871–1910), who studied and eventually died of typhus. Despite the similar name, Rickettsia bacteria do not cause rickets. The disease of rickets takes its name from the Greek word for spine, hrake, and is a vitamin D deficiency disease, not an infectious disease.

References

External links

• Rickettsia (from PATRIC the PathoSystems Resource Integration Center, a NIAID Bioinformatics Resource Center)
• African Tick Bite Fever from the Centers for Disease Control and Prevention