Rickettsia

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Rickettsiae are a group of organisms belonging to the class Alphaproteobacteria, a large and metabolically diverse group of gram-negative bacteria. Within Alphaproteobacteria, the order Rickettsiales comprises three families: Holosporaceae, Anaplasmataceae and Rickettsiaceae, of which Rickettsia spp. are grouped in the latter, along with the monotypic genus Orientia, the scrub typhus agent. Robust phylogenetic analysis further suggests that the abundant free-living marine bacterioplankton Pelagibacter ubique and mitochondria are early-branching groups of the order. Species in the genus Rickettsia are obligate intracellular symbionts of plants, amoebae, arthropods, annelids, vertebrates and likely many other organisms. Most Rickettsia-containing vertebrates are secondary hosts that acquired these bacteria via blood-feeding arthropods or the transdermal inoculation or inhalation of the feces of infected arthropods. Rickettsia spp. are often parasitic in the secondary vertebrate host, and their pathogenicity to some extent has been well studied. In particular, human rickettsial infections are known to cause many diseases, including epidemic typhus (R. prowazekii), murine typhus (R. typhi), murine typhus-like (R. felis), rickettsial pox (R. akari), Rocky Mountain spotted fever (R. rickettsii), Boutonneuse fever (R. conorii), and North Asian tick typhus (R. sibirica). These virulent species of rickettsiae are of great interest both as emerging infectious diseases and for their potential deployment as bioterrorism agents.(Gillespie et al., 2008)

For references and expanded information, please refer to the R. conorii, R. prowazekii, R. rickettsii, and R. typhi PathInfo documents.

Rickettsia conorii

Pathogen and Disease

Mediterranean spotted fever (MSF), also known as boutonneuse fever, is caused by Rickettsia conorii, an obligately intracellular, slow-growing gram-negative bacterium. Unusual rickettsial strains related to R. conorii have been described as belonging to an "R. conorii complex" which includes the Indian tick typhus rickettsia, the Astrakhan fever rickettsia, and the Israeli spotted fever (ISF) rickettsia. The genus Rickettsia is divided into spotted fever and typhus groups. Boutonneuse fever, also known as Marseilles fever and Mediterranean spotted fever, caused by Rickettsia conorii and transmitted by ixodid ticks, especially Rhipicephalus sanguineus, the common dog tick in the Mediterranean.

For references and expanded information, please refer to the R. conorii PathInfo document.

Rickettsia prowazekii

Pathogen and Disease

Rickettsia prowazekii is the etiologic agent of epidemic typhus, which occurs in two clinical forms: the primary febrile illness and recrudescent infection (Brill-Zinsser disease). Epidemic typhus is transmitted by the body louse (Pediculus humanus corporis) and typically occurs during cold-weather months. Epidemic typhus is also known as louse-borne typhus, classic typhus, and sylvatic typhus.

For references and expanded information, please refer to the R. prowazekii PathInfo document.

Rickettsia rickettsii

Pathogen and Disease

R. rickettsii is native to the New World and causes the malady known as Rocky Mountain spotted fever (RMSF). RMSF is transmitted by the bit of an infected tick while feeding on warm-blooded animals, including humans. Wolbach deserves the credit for the first detailed description of the etiologic agent in 1919. He clearly recognized it as an intracellular bacterium which was seen most frequently in endothelial cells. The nucleus was often completely filled with minute particles and often was distended. Although Wolbach recognized its similarity to the agent of typhus and tsutsugamushi fever, he did not regard the designation 'rickettsia' as appropriate. He proposed the name Dermacentroxenus rickettsi. Brumpt felt that the etiologic agent of RMSF, despite some uncertainty about its properties, belonged in the genus Rickettsia and in 1922 proposed the name Rickettsia rickettsii.

For references and expanded information, please refer to the R. rickettsii PathInfo document.

Rickettsia typhi

Pathogen and Disease

Rickettsia typhi, the causative agent of murine typhus, is an obligate intracellular bacterium with a life cycle involving both vertebrate and invertebrate hosts. The rickettsiae are small (ca. 0.4 by 0.9 um), gram-negative, aerobic, coccobacillary, alpha-proteobacteria. R. typhi is one of the leading causes of rickettsioses in the world. Although distributed worldwide, it is most common in warm coastal areas with large rat populations.

Rickettsia are classified into two groups; the spotted fever group (SFG), which includes R. conorii, R. sibirica, and R. rickettsii, and the typhus group, which includes R. prowazekii and R. typhi. Murine typhus is a zoonosis whose enzootic cycle in nature involves vertebrate hosts (mainly commensal rodents) and arthropod vectors fleas (Siphonaptera) and lice (Phthiraptera).

For references and expanded information, please refer to the R. typhi PathInfo document.