Rickettsia honei
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Classification
Higher order taxa
Bacteria; Proteobacteria; Alphaproteobacteria; Rickettsiales; Rickettsiaceae
Species
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NCBI: Taxonomy |
Rickettsia honei
Description and significance
Rickettsia honei belongs to the spotted fever group (SFG), and are etiological agents of typhus and spotted fever in humans. Rickettsiae are transmitted by lice, fleas, tick, and mites. The genus Rickettsia is named after the American pathologist Howard Taylor Ricketts. Despite the name, Rickettsia spp. do not cause rickets (Greek rhakis, "spine"), a disorder of bone development caused by vitamin deficiency. The species name honei is named after Frank Sandland Hone, and early pioneer in Australian rickettsiology. An AFG rickettsiosis-like ailment was identified in 1991 on Flinders Island of Australia by Steonos and was named Flinders Island spotted fever (FISF) (1). The causative agent of FISF was isolated from buffy coat preparations from the blood of two patients and named R. honei. Since then, R. honei has been confirmed in Thailand (1961), and in Texas (in 1998) and in Nepal (2). On each continent it has been associated with a different type of tick (2). Rickettsia are a genus of gram negative, rod shaped bacteria and they are short and often paired rods, 0.3-0.5 x 0.8-2.0 um (1). Rickettsiae retain basic fuchsin when stained by the method of Gimenez (1). The organisms are obligately intracellular and reside free in the cytoplasm of the eucaryotic host cell, where they divide by binary fission (1). Organisms of the genus Rickettsia are typically surrounded in the host cell by an electron-lucent zone that has been proposed to represent a slime layer, which is stabilized by the presence of antibodies (1). During the course of infection, rickettsial morphology can change: older cells can become smaller and more electron dense. SFG rickettsiae usually do not form cacuoles and crystalline structures (1).
Genome and genetics
R. honei is most closely related to R. slovaca and R. rickettsii. R. honei and the Thai tick typhus rickettsia (TT-118) are identical in DNA sequences of the gene encoding the 17 kDa protein, gltA, and ompA, and they have only 1 nucleotide difference in the 16S rRNA gene (1). Thus, R. honei and TT-118 are considered a single species. A Rickettsia genetically identical to R. honei has been detected by PCR in Aponoma hydrosauri ticks from Flinders Island and Tasmania (1). The major antigens of Rickettsia are lipopolysaccharide, lipoprotein, outer membrane proteins, and heat shock proteins (1). The Weil-Felix reaction has been used as a presumptive diagnostic test for rickettsial diseases (1). It is based on the cross-reaction of antibodies to rickettsial antigens from primary rickettsial infections with the somatic antigens of three strains: Proteus vulgaris strains OX19, and OX2, and Proteus mirabilis strain OXK (1). The cross-reactive antigens between Rickettsia and Proteus are most likely present in the LPS. The O-polysaccharides of the LPS of typhus rickettsiae are composed of glucose, glucosamine, quinovosamine, and phosphrylated hexosamine (1). All rickettsiae have a 135-kDa outer membrane protein B (OmpB) that has been identified in all Rickettsia species examined. The sequence of the gene encoding the 17-kDa prtoein gene are conserved among rickettsial species, indicating the importance of it to the survival of the rickettsiae (1). The mol % G + C of the DNA is not determined. Type strain of Rickettsia honei is: RB, ATCC VR-1472. GenBank accession number (16SrRNA): U17645, AF060705 (1).
Nutrition and metabolism
The nutritional requirements of the rickettsiae, as distinct from those of their host cells, are not known (1). Rickettsiae grow in heavily irradiated cells that have lost the ability to divide, and they grow in the presence of a low level of cycloheximide that inhibits host protein syntheses. Under these conditions, rickettsiae incorporate exogenous amino acids and adenine, but not thymidine (1). Thus, rickettsial growth occurs independently of host cell protein synthesis, host cell division, and DNA or RNA synthesis. Rickettsiae require a CO2 enriched atmosphere to grow in chicken embryo cells when an organic buffer is substituted for sodium bicarbonate (1). Macimal rickettsial growth occurs only in host cells with an intracellular proline pool of 1.0 mM or greater (1).
Ecology / Pathology
Ecology: A consistent characteristic of Rickettsia species is their residence in an arthropod host as at least a part of their ecological niche. Transovarian maintenance from one generation of tick, mite, or flea to the next via infected ova that hatch into infected larvae is a factor in the maintenance in nature of all SFG rickettsiae (1). Knowlege of rickettsial ecology is preponderantly based on investigations of medical epidemiology and hematophagous arthropods. The view of the diversity and evolutionary origin of Rickettsia is likely overly anthropocentric (1). Pathology: A number of Rickettsia species cause severe disease in humans. Before the advent of the broad-spectrum antibiotics, epidemic typhus and Rocky Mountain spotted fever had a very high case fatality rate. R.honei causes Flinders Island spotted fever, which is also deadly to humans (1). Rickettsiae are inoculated into the skin in the saliva during feeding by and infected tick, mite, or flea or by scrathing of rickettsia-laden feces deposited by an infected louse or flea (1). Rickettsiae are distributed throughout the body via the bloodstream where they enter their principal target, endothelial cells (1). In the interaction of rickettsiae with host cells, the entry process involves three steps: attachment, internalization, and escape from the phagosome (1). Rickettsiae adhere to the host cell by means of a rickettsial adhesin and a host cell receptor, and cause a number of problems with a humans health. Some symptoms of the disease are listed further on.
Current Research
Flinders Island Spotted Fever Rickettsioses Caused by "marmionii" Strain of Rickettsia honei, Eastern Australia studied seven patients and observed how Flinders Islan spotted fever affected them. They then experimented with Rickettsial Serology, Rickettsia isolation from blood, and Rickettsial PCR on blood (3). Seroconversion, defined as as a 4-fold increases in antibody titer, occurred in only 2 of the 7 patients, although positive titers were seen in 5 of 7 patients (3). In 5 of 6 patients a rickettsia was isolated from blood in Vero cell culture, however 4 of these 5 isolates did not persist in the cell culture after their third passage (3). The remaining isolate has been maintained in continuous culture in only the XTC-2 cell line. Rickettsia honei Infection in Human, Nepal, 2009 reports a case of Rickettsia honei infection in a human in Nepal. The patient had severe illness and many clinical features typical of Flinders Island spotted fever. Diagnosis was confirmed by indirect immunofluorescent assay with serum and molecular biological techniques (4). Flinders Island spotted fever may be an endemic rickettsiosis in Nepal. The patient was 67 and from New Zealand, and had lived in Nepal for 30 years (4). She worked in wild dog protection, and reported removal of a tick two weeks before admission. The pateient had photosensitivity, tinnitus, frontal headache, insomnia, confusion, cough, distress, hypotension, tachycardia, hypozia, and a fever (4). She was treated with oral doxycycline and showed defervescence by day 16. She recovered slowly over 3 months but had persistent tinnitus and residual high-tone hearing loss (4).
References
[1] Bergey, D. H., George M. Garrity, David R. Boone, Don J. Brenner, Richard W. Castenholz, Michael Goodfellow, Noel R. Krieg, James T. Staley, and Paul De Vos. "Genus I. Rickettsia." “Bergey's Manual of Systematic Bacteriology. the Actinobacteria”. C ed. Vol. 2. 2012.
[2] Graves, S., and J. Stenos. "Rickettsia Honei: A Spotted Fever Group Rickettsia on Three Continents." Annals of the New York Academy of Sciences. U.S. National Library of Medicine, June 2003.
[3] Unsworth, Nathan B., John Stenos, Stephen R. Graves, and Antony G. Faa. "Flinders Island Spotted Fever Rickettsioses Caused by "marmionii" Strain of Rickettsia Honei, Eastern Australia - Volume 13, Number 4-April 2007 - Emerging Infectious Disease Journal - CDC." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 04 Jan. 2012.
[4] Murphy, Holly, Aurelie Renvoise, Prativa Pandey, Philippe Parola, and Didier Raoult. "Rickettsia Honei Infection in Human, Nepal, 2009." Emerging Infectious Diseases 17 (2011): 1865-86.
