- Orientia tsutsugamushi
Orientia Orientia tsutsugamushi Scientific classification Kingdom: Bacteria Phylum: Proteobacteria Class: Alpha Proteobacteria Order: Rickettsiales Family: Rickettsiaceae Genus: Orientia Species: O. tsutsugamushi Binomial name Orientia tsutsugamushi
(Hayashi 1920) Tamura et al. 1995
Orientia tsutsugamushi is the causative organism of scrub typhus, and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium). The organism is an obligate intracellular pathogen, which needs to infect eukaryotic cells in order to multiply. The envelope is similar to that of Gram negative bacteria, but it is not easily stained with Gram stain and the Gimenez stain is preferred. There are a large number of serotypes described: these include Karp (which accounts for approximately 50% of all infection), Gilliam (25%), Kato (less than 10%), and Kawasaki, but there exists enormous variability, with eight serotypes being reported in a single field in Malaysia, and many more serotypes continue to be reported. Genetic methods have revealed even greater complexity than had been previously described (for example, Gilliam is further divided into Gilliam and JG types). Infection with one serotype does not confer immunity to other serotypes (no cross immunity). Repeated infection in the same individual is therefore possible, and this complicates vaccine design.
The bacterium was initially categorised in the genus Rickettsia, but is now classed in a separate genus, Orientia, in which it is the only species.
It is 0.5µm wide and 1.2 to 3.0µm long, and is an obligatory intracellular organism that can only be cultured in cell monolayers. The organism is highly virulent and should only be handled in a laboratory with biosafety level 3 facilities.
Antimicrobial therapyFor more details on this topic, see Scrub typhus.
O. tsutsugamushi is sensitive in vitro to doxycycline, rifampicin and azithromycin. It is innately resistant to all β-lactam antibiotics (for example, penicillin) because it lacks a peptidoglycan cell wall. Aminoglycosides (for example, gentamicin) are also ineffective in human infection because the organism is intracellular, and aminoglycosides do not penetrate intracellularly.
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