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 (from Japanese tsutsuga "illness" and mushi "insect") is the causative organism of scrub typhus,[1] and the natural vector and reservoir is probably trombiculid mites (genus Leptotrombidium).[2][3] 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),[3] Gilliam (25%),[3] Kato (less than 10%),[3] and Kawasaki,[4] but there exists enormous variability, with eight serotypes being reported in a single field in Malaysia,[5] and many more serotypes continue to be reported.[6][7][8] 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.

Microbiology

The bacterium was initially categorised in the genus Rickettsia,[1] but is now classed in a separate genus, Orientia, in which it is the only species.[9]

It is 0.5 µm wide and 1.2 to 3.0 µm 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.[10]

Antimicrobial therapy

For 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.[11] Aminoglycosides (for example, gentamicin) are also ineffective in human infection because the organism is intracellular, and aminoglycosides do not penetrate intracellularly.

Vaccine

There are currently no licensed scrub typhus vaccines available. It is now known that there is enormous antigenic variation in Orientia tsutsugamushi strains,and immunity to one strain does not confer immunity to another. Any scrub typhus vaccine should give protection to all the strains present locally, in order to give an acceptable level of protection. A vaccine developed for one locality may not be protective in another locality, because of antigenic variation. This complexity continues to hamper efforts to produce a viable vaccine.

References

  1. 1 2 Watt G, Parola P (October 2003). "Scrub typhus and tropical rickettsioses". Curr Opin Infect Dis 16 (5): 429–36. doi:10.1097/00001432-200310000-00009. PMID 14501995.
  2. ↑ Traub R, Wisseman CL Jr (1974). "The ecology of chigger-borne rickettsiosis (scrub typhus)". J Med Entomol 11 (3): 237–303. doi:10.1093/jmedent/11.3.237. PMID 4212400.
  3. 1 2 3 4 Kelly DJ, Fuerst PA, Ching W-M, Richards AL (2009). "Scrub typhus: The geographic distribution of phenotypic and genotypic variants of Orientia tsutsugamushi". Clin Infect Dis 48 (s3): S203–30. doi:10.1086/596576. PMID 19220144.
  4. ↑ Yamamoto S, Kawabata N, Tamura A, et al. (1986). "Immunological properties of Rickettsia tsutsugamushi, Kawasaki strain, isolated from a patient in Kyushu". Microbiol Immunol 30 (7): 611–20. doi:10.1111/j.1348-0421.1986.tb02988.x. PMID 3095612.
  5. ↑ Shirai A, Tanskul PL, Andre, RG; et al. (1981). "Rickettsia tsutsugamushi strains found in chiggers collected in Thailand". Southeast Asian J Trop Med Public Health 12 (1): 1–6. PMID 6789455.
  6. ↑ Kang JS, Chang WH (1999). "Antigenic relationship among the eight prototype and new serotype strains of Orientia tsutsugamushi revealed by monoclonal antibodies". Microbiol Immunol 43 (3): 229–34. doi:10.1111/j.1348-0421.1999.tb02397.x. PMID 10338191.
  7. ↑ Bakshi D, Singhal P, Mahajan SK, Subramaniam P, Tuteja U, Batra HV (2007). "Development of a real-time PCR assay for the diagnosis of scrub typhus cases in India and evidence of the prevalence of new genotype of O. tsutsugamushi". Acta Trop 104 (1): 63–71. doi:10.1016/j.actatropica.2007.07.013. PMID 17870041.
  8. ↑ Parola P, Blacksell SD, Phetsouvanh R, et al. (2008). "Genotyping of Orientia tsutsugamushi from Humans with Scrub Typhus, Laos". Emerg Infect Dis 14 (9): 1483–1485. doi:10.3201/eid1409.071259. PMC 2603112. PMID 18760027.
  9. ↑ Tamura A, Ohashi N, Urakami H, Miyamura S (1995). "Classification of Rickettsia tsutsugamushi in a new genus, Orientia gen. nov., as Orientia tsutsugamushi comb. nov". Int J Syst Bacteriol 45 (3): 589–591. doi:10.1099/00207713-45-3-589. PMID 8590688.
  10. ↑ Centers for Disease Control and Prevention (1999). "Section VII-E: Rickettsial Agents". Biosafety in Microbiological and Biomedical Laboratories (BMBL) (4th ed.). Washington DC: U.S. Government Printing Office. pp. 149–53. Retrieved 13 Mar 2009.
  11. ↑ Amano K, Tamura A, Ohashi N, Urakami H, Kaya S, Fukushi K (1987). "Deficiency of peptidoglycan and lipopolysaccharide components in Rickettsia tsutsugamushi". Infect Immun 55 (9): 2290–2. PMC 260693. PMID 3114150.
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