Orientia tsutsugamushi, the agent of scrub typhus

Abstract

Orientia tsutsugamushi is a mite-borne bacterium belonging to the family Rickettsiaceae and is responsible for the disease scrub typhus in humans. It is an obligate intracellular parasite of trombiculid mites, in which natural transmission is maintained from the female to its eggs (transovarial transmission) and from the eggs to adults (transstadial transmission). With a genome of only 2.0–2.7 Mb, it has the most repeated DNA sequences among bacteria. It is transmitted by mite larvae (chiggers) from rodents, the natural hosts of mites, to humans through accidental bites. Naosuke Hayashi first described it in 1920, giving it the name Theileria tsutsugamushi, but it was renamed to Orientia tsutsugamushi in 1995, owing to its unique properties. Unlike other Gram-negative bacteria, its cell wall lacks lipophosphoglycan and peptidoglycan. It instead has a unique 56-kDa type-specific antigen (TSA56), which gives rise to many strains (sub-types) of the bacterium such as Karp, Gilliam, Kato, Shimokoshi, Kuroki, and Kawasaki. It is most closely related to Candidatus Orientia chuto, a species described in 2010. Primarily indicated by undifferentiated febrile illnesses, the infection can be complicated and often fatal. Diagnosis is difficult and requires laborious detection methods such as the Weil–Felix test, rapid immunochromatographic test, immunofluorescence assays, ELISA, or PCR. Eschar, if present on the skin, is a good diagnostic indicator. One million infections are estimated to occur annually in the endemic region called the Tsutsugamushi Triangle, which covers the Russian Far East in the north, Japan in the east, northern Australia in the south, and Afghanistan in the west. However, infections have also spread to Africa, Europe and South America. Antibiotics such as azithromycin and doxycycline are the main prescription drugs. There is no vaccine for the infection.