Orientia

Orientia is a genus of Gram-negative, obligate intracellular bacteria in the family Rickettsiaceae, order Rickettsiales. Its best-known species, Orientia tsutsugamushi, is the etiologic agent of scrub typhus, a vector-borne febrile illness in the Asia-Pacific region. The genus was established in 1995 to accommodate what had been called Rickettsia tsutsugamushi; recent taxonomic work has recognized additional Orientia species, including Orientia chuto, described from the Arabian Peninsula.

Orientia bacteria are obligate intracellular parasites that infect endothelial cells and mononuclear phagocytes. The genome of O. tsutsugamushi is unusually large for an rickettsial organism and contains a high density of repetitive elements and mobile genetic sequences, contributing to extensive antigenic diversity among strains.

Transmission occurs via the bite of larval trombiculid mites (chiggers), as free-living larvae that feed transiently on vertebrate hosts. Humans are incidental hosts. Scrub typhus is endemic to the tsutsugamushi triangle, a region spanning parts of Southeast Asia, the western Pacific, and extending into parts of the Middle East; risk is highest in rural, sylvatic environments where mite vectors are common.

Clinical features typically begin after a 1–3 week incubation and include fever, headache, myalgia, and often an eschar at the bite site. Lymphadenopathy and atypical pneumonia, hepatitis, or CNS involvement can occur in more severe cases.

Diagnosis relies on serology, with indirect immunofluorescence assay commonly used as a reference test, supported by molecular methods such as PCR. Treatment is with doxycycline or tetracycline; azithromycin is an alternative, particularly in pregnant women or children. In severe cases or when doxycycline is unsuitable, other antibiotics such as chloramphenicol may be used.

See also: Rickettsiales, scrub typhus.