Genus Orientia contains Orientia tsutsugamushi, the causative agent of scrub typhus.
O. tsutsugamushi, formerly known as R. tsutsugamushi, is thecausative agent of scrub typhus. The condition is transmitted to humans by the mite Leptotrombidium akamushi and also pos-sibly by Leptotrombidium deliense. Four stages of development usually take place in the life cycle of the mite. Of these stages, only larval stage (chigger) is infectious to humans and other mammals because these stages require blood meal for further development.
O. tsutsugamushi shows a remarkable antigenic heterogene-ity. Three major serotypes are recognized. These are Kart, Gili, and Kata types. The organism is transmitted to humans by chiggers that live and reproduce in the soil and in the scrub vegetation.
Scrub typhus is restricted in its geographical distribution. The condition is present in the western Pacific regions, Australia, and Eastern Asia. Once the mice are infected, they act as reservoir of O. tsutsugamushi. Rats, mice, and larger mam-mals are the usual hosts. Humans are the accidental hosts. The mites feed on the blood of animals only once during the cycle of development, hence are not believed to be an impor-tant reservoir for human diseases. The infection is transmit-ted from rats to the progeny by transovarial transmission.
Scrub typhus is a mild and self-limiting disease, but if untreated it has a fatality of 7%. The incubation period on an average varies from 10 to 12 days. The condition manifests with severe headache, fever, myalgia, and rash. The maculopapular rash develops initially on the trunk and later on the limbs, and is present in nearly 50% of the patients. In untreated patients, fever disappears after 2–3 weeks. Splenomegaly, CNS complica-tions, generalized lymphadenopathy, and heart failure are the complications of this condition.
Tetracyclines, doxycycline, or chloramphenicol are highly effective in treatment of scrub typhus. An effective vaccine is not available. Use of insect repellants and wearing protective clothings to avoid exposure to chiggers prevent the disease.