Unresolved Problems Related to Scrub Typhus: A Seriously Neglected Life-Threatening Disease
Ecology and epidemiology. Scrub typhus is a life-threatening infectious disease that presents as an acute undifferentiated febrile illness. Its agent, Orientia tsutsugamushi, is an obligately intracytosolic bacterium that is transmitted by feeding larval trombiculid mites, which are the reservoir of the agent and the only life stage that feeds on a vertebrate host. 1 Nymphal and adult trombiculid mites live in the soil and feed on the eggs of insects. Mites maintain the organisms by transovarian transmission as well as transtadial transmission through the mite's lifecycle. Although mites transmit O. tsutsugamushi to vertebrate hosts such as rodents, only a small proportion of uninfected mites acquire Orientia during feeding on infected animals, and the ingested Orientia do not establish disseminated infection in the mites and are not transmitted transovarially to the next generation. 4 -7 It seems that chigger cofeeding on rodents is more relevant for effective mouseto-mite transmission of Orientia than feeding on rickettsemic hosts. Thus, the true role of rodents as reservoirs requires additional investigation, because there may be marked genetic variability in chiggers with respect to the ability to acquire rickettsiae by feeding. Also, rodents may be better considered as dead-end hosts just as humans are rather than a reservoir. However, the evolutionary selection of tremendous antigenic diversity of the immunodominant major 56 kDa surface protein presents an enigma if immune variation plays no role in survival of O. tsutsugamushi. 9 The potential immunomodulatory effects of the saliva of larval mites on the pathogenesis of and immunity to infection with O. tsutsugamushi remain undetermined.