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Scrub Typhus (Orientia tsutsugamushi)
Scrub typhus is an important cause of acute febrile illness in South and East Asia and the Pacific and could be emerging in the Middle East and South America. The causative agent is distinct from, but related to, Rickettsia species. The infection is transmitted via chigger (larval mite) bites and involves many antigenically diverse strains of Orientia tsutsugamushi , hampering vaccine development.
Etiology
The causative agent of scrub typhus, or tsutsugamushi fever, is O. tsutsugamushi, which is distinct from other spotted fever and typhus group rickettsiae (see Table 255.1 in Chapter 255 ). O. tsutsugamushi lacks both lipopolysaccharide and peptidoglycan in its cell wall. Like other vasculotropic rickettsiae, O. tsutsugamushi infects endothelial cells and causes vasculitis, the predominant clinicopathologic feature of the disease. However, the organism also infects macrophages and cardiac myocytes. A new Candidatus species, Orientia chuto , was isolated from a patient in the Middle East, and definitive evidence of infection based on serology and/or PCR amplification of O. tsutsugamushi genes from acute phase blood suggests a wider range for scrub typhus and related infections.
Epidemiology
More than 1 million infections occur each year, and it is estimated that more than 1 billion people are at risk. Scrub typhus occurs mostly in Asia, including areas delimited by Korea, Pakistan, and northern Australia. Outside these tropical and subtropical regions, the disease occurs in Japan, the Primorsky of far eastern Russia, Tajikistan, Nepal, and nontropical China, including Tibet. Cases imported to the United States and other parts of the world are reported. Endemic scrub typhus has historically been confined to Asia and Oceania and the tsutsugamushi triangle; however, Orientia may be distributed more broadly, with confirmed cases in South America and possible cases in Africa. Most infections in children are acquired in rural areas. In Thailand and Sri Lanka, scrub typhus is the cause of 1–8% of acute fevers of unknown origin. Infections are most common during rainy months, usually June through November. Reported cases in boys are higher than in girls.
Transmission
O. tsutsugamushi is transmitted via the bite of the larval stage (chigger) of a trombiculid mite (Leptotrombidium), which serves as both vector and reservoir. Vertical transovarial transmission (passage of the organism from infected mites to their progeny) is the major mechanism for maintenance in nature. Because only the larval stage takes blood meals, a role for horizontal transmission from infected rodent hosts to uninfected mites has not been proved, but transmission among co-feeding larval mites is a possibility. Multiple serotypes of O. tsutsugamushi are recognized, and some share antigenic cross reactivity; however, they do not stimulate protective cross-immunity.
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