Etiology

The causative agent of scrub typhus, or tsutsugamushi fever, is Orientia tsutsugamushi, which is distinct from other spotted fever and typhus group rickettsiae (see Table 220-1). 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 cardiac myocytes and macrophages.

Epidemiology

Approximately 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 Primorye of far eastern Russia, Tajikistan, Nepal, and nontropical China, including Tibet. Cases imported to the USA and other parts of the world are reported. Most infections in children are acquired in rural areas. In Thailand, 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. Transovarial transmission (passage of the organism from infected ticks to their progeny) and transmission of the organism from infected animals to mites both occur. Because only the larval stage takes blood meals, a role for horizontal transmission from infected rodent hosts to uninfected mites has not been proved. Multiple serotypes of O. tsutsugamushi are recognized, and some share antigenic cross reactivity; however, they do not stimulate protective cross immunity.

Pathology and Pathogenesis