Changing Dynamics of Human–Rickettsial Interactions

Fluctuations in the reported incidence of scrub typhus, Rocky Mountain spotted fever (RMSF), and Mediterranean spotted fever have been noted over the past century since their recognition as arthropod-borne infections, with decadelong periods between the peaks and troughs. In Japan, classic Leptotrombidium akamushi-transmitted scrub typhus occurred with a high incidence before the 1950s, and thereafter decreased dramatically, essentially disappearing by the early 1980s, at which time a tremendous rise in scrub typhus transmitted by Leptotrombidium pallidium and Leptotrombidium scutellare mites occurred. 1 The report by Jeung and others in this issue 2 addresses scrub typhus in South Korea, where the first case was diagnosed in 1985 and a rise to 10,365 cases was reported in 2013. By studying the times of peak incidence in provinces representing northern, central, and southern Korea over a 13-year-period during which the number of cases increased tremendously, they noted that the peak occurred progressively later in the year. Increase in the density of L. scutellare, the vector of Orientia tsutsugamushi, in northern and central Korea correlated with the increased incidence of disease. The possibility of global climate change as a factor contributing to these changes is supported by the effect of longer seasonal warm temperatures and L. scutellare abundance in temperate regions of Korea. 3 Trombiculid mites feed particularly on rodent hosts such as Rattus species, and no longer remain active when the ground freezes. Because O. tsutsugamushi is apparently maintained solely by transovarian transmission in mites and is transmitted by larvae (the only stage of this arthropod that feeds on a host), the occurrence of scrub typhus depends on the abundance of infected chiggers and their contact with humans. The appearance of scrub typhus in recent years in areas where the illness was nearly forgotten, such as in India, Sri Lanka, the Maldives, and Micronesia, and in regions where it had not been known previously such as north of the Yangtze river in China and in Korea suggests that environmental changes including potential northward spread of infected mites may be occurring. 4–6 Increased diagnosis of scrub typhus in Thailand, Taiwan, Japan, and Laos may represent truly increased incidence, greater awareness,orgreateravailabilityofdiagnosticmethods. Whatever the explanation for increasing incidence and geographic range, scrub typhus begs for greater medical, public health, and scientific attention. Diagnosis is difficult, frequently missed, and would benefit greatly from a low-cost point-of-care diagnostic tool that is effective early in the illness, when therapeutic decisions are being made. With an estimated 1 million cases annually and an average case-fatality rate of 10%, a vaccine that provides cross protection against 70 or more strains of O. tsutsugamushi is a true public health need. 7 Research should begin with determination of protective immune mechanisms and with genome-wide bioinformatic analyses of potential protective antigens. Currently, excellent research on the pathobiology of O. tsutsugamushi–host interactions is being accomplished in Korea, but the development of an effective vaccine is far from near.