Abstract
To clarify the geographical distribution of scrub typhus vectors in Korea, a survey of larval trombiculid mites was conducted from 2005 to 2007 by collecting wild small mammals twice a year (spring and autumn) at 24 sites nationwide. A total of 67,325 mites representing 4 genera and 14 species were collected from 783 trapped rodents, corresponding to a chigger index (number of chigger mites per rodent) of 86.0. The predominant mite species were Leptotrombidium pallidum (52.6%), Leptotrombiduim scutellare (27.1%), Leptotrombidium palpale (8.2%), Leptotrombidium orientale (5.6%), and Neotrombicula tamiyai (1.7%). However, the proportions of L. scutellare in southern areas, including endemic provinces such as Jeollabuk-Do (34.3%), Jeollanam-Do (49.0%), and Gyeongsangnam-Do (88%), were relatively higher than in central Korean regions where L. pallidum was predominant. In autumn, the ratio of L. scutellare increased to 42% while the ratio of L. pallidum decreased. The geographical distribution map of the L. scutellare chigger index was identical to the incidence pattern of scrub typhus, whereas those of overall mites and L. pallidum showed no relationship with case incidence patterns. Distribution mapping analysis shows an identical geographical distribution of L. scutellare and epidemic incidence of scrub typhus in South Korea. L. pallidum could be another vector at all other parts of the Korean peninsula, including the eastern and northern regions that have a low level of scrub typhus incidence.
Highlights
Scrub typhus is caused by the rickettsial bacterium, Orientia tsutsugamushi, which are transmitted through bites from infected larval trombiculid mites [1]
Information about patients in South Korea diagnosed with scrub typhus in 2007 was obtained from the National Notifiable Disease Web Statistics System (NNDWSS) of the Korea Centers for Disease Control and Prevention (KCDC) (Fig. 2A)
The number and C.I. (43,434 and 108.6) of chigger mites in autumn were 1.8-fold and 1.7-fold higher than in spring (23,891 and 62.4). This result suggests that the high density of chigger mites may affect the high incidence rate (92.3%) of scrub typhus in autumn
Summary
Scrub typhus (tsutsugamushi disease) is caused by the rickettsial bacterium, Orientia tsutsugamushi, which are transmitted through bites from infected larval trombiculid mites [1]. We surveyed 24 localities nationwide in spring and autumn from 2005 to 2007 and analyzed the relationship between trombiculid mite geographical distribution and scrub typhus epidemic regions in Korea. Larval trombiculid mites were collected from wild rodents captured at 24 sites nationwide from 2005 to 2007 (Fig. 1 and S1 Table).
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