Coincidence between Geographical Distribution of Leptotrombidium scutellare and Scrub Typhus Incidence in South Korea

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.

Severe Fever with Thrombocytopenia Syndrome in Patients Suspected of Having Scrub Typhus

“…H. longicornis ticks are widespread in South Korea and their density is high during May–August, when temperatures are usually warm ( 10 ). In contrast, Leptotrombidium scutellare mites are major scrub typhus vectors with high density during autumn (scrub typhus season) ( 12 ). Therefore, for patients with fever and a history of bug bites, physicians in South Korea tend to suspect SFTS during summer and scrub typhus during autumn.…”

Effect of Latitude and Seasonal Variation on Scrub Typhus, South Korea, 2001–2013

“…Physicians who diagnose suspected or confirmed cases have reported these cases to the Korea Centers for Disease Control and Prevention (KCDC) since 1994. 9,10 This study used raw data obtained from the National Notifiable Diseases Surveillance System website of KCDC from 2001 to 2013. A total of 69,210 scrub typhus cases were analyzed according to region, month, and year of incidence.…”

“…More than 4,500 cases have been reported annually since 2004, and 10,365 cases were reported nationwide in 2013. 8,9 As a result, scrub typhus has become the most common rickettsial disease in the country. Although the number of scrub typhus patients has increased in South Korea, 10 the geographical and monthly distributions of this disease are poorly understood.…”

“…12,13 Interestingly, the distribution and chigger index (CI) map of L. scutellare was identical to the incidence pattern of scrub typhus, whereas the CI map of L. pallidum and chigger mites overall showed no relationship with the incidence pattern. 9 The increased density of L. scutellare may affect the high autumn incidence of scrub typhus. 9 One previous report suggested that chigger abundance may be primarily influenced by temperature in temperate areas and precipitation in tropical regions.…”

“…9 The increased density of L. scutellare may affect the high autumn incidence of scrub typhus. 9 One previous report suggested that chigger abundance may be primarily influenced by temperature in temperate areas and precipitation in tropical regions. 5 Because South Korea has both temperate and subtropical regions, it is likely that temperature and humidity are important factors in the incidence of scrub typhus.…”