The purpose of larval chigger mite surveillance was to analyze and identify small mammal hosts, larval chigger mite host preferences, and seasonal abundance of larval chigger mites associated with the seasonal prevalence of scrub typhus (prevalence of scrub typhus is reported separately), in addition to determine the northern limits of L. scutellare. Areas surveyed in this study were geographically separated and ecologically variable. Small mammals, including rodents and soricomorphs were live captured from Hwaseong-gun (1 site), Gyeonggi-do (Province) (37°02'25.3" N, 126°52'11.0" E) and Asan city (1 site), Chungcheongnam-do (36°45'43.2" N, 126°52'04.2" E) in the west central region, and Jangseong-gun (9 sites), Jeollanam-do (36°45'43.2" N, 126°52'04.2" E) in the southwestern region of Korea from April-November 2009 (Fig. 1).

Sherman® live capture traps (7.7×9×23 cm), i.e., aluminum folding traps (H.B. Sherman, Tallahassee, Florida, USA) baited with peanut butter and rolled oats (1:3 ratio by volume) were set in 6 trap lines, each consisting of 10 traps set at 1-3 m intervals. Traps were set at 15:00-17:00 hr and collected the following morning between 06:00-07:00 hr. Traps positive for small mammals were sequentially numbered according to the site and returned to the central laboratory, Konkuk University, Seoul, Korea. Each small mammal was given a unique identification code, anesthetized using CO₂, identified to species, sexed, weighed, and then euthanized by cardiac puncture in accordance with Konkuk University Animal Use Guidelines. Following cardiac puncture, the spleen, kidneys, and liver tissues were removed, and the animal carcasses hung by the heels over a container with water to collect chigger mites as described by Ree et al. [6]. The population densities, by species of larval chigger mite, were calculated as the mean number of larval chigger mites per infested small mammal by species.

Overall, a total of 140 small mammals consisting of 3 species (A. agrarius, M. regulus, and Crocidura lasiura) were collected (Table 1). The trap rates were highest at Hwaseong (23.3%) and lowest at Jangseong (13.7%). Only A. agrarius (n=30) was collected from Hwaseong, while A. agrarius and M. regulus were collected from Asan, and A. agrarius, M. regulus, and C. lasiura were collected at Jangseong. A. agrarius is the most commonly collected rodent throughout most areas of Korea [5], and during this study it accounted for 92.1%, while C. lasiura and M. regulus accounted for 6.4% and 1.4% of the small mammals collected, respectively. Small mammals were trapped during the spring/early summer (April-June) and again during the fall (September-November) (Table 1). Overall, larval chigger mite infestation rates for A. agrarius were 72.1%, 67.9%, and 86.7% for Asan, Jangseong, and Hwaseong, respectively (Table 1). The mean larval chigger mite infestation rate for A. agrarius was 73.1%, ranging from a low level of 44.4% (June) to a high level of 100% (November) (Table 1). Seasonal larval chigger mite indices decreased from 27.3 in April to 4.9 in June, then increased from 95.2 in September to high levels of 180.0 and 169.3 in October and November, respectively. Only 1/2 M. regulus was infested with larval chigger mites during October when larval chigger indices were high for A. agrarius. Chigger indices for M. regulus and C. lasiura captured during October were similar, 73.0% and 77.0% respectively.

A total of 5,088 L. pallidum and 3,422 L. scutellare were collected from A. agrarius (Tables 2, 3). The proportions of L. pallidum and L. scutellare collected from each site were highest during the late fall (October and November). Other larval chigger mites only accounted for 0.3% of the total number collected at Hwaseong, 16.6% at Asan, and 7.6% at Jangseong. The greatest diversity of larval chigger mites was observed at Asan, with 10 larval chigger mite species, while there were only 6 and 4 species observed at Jangseong and Hwaseong, respectively. Both Hwaseong and Asan had the highest proportion of L. pallidum, accounting for 95.8% and 61.2% of all chigger mites collected, respectively (Table 3). For A. agrarius, the lowest L. scutellare mite indices were observed at Hwaseong (3.2), the most northern collection site. The larval chigger mite indices increased for L. scutellare collected from A. agrarius at Asan (22.6), south of Hwaseong, and were highest at Jangseong (74.2), the most southern collection site.

In this study, as well as others, larval chigger indices were highest prior to and during periods when peak numbers of scrub typhus cases were observed [2]. These studies provide evidence that L. scutellare is the principal vector in the southern region, while L. pallidum is the principal vector in the northern region of Korea, and with transmission of O. tsutsugamushi correlated with high larval chigger mite indices for both species in the fall [7,8]. It has been shown that the northern limit line of L. scutellare is reported for areas where the annual mean temperature is greater than 10℃ [9]. During this study, low numbers of L. scutellare were collected where the mean temperature was <10℃. According to a recent report, L. scutellare was also collected at Songsan (chigger index, 0.4) and Jangan (3.9), Gyeonggi-do [1].

The effect of global warming may have contributed to the increase of scrub typhus cases in Korea over the past several years by extending the geographical range of L. scutellare. Continued surveillance of small mammals and associated larval chigger mites should be conducted to determine the northern limit of L. scutellare, as well as the southern limit of L. pallidum and potential changes in geographical distributions due to global warming. In addition, detailed studies that identify specific habitats, hosts of larval chigger mites, and associated risks for transmission of O. tsutsugamushi are needed to develop disease risk assessments.