Larval Chigger Mites Collected from Small Mammals in 3 Provinces, Korea

A total of 9,281 larval chigger mites were collected from small mammals captured at Hwaseong-gun, Gyeonggi-do (Province) (2,754 mites from 30 small mammals), Asan city, Chungcheongnam-do (3,358 mites from 48 mammals), and Jangseong-gun, Jeollanam-do (3,169 for 62 mammals) from April-November 2009 in the Republic of Korea (= Korea) and were identified to species. Leptotrombidium pallidum was the predominant species in Hwaseong (95.8%) and Asan (61.2%), while Leptotrombidium scutellare was the predominant species collected from Jangseong (80.1%). Overall, larval chigger mite indices decreased from April (27.3) to June (4.9), then increased in September (95.2) and to a high level in November (169.3). These data suggest that L. pallidum and L. scutellare are the primary vectors of scrub typhus throughout their range in Korea. While other species of larval chigger mites were also collected with some implications in the transmission of Orientia tsutsugamushi, they only accounted for 11.2% of all larval chigger mites collected from small mammals.

Serosurveillance of Scrub Typhus in Small Mammals Collected from Military Training Sites near the DMZ, Northern Gyeonggi-do, Korea, and Analysis of the Relative Abundance of Chiggers from Mammals Examined

Comprehensive quarterly serosurveillance on scrub typhus in small mammals collected from military training sites located near the Demilitarized Zone (DMZ), northern Gyeonggi-do (Province), ROK was conducted to determine the potential rodent-borne and associated ectoparasite disease risks to military personnel. A total of 1,196 rodents and insectivores representing 8 species, Apodemus agrarius (87.3%, n = 1,044), Mus musculus (5.4%, n = 65), Crocidura lasiura (3.3%, n = 40), Microtus fortis (2.6%, n = 31), Micromys minutus (0.3%, n = 4), Tscherskia triton (0.3%, n = 4), Rattus norvegicus (0.3%, n = 4), and Myodes regulus (0.3%, n = 4) were assayed for the presence of antibodies to Orientia tsutsugamushi. O. tsutsugamushi antibodies were detected in 6 of 8 species and seroprevalence determined; A. agrarius (45.6%), M. musculus (23.1%), M. fortis (48.4%), M. minutus (50.0%), T. triton (50.0%), and R. norvegicus (25.0%). A total of 31,184 chigger mites collected from 508 rodents and insectivores were slide-mounted and 10 species belonging to 4 genera were identified. Leptotrombidium pallidum (53.4%) was the most frequently collected, followed by L. palpale (15.7%), Neotrombicula tamiyai (14.3%), L. orientale (10.7%), L. zetum (3.1%), Walchia fragilis (2.1%), and L. gemiticulum (0.8%), while the remaining 3 species, L. subintermedium, N. gardellai, and Euschoengastia koreaensis were rarely observed (prevalence < 10%). In contrast to previous surveys, higher chigger indices of the primary scrub typhus vectors, L. pallidum (165.4), L. orientale (45.0), and L. palpale (21.4), were observed during the spring season.

Virulence of Malaysian isolates of Orientia tsutsugamushi in mice.

The pathogenicity of Malaysian isolates of Orientia tsutsugamushi was investigated by a mouse virulence assay. The isolates could be differentiated as low (4 isolates), moderately (3 isolates) and highly virulent (2 isolates) based on the different responses in infected mice. No direct correlation between severity of human scrub typhus infections and virulence of the O. tsutsugamushi in mice was observed. Mice infected with virulent strains of O. tsutsugamushi showed splenomegaly, ascitis accumulation and enlargement of kidneys and livers whereas avirulent O. tsutsugamushi strains were asymptomatic and exhibited ruffled fur for a short period after infection. There was low antibody response in mice infected with isolates of low pathogenicity as compared with those of highly virulent isolates. Upon dissection of the infected mice, enlargement of mouse organs such as spleen, kidney and liver was noted. Presence of rickettsemia in mice was confirmed by the growth of O. tsutsugamushi in the L929 cells when inoculated with blood from infected mice. O. tsutsugamushi was also cultured from the peritoneal exudates of the infected mice. However, DNA of O. tsutsugamushi was only detected in the peritoneal exudates (by PCR) and blood (by cell culture) and not from other tissue samples.

Investigation of the role of Blankaartia acuscutellaris (Acari: Trombiculidae) as a vector of scrub typhus in central Thailand.

Monthly collections of rodents were conducted in Phitsanulok Province, central Thailand in 1993 to investigate the role of the mite Blankaartia acuscurellaris as a vector of scrub typhus. Overall, a total of 41 rodents were collected and examined for the presence of the red colored larvae of B. acuscutellaris and yellow larvae of Leptotrombidium deliense and Ascoshoengastia sp. A total of 787 B. acuscutellaris and 1390 yellow larvae were placed into pools, triturated and isolation of Orientia tsutsugamushi attempted in laboratory mice. The sera of 8 of the collected rodents had elevated antibodies to O. tsutsugamushi indicating active infections; however, O. tsutsugamushi was not isolated from rodent tissues or pools of larvae. The results of this survey suggest that B. acuscutellaris may not be an important vector of scrub typhus, but more studies are needed in endemic areas.

Early detection of Rickettsia tsutsugamushi in peripheral monocyte cultures derived from experimentally infected monkeys and dogs.

Rickettsia tsutsugamushi organisms were identified in Giemsa and fluorescent antibody stained monocyte cell cultures derived from experimentally infected monkeys and dogs. The identification of organisms in monocyte cell cultures compared favorably with the standard technique of mouse inoculation.