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Osong Public Health Res Perspect > Volume 1(1); 2010 > Article
Kim, Kim, and Lee: Epidemiological Characteristics of Scrub Typhus in Korea, 2009

Abstract

Objectives

Currently, the incidence of scrub typhus has increased in urban areas. In this study, we described the epidemiological characteristics of scrub typhus cases reported in the urban areas of Korea in 2009.

Methods

We analysed the case investigation reports of scrub typhus cases that were collected in Korea in 2009. Specially, the different risk factors such as fieldwork and outdoor activity were compared to urban and rural areas, and six urban cities. Statistical analysis was performed using χ2 test.

Results

A total of 4,461 cases (including 1,663 suspected cases) were analysed in this study. Among these, the case reports of 4,254 cases had complete addresses. The cases with outdoor activities were 720 (85.2%) in urban areas. In Daegu and Daejeon, the number of cases participated in outdoor activities was 32 (34.4%) and 23 (31.5%), respectively. In other urban areas, cases with outdoor activities were more than 85%.

Conclusion

The most common infection risk factor was outdoor activity in urban areas. However, the proportion and distribution of outdoor activities were different in urban areas. These results will be used to establish strategies for effective prevention and management in urban areas.

Key words

epidemiology; korea; Orientia tsutsugamushi; scrub typhus

Introduction

Scrub typhus (tsutsugamushi disease) is a febrile illness caused by rickettsial bacteria such as Orientia tsutsugamushi.1 Scrub typhus is categorized in Group III of the notifiable communicable diseases. In Korea, scrub typhus is the most common rickettsial disease, with more than 6,000 cases reported annually since 2005 and 4,995 cases reported in 2009.
According to the National Notifiable Disease Surveillance System, the proportion of cases identified in farmers decreased from 43.3% in 2001 to 25.0% in 2009. According to National Notifiable Disease Surveillance System, the scrub typhus cases in urban areas increased from 388 (20.0%) in 2002 to 1,345 (26.9%) in 2009. However, most of these studies have been performed in rural areas that are endemic for scrub typhus, and little attention has been paid to cases in urban areas. This study aimed to describe the epidemiological characteristics of scrub typhus cases in urban areas.

Materials and Methods

Investigation of a scrub typhus case (including suspected cases) was legally conducted by the public health care center officials. The investigation results were reported to the Korea Center for Disease Control and Prevention (KCDC) through web system. The case investigation report included demographics, clinical factors, diagnosis, and infection risk factors within 1 month before the onset of scrub typhus.
We analysed the case investigation reports of scrub typhus cases that were collected in Korea in 2009. The cases were classified into two groups according to their area of residence. The urban areas in this investigation included Seoul, Busan, Daegu, Inchon, Gwangju, Daejeon, and Ulsan, whereas the rural areas included Gangwon, Gyeonggi, Chungbuk, Chungnam, Jeonbuk, Jeonnam, Gyeongbuk, Gyeongnam, and Jeju. A total of 4,461 case investigation reports (including 1,663 suspected cases) were collected in 2009. The number of case reports with complete addresses was 4,254, which include 1,028 from urban areas and 3,226 from rural areas (Figure 1).
All the cases were classified by occupation as either farmers or non-farmers. “Farmers” were defined as farmers, forest workers, stock breeding workers, and landscaping workers. “Non-farmers” were defined as officers (students, office workers, and experts), housewives, unemployed, and outdoor workers (fisheries, mining industry employees, construction workers, etc.).
Infection risk factors included fieldwork and outdoor activities. “Fieldwork” was defined as continuous agricultural, forestry, or stock breeding work, as well as working in vegetable gardens. “Outdoor activity” was defined as the activities that non-farmers irregularly participate in, which may be related to agriculture (such as assisting farmer relatives) or leisure (such as climbing, gathering chestnuts, mowing around graves, etc.).
In outdoor activities, the government-run public work projects provide temporary employment to people by public sector. This occupation involved mainly maintenance of hiking trails, deforestation, and cutting grass. In case where the occupation was not reported but the infection risk factor was, the occupation category classified by describing and frequency of infection risk factors.
In this study, we described the epidemiological characteristics of scrub typhus cases that occurred in 2009 according to gender and region. Scrub typhus cases from Ulsan were excluded from the analysis in urban areas because 35% of these cases did not have infection risk factors.
The analyses were conducted using SAS statistical package version 9.1.3 (SAS Inc., Cary, NC, USA). Statistical analysis was performed using χ2 test, and a p value < 0.05 was considered as significant.

Results

3.1 Epidemiological characteristics (N = 4,461)

A total of 4,461 cases (including 1,663 suspected cases) were analysed in this study. Among these, the case reports of 4,254 cases had complete addresses. Most of the cases were in the age range of 60–69 years in both males (N = 277; 26.9%) and females (N = 856; 26.5%).
The farmer’s occupation was the most common occupation as 1,982 (47.8%) cases, in both male 750 (50.4%) and female 1,232 (47.0%) cases (Table 1). Between October and November, 4,129 (90.7%) cases were reported (Table 1).
A total of 2,060 (53.4%) cases participated in outdoor activities. The infection risk factor was not recognized in 583 (13.1%) cases. A total of 41 (7.0%) scrub typhus cases had workplaces or residences either located near a mountain/hill or infested with mice. Furthermore, three (0.5%) cases did not participate in any outdoor activities although a member of their family did. It was impossible to determine the infection risk factor in 539 (12.0%) cases. The median duration between symptom onset and diagnosis was 5 days (range, 0–117 days) (Table 1).

3.2 Epidemiological characteristics by region (N = 4,254)

The number of scrub typhus cases aged below 50 years was 265 (25.8%) and 564 (17.5%) in urban and rural areas, respectively. The number of farmer cases in urban and rural areas was 149 (16.4%) and 1,752 (58.1%), respectively (Table 2). The number of cases participated in outdoor activities in urban and rural areas was 720 (85.2%) and 1,235 (43.4%), respectively (Table 2).

3.3 Epidemiological characteristics by urban areas (N = 1,028)

The proportion of cases aged below 50 years was more than 20% in all urban areas. Inchon had the highest proportion of cases aged below 50 years (N = 36; 45.6%). Outdoor activity was observed in more than 70% of the cases in all urban areas. In particular, 98 (98.0%) cases had a history of outdoor activity in Seoul (Table 3). The distribution of outdoor activities differed between urban areas. In Daegu and Daejeon, the number of cases participated in fieldwork was 32 (34.4%) and 23 (31.5%), respectively, which was higher than that in other urban areas. Among all urban areas, Busan had the highest number of cases (N = 33; 14.2%) participated in government-run public work projects, and their occupations included outdoor activities (Table 3).

Discussion

Recently, the incidence of scrub typhus has increased in the urban areas. In this study, we described the epidemiological characteristics of scrub typhus cases reported in the urban areas of Korea in 2009. A total of 4,129 (90.7%) cases occurred between October and November 2009. Of these, 2,784 (62.4%) were female and 2,650 (59.4%) were 60 years or older. These results were consistent with those of the previous studies.1–6 The proportion of scrub typhus cases in Chungnam, Jeonnam, and Jeonbuk areas was much more than that in any other region. A previous study correlated with higher mean chigger indices of Leptotrombidium pallidum and Leptotrombidium scutellare.7 There were no significant differences between males and females in the seasons; however, a previous study6 reported that the number of female cases increases during early autumn, whereas the number of male cases increases in late autumn.
There were significant differences in age, occupation, and infection risk factors between urban and rural areas (p < 0.001). Of the cases that were analysed in urban areas, 617 (60.0%) were female, 757 (83.6%) were non-farmer, and 524 (51.0%) were below 60 years old. Previously,3,4 farmers and elders (≥60 years of age) were considered a high-risk group. However, our results suggest that even more attention should be paid to the non-farmers.
The most common infection risk factor was outdoor activity in urban areas. The distribution of outdoor activities differed between urban areas. The proportion of fieldwork in Daegu and Daejeon was higher as compared with that in other urban areas. The proportion of government-run public work projects of outdoor activities was the highest in Busan among the urban areas. Thus, control measures need to be established according to regional epidemiological characteristics. Of the 46 cases in Busan, 33 (71.7%) participated in government-run public work projects The government-run public work projects included many occupations, such as maintenance of hiking trails, deforestation, and cutting grass. This occupation characteristic could result in high exposure to chigger mites.
Our study has some limitations. This study was a descriptive study; therefore, we could not find out risk factors and the exact infection pathway. Moreover, cases were not classified as suspected and confirmed case.
This article described the epidemiological characteristics of scrub typhus cases in urban areas of Korea. Our results suggest that a scrub typhus education and surveillance system is needed, particularly in urban areas. Further works are required to determine the infectious risk factors in urban areas by laboratory confirmed case-control study.

Notes

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

References

1. Kweon S.S., Choi J.S., Lim H.S.. A community-based case-control study of behavioral factors associated with scrub typhus during the autumn epidemic season in South Korea. Am J Trop Med Hyg 80(3):2009;442-446.
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2. Kweon S.S., Choi J.S., Lim H.S.. Rapid increase of scrub typhus, South Korea, 2001-2006. Emerg Infect Dis 15(7):2009;1127-1129.
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3. Lim H.S., Lee K., Jung C., Cheong H.K.. A study on the epidemiologic characteristics of scrub typhus in Gyeongsangbuk-do, 1999–2001. Korean J Epidemiol 27:2005;70-79.

4. Lim H.S., Yoo S.J.. Epidemiological characteristics of scrub typhus in Gyeongsangbuk do, 2006. Dongguk J Med (15):2008;26-36.

5. Chang J.G., Park P.G., Lee H.S.. The study of 46 cases of tsutsugamushi disease in Young-Dong region in Gang-Won-Do. J Infect Chemother 35(3):2003;138-144.

6. Kong W.S., Shin E.H., Lee H.. Time-spatial distribution of scrub typhus and its environmental ecology. J Korean Geogr Soc 42(6):2008;82-95.

7. Lee I.Y., Kim H.C., Seo J.H.. Geographical distribution and relative abundance of vectors of scrub typhus in the Republic of Korea. Korean J Parasitol 47(4):2009;381-386.
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Figure 1
Study population flow chart.
gr1
Table 1
General characteristics of scrub typhus cases between January and December 2009 in Korea
Item Male Female Total p
Age (yr)
 <50 415 (24.8) 453 (16.3) 868 (19.5) <0.001
 50–59 342 (20.4) 600 (21.6) 942 (21.1)
 60–69 431 (25.7) 741 (26.6) 1,172 (26.3)
 ≥70 488 (29.1) 990 (35.6) 1,478 (33.1)



Occupation
 Farmers 750 (50.4) 1,232 (47.0) 1,982 (47.8) 0.037
 Non-farmers 739 (49.6) 1,390 (53.0) 2,129 (52.2)



Region
 Urban
 Seoul 49 (3.1) 58 (2.2) 107 (2.5)
 Busan 112 (7.0) 176 (6.6) 288 (6.8)
 Daegu 36 (2.3) 65 (2.4) 101 (2.4)
 Inchon 32 (2.0) 47 (1.8) 79 (1.9)
 Gwangju 41 (2.6) 52 (2.0) 93 (2.2)
 Daejeon 73 (4.6) 95 (3.6) 168 (3.9)
 Ulsan 68 (4.3) 124 (4.7) 192 (4.5)
 Sub-total 411 (25.7) 617 (23.2) 1,028 (24.2)



 Rural
 Gangwon 7 (0.4) 31 (1.2) 38 (0.9)
 Gyeonggi 155 (9.7) 186 (7.0) 341 (8.0)
 Chungbuk 50 (3.1) 120 (4.5) 170 (4.0)
 Chungnam 233 (13.9) 380 (14.3) 603 (14.2)
 Jeonbuk 233 (14.6) 417 (15.7) 650 (15.3)
 Jeonnam 209 (13.1) 429 (16.2) 63 8 (15.0)
 Gyeongbuk 114 (7.1) 199 (7.5) 313 (7.4)
 Gyeongnam 192 (12.0) 268 (10.1) 460 (10.8)
 Jeju 6 (0.4) 7 (0.3) 13 (0.3)
 Sub-total 1,189 (74.3) 2,037 (76.8) 3,226 (75.8)



Month
 January–September 131 (7.8) 192 (6.9) 323 (7.3) 0.486
 October 907 (54.3) 1,482 (53.3) 2,389 (53.7)
 November 600 (35.9) 1,049 (37.7) 1,649 (37.0)
 December 33 (2.0) 58 (2.1) 91 (2.0)



Infection risk factor
 Only fieldwork 673 (46.3) 1,135 (46.8) 1,808 (46.6) 0.706
 Only outdoor activity 692 (47.7) 1,161 (47.9) 1,853 (47.8)
 Both fieldwork and outdoor activity 87 (6.0) 130 (5.4) 217 (5.6)



Non-recognized infection risk factor
 Impossible to estimate 198 (88.4) 341 (95.0) 539 (92.5) 0.003
 Possible to estimate 26 (11.6) 18 (5.0) 44 (7.5)



Major symptoms
 Fever 1,450 (86.5) 2,433 (87.4) 3,883 (87.0)
 Chill 1,052 (62.8) 1,739 (62.4) 2,791 (62.6)
 Eschar 1,254 (74.8) 2,167 (77.8) 3,421 (76.7)

Data n (%).

χ2 test.

Fieldwork was defined as continuous agricultural, forestry, or stock breeding work, as well as working in vegetable gardens.

Outdoor activity was defined as the activities that non-farmers irregularly participate in, which may be related to agriculture (such as assisting farmer relatives) or leisure (such as climbing, tracking, gathering chestnuts, mowing around graves, etc.).

Table 2
Epidemiological characteristics of scrub typhus cases by region between January and December 2009 in Korea
Item Urban* Rural Total p
Gender
 Male 411 (40.0) 1,189 (36.9) 1,600 (37.6) 0.072
 Female 617 (60.0) 2,037 (63.1) 2,654 (62.4)



Age (yr)
 <50 265 (25.8) 564 (17.5) 829 (19.5) <0.001
 50–59 259 (25.2) 620 (19.2) 879 (20.7)
 60–69 277 (26.9) 856 (26.5) 1,133 (26.6)
 ≥70 227 (22.1) 1,185 (36.7) 1,412 (33.2)



Occupation
 Farmers 149 (16.4) 1,752 (58.1) 1,901 (48.5) <0.001
 Non-farmers 757 (83.6) 1,261 (41.9) 2,018 (51.5)



Month
 January–September 76 (7.4) 202 (6.3) 278 (6.5) 0.514
 October 541 (52.7) 1,729 (53.7) 2,270 (53.4)
 November 385 (37.5) 1,227 (38.1) 1,612 (38.0)
 December 24 (2.3) 63 (2.0) 87 (2.0)



Infection risk factor
 Only fieldwork 125 (14.8) 1,612 (56.6) 1,737 (47.0) <0.001
 Only outdoor activity§ 696 (82.4) 1,051 (36.9) 1,747 (47.3)
 Both fieldwork and outdoor activity 24 (2.8) 184 (6.5) 208 (5.6)



Outdoor activities
 Non-farmers irregularly participate in agricultural activities 241 (33.5) 484 (39.2) 725 (37.1) <0.012
 Climbing and tracking 135 (18.8) 199(16.1) 334 (17.1) 0.135
 Gathering chestnuts 100 (13.9) 186 (15.1) 286 (14.6) 0.479
 Government-run public work projects 46 (6.4) 57 (4.6) 103 (5.3) 0.090
 Mowing around graves 53 (7.4) 81 (6.6) 134 (6.9) 0.498
 Others 188 (26.1) 298 (24.1) 486 (24.9) 0.328

Data n (%).

Includes Seoul, Busan, Daegu, Inchon, Gwangju, Daejeon, and Ulsan.

χ2 test.

Fieldwork was defined as continuous agricultural, forestry, or stock breeding work, as well as working in vegetable gardens.

§ Outdoor activity was defined as the activities that non-farmers irregularly participate in, which may be related to agriculture (such as assisting farmer relatives) or leisure (such as climbing, tracking, gathering chestnuts, mowing around graves, etc.).

Government-run public work projects provide temporary employment to people by public sector.

Table 3
Epidemiological characteristics of scrub typhus cases between January and December 2009 in urban areas
Item Seoul Busan Daegu Inchon Gwangju Daejeon
Gender
 Male 49 (45.8) 112 (38.9) 36 (35.6) 32 (40.5) 73 (43.5) 41 (44.1)
 Female 58 (54.2) 176 (61.1) 65 (64.4) 74 (59.5) 95 (56.5) 52 (55.9)



Age (yr)
 <50 36 (36.6) 64 (22.2) 22 (21.8) 36 (45.6) 38 (22.6) 30 (32.3)
 50–59 24 (22.4) 89 (30.9) 26 (25.7) 14 (17.7) 40 (23.8) 15 (16.1)
 60–69 28 (26.2) 74 (25.7) 28 (27.7) 15 (19.0) 44 (26.2) 26 (28.0)
 ≥70 19 (17.8) 61 (21.2) 25 (24.8) 14 (17.7) 46 (27.4) 22 (23.7)



Month
 January–September 9 (7.8) 18 (6.2) 15 (14.6) 13 (15.9) 7 (4.1) 10 (9.3)
 October 81 (69.8) 126 (43.6) 58 (56.3) 40 (48.8) 105 (61.8) 67 (62.6)
 November 26 (22.4) 137 (47.4) 29 (28.2) 26 (31.7) 55 (32.4) 26 (24.3)
 December 0 (0.0) 8 (2.8) 1 (1.0) 3 (3.7) 3 (1.8) 4 (3.7)



Infection risk factor
 Only fieldwork 2 (2.0) 25 (9.7) 25 (27.0) 7 (11.1) 22 (14.1) 20 (27.4)
 Only outdoor activity 97 (97.0) 229 (89.1) 62 (66.0) 54 (85.7) 128 (82.1) 50 (68.5)
 Both fieldwork and outdoor activity 1 (1.0) 3 (1.2) 7 (7.4) 2 (3.2) 6 (3.8) 3 (4.1)



Outdoor activities
 Non-farmers irregularly participate in agricultural activities 24 (24.5) 94 (40.5) 28 (40.6) 19 (33.9) 30 (22.4) 17 (32.1)
 Climbing and tracking 19 (19.4) 44 (19.0) 12 (17.4) 5 (8.9) 30 (22.4) 10 (18.9)
 Gathering chestnuts 14 (14.3) 18 (7.8) 5 (7.2) 9 (16.1) 28 (20.9) 7 (13.2)
 Government-run public work projects 1 (1.0) 33 (14.2) 1 (1.4) 1 (1.8) 6 (4.5) 0 (0.0)
 Mowing around graves 17 (17.3) 10 (4.3) 1 (1.4) 9 (16.1) 8 (6.0) 6 (11.3)
 Others 31 (31.6) 48 (20.7) 29 (40.6) 18 (32.1) 29 (21.6) 19 (34.0)

Data n (%).

Fieldwork was defined as continuous agricultural, forestry, or stock breeding work, as well as working in vegetable gardens.

Outdoor activity was defined as the activities that non-farmers irregularly participate in, which may be related to agriculture (such as assisting farmer relatives) or leisure (such as climbing, tracking, gathering chestnuts, mowing around graves, etc.).

Government-run public work projects provide temporary employment to people by public sector.



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