Surveillance of Chiggers Density in the Republic of Korea, 2022

Abstract

Scrub typhus is an acute febrile disease in autumn. Scrub typhus is spread to people through bites of infected chiggers (larval mites) with Orientia tsutsugamushi. The symptoms are fever, headache, and sometimes rash. In order to survey occurrence of chiggers in the Republic of Korea, chiggers were collected weekly at 18 locations nationwide from August 24 to December 14 in 2022. The number of chiggers collected per trap was calculated for comparison. The period of increasing of chiggers was hurried by 2 weeks compared to 2021. Moreover, after showing the highest occurrence (week 45), it decreased gradually. Of the total 3,723 chiggers were identified as 18 species in 5 genera. Among them, Leptotrombidium scutellare was the dominant species with 1,418 specimens (37.2%). Given the recent an increase in the number of people working outdoors and aging of the rural population, there is a high concern that the number of scrub typhus patients will increase. Therefore, if it is suspected symtoms after being bitten by a mite, it is important to get diagnosed early. And also, because the pattern of vector occurrence has been effected due to climate change, it is important to monitor for mites continuously.

Key messages

① What is known previously?

In Korea, eight species among a total of 59 species of chiggers were reported to transmit scrub typhus.

② What new information is presented?

Trap index in 2022 was 10.58, an decrease of 27.3% compared to 14.55 in 2021. Of the total 3,723 chiggers were identified as 18 species in 5 genera. Among them, Leptotrombidium scutellare is the dominant species.

③ What are implications?

The number of patients steadily increases after autumn, when the density of Chiggers increases, and the increase in patients in autumn appears to be closely related to the density of larvae.

Introduction

The importance of public health is highlighted amid the rising incidence of infectious disease vectors and endemics caused by climate change. One such disease is scrub typhus, which is transmitted by chigger larvae infected with Orientia tsutsugamushi attaching to the human body and feeding on bodily fluids. Common symptoms include fever, crusting, macular rash, and swollen lymph nodes. Taxonomically, the vector chiggers belong to the order Acari, suborder Prostigmata, and family Trombiculidae [1,2]. Among the 59 species of chiggers found in the Republic of Korea (ROK), 8, including Leptotrombidium scutellare and Leptotrombidium pallidum, have been identified as vectors of scrub typhus [1]. Scrub typhus is classified as a class 3 nationally notifiable infectious disease and is an acute febrile illness that primarily occurs in Asia. Approximately 1 million people are diagnosed with scrub typhus worldwide each year, with an estimated 1 billion people at risk of infection [3-5]. Most patients infected with O. tsutsugamushi can recover with appropriate antibiotic treatment, but some fatalities have been reported in the ROK [6]. Scrub typhus initially emerged in the ROK during the Korean War in 1951. Since being designated as a nationally notifiable infectious disease in 1994, patient surveillance has been ongoing [5,7]. The Division of Vectors and Parasitic Diseases of the Korea Disease Control and Prevention Agency (KDCA) conducts annual nationwide surveys that examine the distribution and incidence of chiggers and provide valuable information on the timing of onset and trends. Based on the chigger incidence density investigated by the Climate Change Response Vector Surveillance Regional Centers and the Jeollabuk-do Research Institute of Public Health and Environment in 2022, this study aimed to provide fundamental data for the prevention and management of vector-borne diseases.

Methods

The weekly survey data between August 24, 2022, and December 14, 2022, provided by the 16 Climate Change Response Vector Surveillance Regional Centers managed by the KDCA and the Jeollabuk-do Research Institute of Public Health and Environment were used in this study. Eighteen regions were chosen for the survey: two regions in Gangwon Province (Cheolwol and Gangneung-si), three in Gyeonggi Province (Hwaseong-si, Paju-si, and Yeoju-si), one in North Chungcheong Province (Okcheon-gun), two in South Chungcheong Province (Boryeong-si and Yesan-gun), three in North Jeolla Province (Buan-gun, Jinan-gun, and Jeongeup-si), two in South Jeolla Province (Suncheon-si and Boseong-gun), two in North Gyeongsang Province (Gimcheon-si and Yeongdeok-gun), two in South Gyeongsang Province (Geoje-si and Hapcheon-gun), and one in Jeju Island (Jeju-si).

The selected survey locations were classified into four distinct environments where people are more likely to come into contact with chiggers: fields, rice paddies, grassy areas, and waterways. Chiggers were collected using a device developed by the KDCA (patent no. 10-1555975) and identified. The collection device had an adhesive tape attached, and a chigger attractant was placed in the center of the device. Twenty of these collection devices were set up in each of the survey areas, with five in each environment. The tape was collected every Wednesday during the survey period. The collected tape samples were observed under a dissecting microscope at each regional center, and organisms suspected to be chiggers were mounted on slides and classified and identified under an optical microscope. The weekly incidence of chiggers was quantified by converting the number of chiggers collected into a per-trap index (weekly trap index: number of organisms/number of traps) for each week. The surveillance data by region and week were compared and analyzed against those obtained from previous years. The climatic data for each week during the survey period were obtained from the Korea Meteorological Administration’s Weather Data Open Portal. The data were collected from the nearest multipoint weather observation station to each survey location [8]. Information regarding scrub typhus patients (weekly or monthly number of patients) was sourced from the KDCA Infectious Disease Portal [9].

Results

According to the 2022 chigger surveillance results, the chigger activity increased 2 weeks earlier compared with that in 2021. In contrast to the observations in 2021, when two distinct peaks occurred in chigger counts, a different pattern was observed in 2022; chiggers initially appeared in week 36, gradually increased from week 40, rapidly surged from week 43, peaked in numbers in week 45 (568 organisms; trap index: 1.58), and then gradually declined. Throughout the 2022 chigger survey period, the cumulative trap index was 10.58, reflecting a decrease of approximately 27.3% from the 2021 figure of 14.55 (Figure 1).

Figure 1. Comparison of weekly chiggers density, 2021–2022.

Temp., temperature

In 2022, 3,723 chiggers from 5 genera and 18 species were collected (of which 84 chiggers could not be identified). By region, the highest number of chiggers were collected in Cheolwon, Gangwon Province (n=670; 17.6%), followed by Jeju-si, Jeju Island (n=548; 14.4%), and Geoje, South Gyeongsang Province (n=410; 10.8%). Chiggers were first observed in Cheolwon, Gangwon Province on week 36. At the time of collection, the average temperature in Cheolwon was 20.2°C, which was 1.6°C lower than the overall average temperature across all locations. Subsequently, chiggers were initially observed in Okcheon, North Chungcheong Province in week 40, South Gyeongsang Province (Geoje and Hapcheon) in week 42, and Jeju Island in week 43, showing a more delayed appearance with decreasing latitude (Table 1).

Table 1. Total number of chiggers in 2022.

Region	August		September					October					November						December		Total
	36 wk		37 wk	38 wk	39 wk	40 wk		41 wk	42 wk	43 wk	44 wk		45 wk	46 wk	47 wk	48 wk	49 wk		50 wk	51 wk
Cheorwon	2		7	6	2	32		72	44	62	88		53	75	79	65	49		6	28	670
Gangneung	0		0	0	0	3		8	5	34	46		78	62	75	49	16		10	12	398
Hwaseong	0		0	1	0	0		0	0	10	22		29	14	32	24	11		0	0	143
Paju	0		0	0	0	0		0	4	11	12		13	8	3	0	2		0	0	53
Yeoju	0		0	2	0	1		1	6	30	43		36	41	25	38	16		2	1	242
Okcheon	0		0	0	0	1		1	7	5	32		22	22	21	21	13		9	1	155
Boryeong	0		0	0	0	0		0	0	0	3		4	4	5	0	0		4	2	22
Yesan	0		0	0	0	0		0	8	12	29		26	39	27	30	16		4	2	193
Buan	0		0	0	0	0		1	0	1	1		10	6	5	3	5		0	2	34
Jinan	0		0	0	0	1		0	11	24	39		38	34	14	10	9		3	2	185
Jeongeup	0		0	0	0	0		0	1	16	46		34	34	21	20	20		19	17	228
Suncheon	0		0	0	0	0		0	0	0	0		1	0	0	9	0		0	0	10
Boseong	0		0	1	0	3		1	7	1	1		15	15	9	4	0		0	0	57
Gimcheon	0		0	0	0	0		0	0	3	9		52	28	24	15	13		14	27	185
Yeongdeok	0		0	0	0	1		1	0	3	9		8	26	29	28	18		32	35	190
Geoje	0		0	0	0	0		0	8	18	47		44	34	42	65	87		45	20	410
Hapcheon	0		0	0	0	0		0	1	18	13		7	2	7	13	3		7	13	84
Jeju	0		0	0	0	0		0	0	55	53		98	120	93	35	35		38	21	548
Total	2		7	10	2	42		85	102	303	493		568	564	511	429	313		193	183	3,807
Trap index*	0.01		0.02	0.03	0.01	0.12		0.24	0.28	0.84	1.37		1.58	1.57	1.42	1.19	0.87		0.54	0.51

*Trap Index (no. of chiggers/traps)

After excluding 84 chiggers that could not be identified, 3,723 out of the 3,807 collected chiggers were examined. The dominant species were L. scutellare (n=1,418; 37.2%), Leptotrombidium palpale (n=662; 17.4%), Neotrombicula tamiyai (n=616; 16.2%), L. pallidum (n=511; 13.4%), and Neotrombicula kwangneungensis (n=254; 6.7%).

Discussion

In the fall of 2022 (weeks 36–51), 3,807 chiggers were collected, demonstrating a 17.9% reduction (n=829) from the 4,636 specimens collected in 2021. The decrease in chigger incidence is attributed to several factors, including a notable increase in the average temperature in week 47, which was 3.7°C higher (11.7°C) than that in 2021 (8.0°C); a decrease in temperature in week 50 (3.3°C) compared with that in 2021 (4.3°C); and a reduction in chigger incidence when the temperature falls below 5°C [10]. The 2022 chigger surveillance results showed a gradual increase in chigger activity from week 40, with a sharp spike observed in week 43. The timing of chigger occurrence in 2022 was approximately 2 weeks earlier than that in the previous year, and the surge in the incidence was similar to the expected rate (Figure 1). Comparing the average temperatures in 18 survey locations, the average temperature (17.1°C) was approximately 3.4°C lower than that in the previous year (20.5°C) from week 40 to week 42. This temperature change is possibly the cause of the earlier onset of increased chigger activity, which was 2 weeks earlier than that in the previous year. In 2022, chiggers tended to emerge from high-latitude regions and surge when the average temperature in the fall dropped to 15°C (Figure 1). Additionally, chigger larvae tend to start their attachment activity at temperatures above 10°C, and their numbers increase when temperatures drop below 20°C, indicating a close relationship between temperature and chigger larval activity [10,11].

In 2022, all eight chigger species known to transmit O. tsutsugamushi were identified: L. scutellare, L. palpale, L. pallidum, Neotrombicula japonica, Helenicula miyagawai, Leptotrombidium orientale, Euschoengastia koreansis, and Leptotrombidium zetum [12]. Of the three most frequently detected species, L. scutellare and L. pallidum peaked in weeks 44 and 45 (n=262 and n=106, respectively), while L. palpale peaked on week 48 (n=128) (Figure 2). By season, L. scutellare emerges in the fall, L. pallidum in the spring and fall, and L. palpale in late fall [13].

Figure 2. Comparison of major species density.

We compared the chigger surveillance results obtained using the chigger collection devices and incidence trends of scrub typhus between 2021 and 2022. In 2021, both chigger and scrub typhus incidence rates increased concurrently in week 43. In 2022, a gradual increase was observed in the chigger activity from weeks 40 to 42, followed by a sharp spike in the number of chiggers in week 43 (Figure 3). Subsequently, the number of scrub typhus cases increased significantly during weeks 45 and 46 (Figure 3). Although the cumulative trap index for chiggers in 2022 (10.58) dropped by 27.35% from that in 2021, the annual number of scrub typhus cases in 2022 (6,232 patients) increased by 5.4% compared with that reported in the previous year (5,915 patients). The similarity in the timing of the increase in chigger numbers and scrub typhus cases with an incubation period of 1–3 weeks suggests an association between the occurrence of chiggers and the increase in scrub typhus cases [14]. The increase in scrub typhus cases despite a decrease in chigger activity in 2022 compared with that in 2021 may be attributed to other factors such as changes in people’s activity patterns, as evidenced by patients’ epidemiological data (KDCA, 2022). From temporary occupational exposure cases, the number of cases from exposure during outdoor activities increased significantly from 694 cases in 2021 to 1,609 cases in 2022. This finding suggests that other variables, including people’s activity patterns, may have influenced these results.

Figure 3. Comparison of weekly chiggers density and patients, 2021–2022.

Scrub typhus is an infectious disease primarily prevalent in Asian regions, and new cases consistently occur in the ROK as well, particularly among older adults in rural areas. Given the increasing proportion of older adults (≥65 years) in rural areas calls, it is imperative to raise awareness and implement heightened precautions against this infectious disease. Additionally, as the risk of contracting coronavirus disease 2019 has diminished, engagement in outdoor activities has increased, leading to a rise in scrub typhus cases even in suburban areas [7]. Therefore, it is crucial to remain vigilant and proactively seek information regarding chigger activity. Chigger bites should be averted during the fall chigger larval season by minimizing exposure to such environments and adhering to preventive measures. If symptoms develop after engaging in outdoor activities, seeking immediate medical attention is crucial, as the mortality rate for untreated cases ranges 0% to 30% depending on the patient’s immune system.