Surveillance of Tick Populations in the Republic of Korea in 2024

Abstract

Objectives

Hard ticks are major vectors carrying various pathogens in the Republic of Korea (ROK), including the severe fever with thrombocytopenia syndrome (SFTS) virus. Recent findings suggest ticks have the potential to carry multiple infectious agents. We assessed hard tick distribution to improve tick-borne disease risk assessment by providing up-to-date information regarding tick density and pathogen presence.

Methods

From April to November 2024, ticks were collected monthly from 16 sites spanning four high-risk environments (mountain roads, graves, copses, and grasslands). Twelve traps (three per environment) baited with dry ice were placed at each site. Tick density was measured using the average number of ticks collected per trap for comparison with previous years.

Results

In total, 71,924 hard ticks representing three genera and six species were collected. Haemaphysalis longicornis, the primary vector of SFTS, accounted for 97.3% of the identified ticks (excluding larvae). The overall hard tick index in 2024 was 46.8, with the highest levels observed in August and September and the lowest levels in October and November.

Conclusions

The 2024 hard tick index was 46.8, representing a 30.0% increase over 2023 (36.0) and a 13.9% increase over the 2020 to 2022 average (41.1). The increase in the hard tick index is presumed to be due to the rise in average temperature and a sharp increase in tick density in certain regions. As the risk of vector-borne diseases increases due to climate change in ROK, it is essential to adhere to prevention guidelines to avoid infectious diseases.

Key messages

① What is known previously?

Severe fever with thrombocytopenia syndrome (SFTS) is legally designated an infectious disease in the Republic of Korea necessitating government management and supervision, and is transmitted by hard ticks carrying the SFTS virus.

② What new information is presented?

In 2024, we recorded a hard tick index (trap index; total number of hard ticks collected/number of traps) of 46.8, representing a 30.0% increase compared to that in 2023 (36.0) and a 13.9% increase compared to the average of normal years (41.1) from 2020 to 2022.

③ What are implications?

SFTS, a high mortality vector-borne disease currently lacking treatment or vaccine, poses increasing risk owing to climate change impact on vector range and density. Thus, adherence to prevention guidelines and continuous surveillance of the vector density and pathogen prevalence are crucial.

Introduction

Severe fever with thrombocytopenia syndrome (SFTS) is one of the most prominent tick-borne zoonotic diseases. It is caused by the SFTS virus (SFTSV or Bandavirus dabieense), which belongs to the family Phenuiviridae and genus Bandavirus. SFTS is primarily transmitted by ticks. SFTS was first reported in China in 2009 and has been mainly reported in East Asian countries, such as the Republic of Korea (ROK), Japan, and China. Recently, cases have also been documented in Southeast Asia, including Thailand and Vietnam, and Southwest Asia, including the United Arab Emirates and Pakistan [1-3].

Since the first patient was reported in the ROK in 2013, it has been designated as a notifiable infectious disease and managed [4]. From 2013 to 2024, a total of 2,065 SFTS cases occurred in the ROK, resulting in 381 deaths with a fatality rate of approximately 18.5% [5]. Ticks belong to the class Arachnida, family Ixodidae, and 44 species in five genera have been reported in the ROK, among which the main vectors of SFTS include Haemaphysalis longicornis, Haemaphysalis flava, Amblyomma testudinarium, and Ixodes nipponensis [6].

Ticks are hematophagous ectoparasites that transmit a variety of viral, bacterial, and protozoan pathogens. They are important arthropod vectors in veterinary and public health. They parasitize not only wild rodents, deer, and humans, but also reptiles and birds, and transmit pathogens during the blood-feeding process as they attach to hosts while wildlife move. Three-host ticks are known to feed on three different host animals in each stage (larvae, nymphs, and adults) of their life cycle. Notably, certain tick species exhibit both parthenogenetic and bisexual reproduction, allowing for rapid population growth and wide geographic spread. Additionally, transovarial transmission of pathogens from adult females to their offspring is also possible [7,8].

Most cases are infected by the bite of a tick that carries the virus; however, rare cases have been reported by contact with bodily fluids and blood from infected humans or animals [9,10]. Ticks have been identified as the possible vectors of several infectious diseases, including Lyme disease, rickettsioses, typhus fever, anaplasmosis, babesiosis, and SFTS [11-14]. Ongoing investigations and analyses of the changes in tick density and pathogen-carrying capacity are necessary. The Division of Vectors and Parasitic Diseases at the Korea Disease Control and Prevention Agency operates the “regional centers for vector surveillance against climate change” to monitor vector populations nationwide, with the aim of providing a basis for analyzing the distribution and density of hard ticks in 2024.

Methods

1. Collection Regions and Environments

This study was conducted jointly by 16 regional centers for vector surveillance against climate change and the Gyeongbuk Regional Center for Disease Control and Prevention. The newly surveyed areas in 2024, Yeongdeok-gun and Gunwi-gun in Gyeongsangbuk-do, were excluded to allow for comparisons with previous data. The surveillance regions included Inje-gun, Samcheok-si, Gangwon-do; Ganghwa-gun, Incheon; Ulju-gun, Ulsan; Pocheon-si, Gwangju-si, Gyeonggi-do; Chungju-si, Chungcheongbuk-do; Nonsan-si, Dangjin-si, Chungcheongnam-do; Gochang-gun, Jeollabuk-do; Gokseong-gun, Boseong-gun, Jeollanam-do; Gimcheon-si, Andong-si, Gyeongsangbuk-do; Jinju-si, Gyeongsangnam-do; and Jeju-si, Jeju-do. The regional surveys were conducted in four different environments where people are more likely to come into contact with ticks: mountain roads, graves, copses, and grasslands.

2. Collection Periods and Methods

Ticks were collected on the 3rd weekday of each month from April through November 2024. A total of 12 traps were set up at each region (three traps per environment) using dry ice as an attractant. Traps were set at 10:00 AM on the day of collection and retrieved at 10:00 AM the following day.

3. Identification and Analysis

The collected ticks were identified under a dissecting microscope using the identification key by Yamaguti et al. [15]. Larvae were classified only to the genus level, as accurate species-level identification was not possible. Tick density was calculated as the average number of ticks per trap per collection (tick index: number of individuals/number of traps). The number of tick individuals is a count that includes all the developmental stages (adults, nymphs, and larvae). To assess temporal changes in tick density, we compared tick index data with historical surveillance data collected during the same seasons and from the same regions. Distributions by tick species were plotted using the Spatial Analyst Tool analysis method in ArcGIS 9.0 (Environmental Systems Research Institute) using Inverse Distance Weighted based on the tick species index.

Results

In 2024, a total of 71,924 individuals from six species in three genera were collected during the tick survey (Table 1). An average of 4,495 ticks were collected per region. By region, the tick density was the highest in Inje-gun, Gangwon-do, with 24,096 individuals, and the lowest in Pocheon-si, Gyeonggi-do, with 246 individuals. The periods with the highest density in each region were mostly August and September, when a large number of larvae were collected. Of the remaining ticks collected, excluding larvae, H. longicornis, the primary vector of SFTS, was identified as the dominant species at 97.3% (36,075 individuals). By environment, ticks were most abundant in grasslands with 29,104 individuals (40.5%), followed by graves with 18,251 (25.4%), copses with 13,826 (19.2%), and mountain roads with 10,743 individuals (14.9%) (Table 2).

Table 1. Total number of collected ticks by region/species.

Region		Haemaphysalis longicornis a)	Haemaphysalis flava a)	Haemaphysalis japonicaa)	Haemaphysalis formosensisa)	Amblyomma testudinarium a)	Ixodes nipponensisa)	Larva	Total (%)
Gangwon State	Inje-gun	9,081	142	10	0	0	3	14,860	24,096 (33.5)
	Samcheok-si	5,081	25	0	0	0	8	7,588	12,702 (17.7)
Incheon	Ganghwa-gun	1,018	118	0	0	0	1	122	1,259 (1.8)
Ulsan	Ulju-gun	400	0	0	0	0	0	23	423 (0.6)
Gyeonggi-do	Pocheon-si	228	6	0	0	0	0	12	246 (0.3)
	Gwangju-si	2,311	59	0	0	0	2	1,263	3,635 (5.1)
Chungcheongbuk-do	Chungju-si	2,560	8	0	0	0	1	2,283	4,852 (6.7)
Chungcheongnam-do	Nonsan-si	1,200	15	0	0	0	0	214	1,429 (2.0)
	Dangjin-si	3,796	71	0	0	0	6	1,443	5,316 (7.4)
Jeonbuk State	Gochang-gun	2,628	169	0	0	48	0	3,601	6,446 (9.0)
Jeollanam-do	Gokseong-gun	405	100	0	0	3	1	1,677	2,186 (3.0)
	Boseong-gun	874	102	0	2	6	0	379	1,363 (1.9)
Gyeongsangbuk-do	Gimcheon-si	1,037	7	0	0	2	1	179	1,226 (1.7)
	Andong-si	189	29	0	0	0	0	47	265 (0.4)
Gyeongsangnam-do	Jinju-si	621	19	0	0	2	2	1	645 (0.9)
Jeju Special Self-Governing Province	Jeju-si	4,646	38	0	0	0	0	1,151	5,835 (8.1)
Total (%)		36,075 (50.2)	908 (1.3)	10 (0.0)	2 (0.0)	61 (0.1)	25 (0.0)	34,843 (48.4)	71,924 (100.0)

^a)Including adults and nymphs.

Table 2. Total number of collected ticks by environment/species.

Species	Graves	Mountain roads	Copses	Grasslands	Total (%)
Haemaphysalis longicornisa)	5,335	4,414	7,593	18,733	36,075 (50.2)
Haemaphysalis flavaa)	303	261	163	181	908 (1.3)
Haemaphysalis japonicaa)	2	4	1	3	10 (0.0)
Haemaphysalis formosensisa)	2	0	0	0	2 (0.0)
Amblyomma testudinariuma)	13	11	15	22	61 (0.1)
Ixodes nipponensisa)	6	5	4	10	25 (0.0)
Larva	12,590	6,048	6,050	10,155	34,843 (48.4)
Total (%)	18,251 (25.4)	10,743 (14.9)	13,826 (19.2)	29,104 (40.5)	71,924 (100.0)

^a)Including adults and nymphs.

Based on developmental stage, adult ticks first appeared in April and reached peak density in July, while nymphs exhibited a high incidence in spring and early summer, with the highest density recorded in May (Figure 1). For larvae, although there were regional differences, their density gradually began to increase in July, with the highest number of individuals collected in August and September. In particular, approximately 89.2% of the ticks collected in September were larvae. Six tick species from three genera were collected, with H. longicornis collected in all regions and H. flava collected in all regions, except Ulsan (Figure 2). I. nipponensis was collected from nine of the 16 regions, with no more than 10 individuals collected per region. As a species predominantly collected in the southern part of the country, A. testudinarium was collected from five locations. Haemaphysalis japonica and Haemaphysalis formosensis were exclusively found in Inje-gun (10 individuals) and Boseong-gun (2 individuals), respectively. In 2024, the overall tick index was 46.8, with August and September having the highest value and October and November having the lowest value (Figure 3).

Figure 1. Monthly density of collected hard ticks by developmental stage.

Figure 2. Nationwide geographical distribution map of hard ticks in 2024.

(A) Total hard ticks, (B) Haemaphysalis longicornis, (C) Haemaphysalis flava, (D) Amblyomma testudinarium, (E) Ixodes nipponensis, (F) Haemaphysalis japonica, (G) larva.

Figure 3. Comparison of monthly hard tick density.

The data from 2020 to 2024 were based on tick surveillance conducted by the Division of Vectors and Parasitic Diseases, Korea Disease Control and Prevention Agency, at the same collection sites and during the same period each year (the weekday of the third week of each month). SFTS=severe fever with thrombocytopenia syndrome.

Discussion

The overall tick index for 2024 was 46.8, representing a 30.0% increase from the 2023 index (36.0) and a 13.9% increase from normal year (41.1). The collected hard ticks showed the highest numbers for each developmental stage (nymphs, adults, and larvae) in May-June, June-August, and August-September, respectively, consistent with findings from previous studies [16]. Adult hard ticks primarily lay eggs in the summer, and because larvae hatched from these eggs begin their active phase to develop into nymphs, the larval population density is characteristically high from August to September. Furthermore, since ticks typically overwinter in the nymph stage, their numbers tend to increase in the spring [16].

In 2024, the average temperature was 14.5℃, 2.0℃ above normal, and 0.8℃ higher than the hottest year on record, 2023, which was 13.7℃ [17]. Specifically, the average temperature in April, which was 2.8℃ higher than normal and 1.7℃ higher than the previous year, appears to have been the primary factor contributing to the increase in tick density. The notable rise in tick density observed in the Gangwon-do and Chungcheong-do provinces compared to 2023 is also considered as a contributing element to the overall increase in tick density. The increased density of ticks in these regions requires further investigation, including an analysis of the collection environment and changes in environmental factors. The highest number of ticks was observed in August and September, when the density increased due to the emergence of larvae. In contrast to the normal and previous years, 2024 saw the highest number of ticks in August, which could be interpreted as an aspect of climate change. In August, the average temperature (27.9℃), average maximum temperature (33.0℃), and average minimum temperature (24.1℃) were the highest since observation, and in particular, the average maximum temperature was 2.1℃ higher than that of the previous year. This increase was analyzed as a potential factor influencing the growth rate of ticks, resulting in high density. The tick density in July may have been affected by the rainy season, with increased precipitation causing a sharp drop in density. Consequently, it is hypothesized that the prevalence of ticks is contingent on climatic variables, including temperature and precipitation.

In 2024, six tick species from three genera were collected, and H. longicornis and H. flava were found to be distributed nationwide, while A. testudinarium was mainly distributed in the southern regions of the country, including Jinju-si, Gimcheon-si, Gokseong-gun, Boseong-gun, and Gochang-gun. I. nipponensis was collected in Inje-gun, Samcheok-si, Gwangju-si, Jinju-si, Gimcheon-si, Incheon-si, Chungju-si, Dangjin-si, and Gokseong-gun, with less than 10 individuals per region. H. japonica and H. formosensis were collected only in Inje-gun and Boseong-gun, respectively, which was likely due to the regional characteristics, such as mountainous areas and southern regions. H. longicornis, known to be the primary vector of SFTS, was found to be distributed throughout the country and accounted for a high proportion of the ticks collected (97.3%), excluding larvae. The survey also confirmed that all three species known to be capable of transmitting SFTS (H. flava, A. testudinarium, and I. nipponensis) were present. By environment, ticks were the most abundant in grasslands, with 29,104 individuals (40.5%), with the highest monthly densities in grasslands, except for August and November. This finding aligns with previous findings that identified grasslands as the optimal habitat for ticks [18-20].

There is a close correlation between the occurrence of SFTS cases and tick density. Adult ticks have been observed to lay approximately 3,000 to 8,000 eggs, primarily during the summer months. Transovarial transmission of the SFTSV has been demonstrated to occur from parents to eggs [8]. SFTS cases demonstrate a marked annual peak in October, a phenomenon likely attributable to a combination of factors. These include high densities of tick larvae in August and September, as well as an increased potential for human contact due to increased outdoor activities, such as fall harvesting, visiting of ancestral graves, and camping. Further clinical and epidemiological studies are necessary to ascertain the extent to which each growth stage of the tick contributes to human infection.

SFTS affects approximately 200 people each year and has one of the highest mortality rates of any vector-borne infectious disease, with a fatality rate of approximately 18.5%. However, no treatments or vaccines have been developed to target the virus [21]. Furthermore, as the climate in ROK undergoes a transition from temperate to subtropical, the risk of tick-borne infectious diseases is increasing, necessitating the implementation of preventive measures to avoid tick bites [22]. Since the surveys have been conducted in only 16 regions of the country, they are not representative of the entire country and do not provide results on SFTS infection rates. Nevertheless, national tick surveillance data can be used as the foundational data for local governments to implement tick-borne disease control policies, including SFTS. It is therefore important to protect human health from tick-borne diseases through rigorous surveillance and adherence to preventive measures.