PHWR Public Health Weekly Report

Scrub typhus is an acute febrile illness caused by the bite of a chigger—the larval stage of a trombiculid mite infected with Orientia tsutsugamushi. In the Republic of Korea (ROK), the incidence of scrub typhus typically peaks during autumn (September–November) [1]. Since 2003, the number of reported cases has increased substantially, exhibiting a stepwise escalation over several years. Although a decline was observed following the 2019 revision of case reporting criteria—which excluded suspected cases—the incidence has been rising again since 2021, with approximately 6,000 cases reported annually in recent years [2].

O. tsutsugamushi is an intracellular pathogenic bacterium that predominantly invades endothelial cells and macrophages, leading to systemic vasculitis and a broad spectrum of clinical manifestations. Clinically, scrub typhus is characterized by nonspecific symptoms such as acute fever, headache, myalgia, rash, and lymphadenopathy, along with the formation of an eschar at the site of the chigger bite [3]. An eschar is a necrotic skin lesion that develops at the bite site, characterized by a central area of black necrosis surrounded by an erythematous, papular rash. According to previous studies, an eschar is observed in approximately 46–92% of patients, and its early recognition is crucial for prompt diagnosis and initiation of treatment [4].

The incidence of eschar formation varies significantly depending on geographical characteristics, diversity of the infecting strains, and the patient’s age and immune status [5]. Reported eschar rates differ by country. While low rates are reported in some Southeast Asian regions such as Thailand and Indonesia, relatively high rates have been documented in ROK and Japan [6]. Eschars typically form on thin, moist areas of the skin—such as the trunk, groin, axillae, inframammary region, and behind the ears—making a thorough physical examination essential for their detection [7]. However, some patients do not develop an eschar, which complicates early clinical diagnosis and increases the risk of disease progression and complications. Therefore, accurate diagnosis and effective patient management require both serological tests (e.g., immunochromatographic assay [ICA], indirect immunofluorescence assay [IFA]) and molecular tests such as polymerase chain reaction (PCR) [1,8].

Under the previous surveillance system, a case of scrub typhus could be reported as confirmed if O. tsutsugamushi was isolated and identified from a specimen (e.g., blood, tissue, or eschar) via culture; if a four-fold or greater increase in antibody titer was observed in convalescent-phase serum compared to the acute phase using IFA; or if a specific gene was detected in a specimen. However, an individual could also be reported as a suspected case if specific antibodies were detected in a blood specimen through presumptive diagnostic tests (e.g., IFA, ICA). Consequently, the proportion of suspected cases was disproportionately high, accounting for approximately 95% of all annual reports compared to confirmed cases. Furthermore, since antibody detection tests (e.g., IFA, ICA) measured immunoglobulin M (IgM), IgG, and IgA titers, approximately 100 cases were reported, even during the non-epidemic season for scrub typhus (December to February), suggesting a potential overestimation due to the inability to distinguish past infections from current ones. To address this issue, on September 11, 2024, the Korea Disease Control and Prevention Agency revised the reporting criteria for suspected cases. Under the new criteria, reports must be based not only on the detection of specific antibodies but also on the presence of an eschar, an epidemiological link, and clinical symptoms consistent with scrub typhus.

Therefore, this study aims to evaluate the impact of these changes to the disease surveillance system by comparing case classification, epidemiological characteristics, and diagnostic methods before and after the revision of the scrub typhus reporting criteria.

The incidence of scrub typhus cases (including suspected cases) by sex and age was analyzed for 18,166 individuals reported between 2022 and 2024. Additionally, the monthly incidence of suspected cases was examined using data for 5,239 suspected cases between January and December 2024, as well as 52 preliminary suspected cases reported between January and April 2025. Furthermore, the status, characteristics, and diagnostic testing profiles of cases were analyzed before and after the revision of the reporting criteria, using data from 5,474, 4,804, and 5,271 cases (including suspected cases) that occurred between September 11 and December 31 in 2022, 2023, and 2024, respectively. All analyses were performed using Microsoft Office LTSC Professional Plus 2021 (Microsoft).

1. Monthly Incidence of Suspected Scrub Typhus Cases (January 2024–April 2025)

Although the data for 2025 are preliminary, a comparison of reported suspected cases shows a notable decline. From January to April 2024, the number of suspected scrub typhus cases was 153, 74, 56, and 100, respectively, with a monthly average of 95.8 cases. In contrast, during the same period in 2025, the numbers were 17, 5, 6, and 24, respectively, yielding a monthly average of 13.0 suspected cases. This represents an approximately 86.4% decrease compared to the same period in the previous year (January–April), indicating a substantial overall decline in the number of suspected cases (Figure 1).

Figure 1. Monthly distribution of scrub typhus suspected cases

2. Comparison of Confirmed and Suspected Case Classification after Revision of Reporting Criteria

Following the revision of the scrub typhus reporting criteria on September 11, 2024, case classifications reported between September 11 and December 31 in 2022, 2023, and 2024 were compared. The results showed that the proportion of suspected cases decreased by 12.5 percentage points compared to the same period in 2023, while the proportion of confirmed cases increased correspondingly (Figure 2).

Figure 2. Proportion of cases and suspected cases of scrub typhus

3. Comparison of Eschar Presence by Case Classification (Same Period in 2022–2024 after Revision of Reporting Criteria)

A comparison of eschar presence among cases reported between September 11 and December 31 in 2022, 2023, and 2024 revealed that, following the revision of the reporting criteria, the number of suspected cases in 2024 was 4,255, representing a decrease of 893 cases compared to 2022 and 223 cases compared to 2023. However, the number of suspected cases with a confirmed eschar after September 11, 2024, increased by 1,035 compared to 2022 and by 1,235 compared to 2023 (Table 1). Additionally, an analysis of diagnostic methods for 236 confirmed cases without an eschar or with unconfirmed eschar status in 2024 showed that 93 cases (39.4%) were classified as confirmed through a combination of presumptive diagnosis (antibody detection tests) and confirmatory diagnosis (PCR), while 143 cases (60.6%) were confirmed solely through confirmatory diagnosis (Table 2).

Table 1 Eschar confirmation of cases (confirmed/suspected) by year

Variable	2022				2023				2024
	Eschar	Unconfirmed	Total		Eschar	Unconfirmed	Total		Eschar	Unconfirmed	Total
Suspected cases	3,220 (62.5)	1,928 (37.5)	5,148 (100.0)		3,020 (67.4)	1,458 (32.6)	4,478 (100.0)		4,255 (100.0)	0 (-)	4,255 (100.0)
Confirmed cases	219 (67.2)	107 (32.8)	326 (100.0)		237 (72.7)	89 (27.3)	326 (100.0)		780 (76.8)	236 (23.2)	1,016 (100.0)
Total	3,439 (62.8)	2,035 (37.2)	5,474 (100.0)		3,257 (67.8)	1,547 (32.2)	4,804 (100.0)		5,035 (95.5)	236 (4.5)	5,271 (100.0)

Period: September 11–December 31. Unit: n (%). -=not available..

Table 2 Laboratory diagnosis for patients without eschar in 2024

Diagnostic method	Unconfirmed eschar
Presumptive diagnosis (ICA or IFA) negative/confirmatory diagnosis (PCR) positive	93 (39.4)
Presumptive diagnosis not performed/confirmatory diagnosis (≥4-fold increase in the antibody titer between paired serum samples) positive	11 (4.7)
Presumptive diagnosis not performed/confirmatory diagnosis (PCR) positive	132 (55.9)
Total	236 (100.0)

Period: September 11–December 31. Unit: n (%). ICA=immunochromatographic assay; IFA=indirect immunofluorescence assay; PCR=polymerase chain reaction..

4. Comparison of Eschar Detection Rates by Body Site (Same Period in 2022–2024 after Revision of Reporting Criteria)

Among the cases reported between September 11 and December 31 in 2022, 2023, and 2024, the number of patients with a confirmed eschar was 3,439, 3,257, and 5,035, respectively. For these cases, the respective eschar detection rates by body site in 2022, 2023, and 2024 were as follows: axilla and flank region—27.9%, 26.7%, and 14.5%; trunk (including shoulders and back)—10.2%, 10.7%, and 18.0%; and legs (including thighs and calves)—18.6%, 20.3%, and 22.2% (Figure 3).

Figure 3. Frequency and location of eschar in patients with scrub typhus (2022–2024)

This study evaluated the impact of changes in the surveillance system on case classification and diagnosis by comparing the incidence and epidemiological characteristics of scrub typhus cases before and after the revision of the reporting criteria. The results showed that, following the criteria revision on September 11, 2024, the proportion of suspected cases among all reported cases was 80.7% (4,255 individuals), representing a decrease of 12.5 percentage points from the 93.2% (4,478 individuals) during the same period in 2023. However, the number of suspected cases with a confirmed eschar in 2024 increased by 1,035 compared to 2022 and by 1,235 compared to 2023. This finding supports the conclusion that explicitly including eschar confirmation in the reporting criteria for suspected cases, along with an increased proportion of confirmed cases diagnosed via PCR, has enhanced clinical diagnostic accuracy. Furthermore, the number of suspected cases between January and April 2025 decreased by 86.4% compared to the same period in the previous year. This change indicates that incorporating eschar confirmation into the criteria for suspected cases has improved the capacity to differentiate between recent and non-recent infections, thereby contributing to more accurate case reporting and enhancing the reliability of infectious disease surveillance data.

Therefore, this study is significant in demonstrating that comprehensive criteria for presumptive diagnosis can substantially improve the efficiency and accuracy of infectious disease reporting and surveillance systems. Before the revision, reporting was possible based solely on serological test results (IFA, ICA), which made it challenging to determine the actual scale of recent infections. After the revision, the combined use of eschar confirmation and PCR-based confirmatory diagnosis has reduced unnecessary reporting while improving diagnostic reliability.

The eschar is a pathological necrotic lesion that develops at the site of a chigger bite. Since patients are often unaware of it during the early stages of illness, identifying an eschar through physical examination can serve as a critical diagnostic clue. In this study, the proportion of cases in 2024 without a confirmed eschar was only 4.5%, while 95.5% of patients were diagnosed with a confirmed eschar. This rate is higher than the 60–80% previously reported in ROK [9], suggesting both improved clinician diagnostic skills and strong compliance with the revised reporting criteria.

Moreover, among the 236 confirmed cases in 2024 without an eschar or with unconfirmed eschar status, 93 (39.4%) were identified through both presumptive (IFA, ICA) and confirmatory (PCR) tests, preventing these cases from being overlooked. This highlights the limitations of relying solely on serological tests (IFA, ICA) for diagnosis and underscores the importance of incorporating PCR as a confirmatory diagnostic tool.

Early antibiotic treatment can delay antibody formation, making pathogen detection via PCR a more reliable diagnostic method [10]. Therefore, clinical guidelines should recommend that clinicians confirm the presence of an eschar for presumptive diagnosis. If the eschar is indistinct or absent, a molecular test (PCR) should be performed promptly to identify patients with scrub typhus and ensure timely treatment.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: HJN, JHK, SDL. Data curation: HJN. Formal analysis: HJN, SKL, Dong-Hwi Kim. Methodology: HJN, JHK. Visualization: HJN, SKL. Writing – original draft: HJN, SKL, Do-Hyeong Kim. Writing – review & editing: HJN, JHK, SDL.